High temperature superconductors and pulsed laser deposition methods for making the same

Abstract

This disclosure relates to methods for making compounds of formula (I): LnDm(BxB'1-x)r(ZtZ'1-t)qMpAyA's (I), in which n, m, x, r, t, q, p, y, s, L, D, B, B', Z, Z', M, A, and A' are defined in the specification and methods for making the same. These compounds can exhibit superconductivity at a high temperature. The methods include using pulsed laser deposition.

Description

[0001]Attorney Docket No.: 40218-0028WO1 HIGH TEMPERATURE SUPERCONDUCTORS AND PULSED LASER DEPOSITION METHODS FOR MAKING THE SAME TECHNICAL FIELD This disclosure relates to high temperature superconductors, as well as related methods and devices. BACKGROUND In 1986, Bednorz and Muller surprised the solid-state physics community with their announcement of a new class of superconducting materials having criticaltemperatures (Tc) significantly higher than those achieved previously [Bednorz, et al., Z.Phys. B 64, 189 (1986)]. These materials are ceramics consisting of copper oxide layersseparated by buffer cations. In Bednorz and Muller's original compound (LBCO), the buffer cations are lanthanum and barium. Inspired by their work and motivated by his own critical temperature under pressure measurements, Paul Chu synthesized a similar material in which the buffer ions were yttrium and barium. This material was YBCO, the first superconductor with a Tc above the boiling point of liquid nitrogen (77K) [Wu, etal., Phys. Rev. Lett. 58, 908 (1987)]. The highest critical temperature reported to date is164K, obtained by a mercury-based superconductor at a pressure of 31 GPa. [Putilin, etal., Nature 362, 226 (1993), and Chu, et al., Nature 365, 323 (1993)].SUMMARY This disclosure is based in part on the unexpected discovery that certain metal oxides containing alkali metal ions in their crystal structures are superconductors at extremely high temperatures (e.g., up to about 550K) and such crystals can be made by forming a crystal having a first composition and replacing certain ions in that crystal with other ions, including a halogen ion, by exposure of the first crystal to a powdered salt at elevated temperature. In one aspect, this disclosure features a compound of formula (I): LnDm(BxB’1-x)r(ZtZ’1-t)qMpAyA’s(I), Attorney Docket No.: 40218-0028WO1 in which n is a number from 0 to 3; m is a number from 0 to 6, x is a number from 0.1 to 1; r is a number from 1 to 8; t is a number from 0 to 1; q is a number from 0 to 6; p is a number from 1 to 7; s is a number from 0 to 20; y is a number from 0 to 20; L includes at least one metal ion selected from the group consisting of transition metal ions and post-transition metal ions; D includes at least one element selected from the group consisting of the elements in Groups IIIA and IVA in the Periodic Table; B includes at least one first alkali metal ion; B’ includes at least one first ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; Z includes at least one second alkali metal ion; Z’ includes at least one second ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; M includes at least one transition metal ion; A includes at least one anion; and A’ includes at least one halogen anion. The compound of formula (I) is a crystalline compound. In another aspect, this disclosure features a compound, which is a crystalline metal oxide containing at least one transition metal ion (e.g., Cu ion), at least one alkaline earth metal ion (e.g., Sr or Ca) or at least one rare earth metal ion, and at least one chalcogen anion, in which from 10% to 100% of the at least one alkaline earth metal ion or at least one rare earth metal ion is replaced by an alkali metal ion and from 10% to 100% of the at least one chalcogen anion is replaced by a halogen anion. In another aspect, this disclosure features a method that includes (1) mixing a crystalline metal oxide with an alkali metal halogen salt containing an alkali metal cation and a halogen anion to form a mixture, in which the metal oxide contain at least one transition metal ion and at least one alkaline earth metal ion and the atomic ratio between the alkali metal ion and the at least one alkaline earth metal ion is higher than 1:1; and (2) sintering the mixture at an elevated temperature to form a crystalline compound containing the alkali metal cation and the halogen anion. In another aspect, this disclosure features a method that includes (1) mixing at least one metal oxide (e.g., CuO, Y2O3, and Bi2O3) with an alkali metal halogen salt containing an alkali metal cation and a halogen anion to form a mixture, in which the at least one metal oxide contain at least one transition metal ion; and (2) sintering the Attorney Docket No.: 40218-0028WO1 mixture at an elevated temperature to form a crystalline compound containing the alkali metal cation and the halogen anion. In another aspect, this disclosure features a device that includes a superconducting compound described herein and is superconductive (e.g., exhibiting superconductive properties such as capable of carrying a superconductive current) at a temperature of at least 200K (e.g., at least 273K). In yet another aspect, this disclosure features a composition containing the superconducting compound described herein. Other features, objects, and advantages will be apparent from the description, drawings, and claims. DESCRIPTION OF DRAWINGS FIG.1 is a scheme illustrating an octahedral cluster in the crystal structure of a superconducting compound described herein. FIG.2A is a scheme illustrating the relationship among the energy band of a material, its Fermi level and its corresponding conductance according to the Wilson rule and the present disclosure. FIGs.2B and 2C present the Fermi landscape of a simple metal and a superconductor. FIG.2B shows a simple isotropic 2D metal. The Fermi surface appears as a 1D circle. FIG.2C shows the Fermi landscape of an isotropic 2D superconductor according to the present disclosure. The Fermi volume appears as a 2D ring. FIGs.2D-2G present realistic anisotropic Fermi landscape. FIG.2D is a scheme illustrating measured Fermi landscape of Bi2212 [Norman et. al., Phys. Rev. B, 52, 615 (1995)]. FIG.2E is a scheme illustrating the Fermi landscape of a possible higher temperature superconductor predicted by the inventor. FIGs.2F and 2G are a scheme illustrating the Fermi landscape of a possible even higher temperature superconductor predicted by the inventor. FIGs.3A-3G show known electronic structure results measured by ARPES. FIGs. 4A-4C show known cluster calculations. The energy difference 2 isdisplayed as function of the distance between the buffer ion and the plane. FIG.4A shows Attorney Docket No.: 40218-0028WO1the effect of the buffer ion radius on 2 . FIG. 4B shows the effect of the buffer ion chargeon 2 . FIG. 4C shows the effect of the buffer ion softness on 2 .FIG.4D presents a table listing the properties of known cuprate superconductors. The critical temperature can be explained by the model described herein. FIGs.4E and 4F show the crystal structure of LBCO and YBCO, respectively, for clarity. FIG.5 is a scheme illustrating another exemplary crystal structure of a superconducting compound described herein. FIG.6 is a schematic diagram showing an example of a reaction chamber for forming a superconducting crystal. FIGs.7A-7E are scanning electron microscope (SEM) photographs of an example superconductor composed of Bi2(Sr,Ca)2CuO5 and other oxides. FIG.8 are plots showing results of X-ray Diffraction (XRD) studies of the example shown in FIGs.7A-7E with Rietveld analysis. FIG.9 is a plot of resistance v. current made at 25°C for the example superconductor shown in FIGs.7A-7E. FIG.10 is a plot of magnetic moment v. temperature for the example superconductor shown in FIGs.7A-7E. FIG.11 is a SEM photograph showing microstructure of the granular material of the example superconductor shown in FIGs.7A-7E. FIG.12 is a plot showing temperature dependence of the critical current in the example superconductor shown in FIGs.7A-7E. FIG.13 is a plot showing dependence of the derivative dIC / DT on temperature for the example superconductor shown in FIGs.7A-7E. FIG.14 is a plot showing the dependence of resistance on temperature in the example superconductor shown in FIGs.7A-7E. FIG.15 is a schematic diagram showing an example of a pulsed laser deposition (PLD) system for forming a superconducting crystal via PLD. FIGs.16A and 16B are SEM photographs of an example superconductor composed approximately of Bi2SrK3CaCu2FO10. Attorney Docket No.: 40218-0028WO1 FIGs.17A-17D are SEM photographs of the example superconductor shown in FIGs.16A and 16B after annealing. FIGs.18A and 18B are SEM photographs of example targets for pulsed layer deposition. In the insets, images of the 1” diameter targets, respectively. FIGs.19A and 19B are SEM photographs of an example of a modified BSCCO material. FIG.20A is a photograph of an example tape with a modified BSCCO material. FIG.20B is a plot showing resistivity as a function of temperature for an example tape with a modified BSCCO material (“QDM tape”) in comparison to copper. FIGs.21A and 21B are SEM photographs of example targets for pulsed layer deposition. FIG.22 is a plot from in situ X-ray diffraction measurements conducted on an example modified BSCCO layer. FIG.23 is a plot showing dependence of M / H as a function of temperature for an example modified BSCCO material. FIGs.24A-24F are plots showing the dependence of M / H on applied magnetic field for different temperatures for an example modified BSCCO material. FIG.25 is a plot showing a dependence of resistance on temperature for another example of a modified BSCCO material. FIGs.26A and 26B are SEM photographs of a further example of a modified BSCCO material. Like reference symbols in the various drawings indicate like elements. DETAILED DESCRIPTION This disclosure generally relates to high temperature superconductors (HTS), i.e., compounds exhibiting superconductivity at a high temperature (e.g., from 273K to 550K). In some embodiments, a high temperature superconductor described herein is a compound of formula (I): Attorney Docket No.: 40218-0028WO1 LnDm(BxB’1-x)r(ZtZ’1-t)qMpAyA’s(I), in which n is any number from 0 to 3 (e.g., 0, 1, 2, or 3); m is any number from 0 to 6; x is any number from 0.1 to 1; r is any number from 1 to 8 (e.g., 1, 2, 3, 4, 5, 6, 7, or 8); t is any number from 0 to 1; q is any number from 0 to 6 (e.g., 0, 1, 2, 3, 4, 5, or 6); p is any number from 1 to 7 (e.g., 1, 2, 3, 4, 5, 6, or 7); s is any number from 0 to 20; y is any number from 0 to 20; L includes at least one metal ion selected from the group consisting of transition metal ions and post-transition metal ions; D includes at least one element selected from the group consisting of the elements in Groups IIIA (e.g., B, Al, Ga, In, or Tl) and IVA (e.g., C, Si, Ge, Sn, or Pb) in the Periodic Table; B includes at least one first alkali metal ion; B’ includes at least one first ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; Z includes at least one second alkali metal ion; Z’ includes at least one second ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; M includes at least one transition metal ion; A includes at least one chalcogen anion; and A’ includes at least one halogen anion. The compound of formula (I) is a crystalline compound. In some embodiments, the compound of formula (I) is a single-phase compound. In some embodiments, the compound of formula (I) is a single crystal compound. In general, n, m, x, r, t, q, p, s, and y can be either an integer or a non-integer. In some embodiments, the first alkali metal ion is different from the second alkali metal ion. In some embodiments, the first alkali metal ion is the same as the second alkali metal ion. In some embodiments, the first ion assigned to B’ is different from the second ion assigned to Z’. In some embodiments, the first ion assigned to B’ is the same as the second ion assigned to Z’. In some embodiments, the element assigned to D is different from the metal ion assigned to L. In some embodiment, the element assigned to D is the same as the metal ion assigned to L. The term “alkali metal ion”, as used herein, refers to an ion containing an element selected from group IA of the periodic table, i.e., Li, Na, K, Rb, Cs, and Fr or any combination thereof. In general, the alkali metal ion can have a valence number of +1. In some embodiments, the alkali metal ion can form a molecular cluster having effective Attorney Docket No.: 40218-0028WO1 electric charge of between +1 and zero. In such embodiments, the molecular cluster can include one or more negative ions in the proximity of an alkali metal ion such that the positive charge on the alkali metal ion is compensated by the negative charge on the negative ion. The term “alkaline earth metal ion”, as used herein, refers to a metal ion having a valence number of +2 and containing an element selected from group IIA of the periodic table, i.e., Be, Mg, Ca, Sr, Ba, Ra or any combination thereof. The term “transition metal ion”, as used herein, refers to a metal ion containing an element selected from Groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB and IIB of the Periodic Table. In some embodiments, the transition metal mentioned herein can be Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir, Hg, or any combination thereof. In some embodiments, the transition metal is Cu. In other embodiments, the transition metal is Fe or Zn. The term “post-transition metal ion”, as used herein, refers to a metal ion containing an element selected from Group IIIA, IVA and VA of the Periodic Table. In some embodiments, the post-transition metal mentioned herein can be Al, Ga, In, Tl, Sn, Pb, Bi, Hg or any combination thereof. The term “rare earth metal ion”, as used herein, refers to a metal ion containing an element selected from scandium (Sc), yttrium (Y), the lanthanide series of metals (having atomic numbers from 57-71) and the actinide series of metals (having atomic numbers from 89-103) in the Periodic Table. Examples of the rare earth metals in the lanthanide series include La, Ce, Pr, Sm, Gd, Eu, Tb, Dy, Er, Tm, Nd, Yb, or any combination thereof. Examples of the rare earth metals in the actinide series include Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, or any combination thereof. The term “anion”, as used herein, can include a simple anion, a halide or halogenanion, a chalcogen anion, an organic anion, an oxoanion, a pnictide anion, or any combination thereof. Examples of simple anions include those containing O, S, Se, Te, N, P, As, or Sb as a single atom. Examples of halide or halogen anions include those containing F, Cl, Br, I, At or any combination thereof (such as IBr-3, Cl2I-3, Br2I-3and I2Cl-3). As used herein, the terms “halide anions” and “halogen anions” are used Attorney Docket No.: 40218-0028WO1 interchangeably. Examples of chalcogen anions include those containing O, S, Se, Te or any combination thereof. Examples of organic anions include acetate (CH3COO-), formate (HCOO-), oxalate (C2O4-2), cyanide (CN-) or any combination thereof. Examples of oxoanion include AsO4-3, AsO3-3, CO3-2, HCO3-, OH-, NO3-, NO2-, PO4-3, HPO4-2,SO4-2, HSO4-, S2O3-2, SO3-2, ClO4-, ClO3-, ClO2-, OCl-, IO3-, BrO3-, OBr-, CrO4-2, Cr2O7-2 orany combination thereof. Examples of pnictide anions include those containing N, P, As, Sb, or any combination thereof. In some embodiments, the anion mentioned herein can be an anion containing any combination of F, Cl, Br, I, O, S, Se, Te, N, P, As, and Sb. In some embodiments, the anion mentioned herein can be NCS-, CN-, or NCO-. In some embodiments, L in formula (I) can include Bi, Tl, Cu, or Hg. In some embodiments, D in formula (I) can include C, Si, Ge, Sn, Pb, or Al. In some embodiments, B in formula (I) can include Li, Na, K, Rb, or Cs. In some embodiments, B’ in formula (I) can include La, Mg, Ca, Sr, or Ba. In some embodiments, Z in formula (I) can include Li, Na, K, Rb, or Cs. In some embodiments, Z’ in formula (I) can include Ca or Y. In some embodiments, M in formula (I) can include Cu or Fe. In some embodiments, A in formula (I) can include O, S, Se, P, or As. In some embodiments, A’ in formula (I) can include F, Cl, Br, or I. In some embodiments, the compounds of formula (I) can satisfy the following equation: x*r + t*q = s. In some embodiments, the compound presented herein can include crystal structures of multiple compounds of formula (I). In that case, the compound is called a superstructure or an intergrowth. In some embodiments, the superconducting compounds of formula (I) can be compounds of formula (Ia): LnDm(BxB’1-x)r(ZtZ’1-t)qMpAyA’sA’’u (Ia), in which n, m, x, r, t, p, q, y, s, L, D, B, B’, Z, Z’, M, A, and A’ are defined above, A’’ can include a halogen anion (e.g., F, Cl, Br, or I), and u can be a number from 0 to 20. In such embodiments, p can be any number from 1 to 3. In general, A’’ can be the same as or can be different from A’. Attorney Docket No.: 40218-0028WO1 In some embodiments, the compounds of formula (Ia) can satisfy the following equation: x*r = s. In such embodiments, the compounds of formula (Ia) can also satisfy the following equation: t*q = u. In some embodiments, the superconducting compounds of formula (Ia) can be compounds of formula (II): LnDm(BxB’1-x)r(ZtZ’1-t)qCupOyA’sA’’u (II), in which n, m, x, r, t, p, q, y, s, u, L, D, B, B’, Z, Z’, A’, and A’’ are defined above. In such embodiments, p can be any number from 1 to 3. In some embodiments, the superconducting compounds of formula (Ia) can be compounds of formula (III): LnDm(BxB’1-x)r(ZtZ’1-t)qCu2OyA’sA’’u(III), in which n, m, x, r, t, q, y, s, u, L, D, B, B’, Z, Z’, A’, and A’’ are defined above. In such embodiments, q can be any number from 1 to 2 and r can be any number from 2 to 4. Referring to formula (II), a subset of superconducting compounds are those in which q is 0 or 1 and r is any number between 2 and 6. In such embodiments, L can include Bi, Tl, Cu, Pb, or Hg; n can be between 0 and 4; D can be carbon; m can be any number from 0 to 4; B can include K, Rb, or Cs; B’ can include Sr, Ba, Ca and Y; x can be a number from 0.1 to 1; and p can be 1, 2, or 3. Referring to formula (III), a subset of superconducting compounds are those in which q is 1 and r is 2. In such embodiments, L can include Bi, Tl, Cu, Pb, or Hg; n can be 0, 1, or 2; D can be carbon; m can be any number from 0 to 4; B can include K, Rb, or Cs; B’ can include Sr; x can be a number from 0.1 to 1; t can be 0; and Z’ can include Ca. Referring to formula (III), another subset of superconducting compounds are those in which q is 1, r is 2, and t is a number greater than 0. In such embodiments, L can include Bi, Tl, or Hg; n can be 0, 1, or 2; D can be carbon; m can be any number from 0 to 4; B can include K, Rb, or Cs; B’ can include Sr; x can be a number from 0.1 to 1; and Z’ can include Ca. Referring to formula (II), a subset of superconducting compounds are those in which n is 2, m is a number from 0 to 4, r is a number from 2 to 8, q is a number from 0 to 3, p is 4, L is Bi, B is K, Rb, or Cs, B’s is Sr, Z is K, Rb, or Cs, and Z’ is Ca. Attorney Docket No.: 40218-0028WO1 Referring to formula (II), a subset of superconducting compounds are those in which n is 0, m is a number from 0 to 4, x is 1, t is 1, r is 4, q is 2, p is 4 or 7, B is K, Rb, or Cs, Z is Na. Referring to formula (II), another subset of superconducting compounds are those in which n is 1, m is a number from 0 to 4, x is 1, t is 0 or 1, r is 2, 4, or 6, q is 0, 1, or 2, p is 1, 2, or 3, L is Hg, B is K, Rb, or Cs, Z is Na, and Z’ is Ba. Referring to formula (II), another subset of superconducting compounds are those in which n is 1, 2, or 3, m is a number from 0 to 4, x is 1, t is 0 or 1, r is 2, 4, or 6, q is 0, 1, 2, 3, or 4, p is 1, 2, 3, 4, or 5, L is Tl, B is K, Rb, or Cs, Z is Na, and Z’ is Ba. Referring to formula (II), another subset of superconducting compounds are those in which n is 2, m is a number from 0 to 4, x is 1, t is 1, r is 2, 4, or 6, q is 0 or 2, p is 1 or 3, L is Bi, B is K, and Z is Na. Referring to formula (II), another subset of superconducting compounds are those in which n is a number from 0 to 1, m is a number from 0 to 1, x is a number from 0.1 to 1, r is 2 or 4, t is a number from 0 to 1, q is 0, 1, or 2, p is 2, 3, or 6, L is Y, B is K, Rb, or Cs, B’ is Sr or Ba, Z is Na, K, Rb, or Cs, and Z’ is Y. Referring to formula (II), another subset of superconducting compounds are those in which n is 0 or 1, m is a number from 0 to 1, x is a number from 0 to 1, r is 2 or 4, t is a number from 0 to 1, q is 1, p is 2, 3, 4, 5, or 6, L is Cu, B is K, Rb, or Cs, B’ is Ba, and Z is Na. In some embodiments, the superconducting compounds of formula (I) can be compounds of formula (IV): BMAA’ (IV), in which B, M, A, and A’ are defined above. Examples of such compound include NaCuOF, NaCuOCl, KCuOF, KCuOCl, RbCuOF, RuCuOCl, CsCuOF, and CsCuOCl. In some embodiments, the superconducting compounds of formula (I) can be compounds of formula (V): (BxB’1-x)rZ’qMpAyA’s (V), in which B, Z’, M, A, A’, x, r, q, p, y, and are defined above. In some embodiments, a subset of superconducting compounds of formula (V) are those in which B is K or Rb, B; is Ba, Z’ is Y, M is Cu, A is O, and A’ is F, x is a number of 0.2 to 1, r is a number of 1 to 5, q is a number of 1 to 4, p is a number of 1 to 5, y is a number of 2 to 7, and s is a number of 0.3 to 4. Attorney Docket No.: 40218-0028WO1 Examples of such compound include Y2Rb4.5CuO3.5F3.5, Y1.5Rb3CuO3F3, Y2Rb3Cu2F4O4, Y2.5RbCu2.5O4F1.5, Y4RbCu2O7F0.3, YRb2Cu2.5O4F, YBa1.5K0.25Cu2O3.5F0.25, YBa2KCu4O6F0.5, Y3Ba2KCu2O6F0.5, Y1.5BaKCu1.5O3F2, and Y2KCu5O4F1.5. In some embodiments, B’ is a metal ion having a first atomic number, Z’ is a metal ion having a second atomic number, and the second atomic number is smaller than the first atomic number. For example, B’ can be a metal ion containing Sr or Ba and Z’ can be a metal ion containing Ca. In some embodiments, x in formula (I) ranges from 0.1 to 1 (e.g., from 0.2 to 1, from 0.3 to 1, from 0.4 to 1, from 0.5 to 1, from 0.55 to 1, from 0.6 to 1, from 0.65 to 1, from 0.7 to 1, from 0.75 to 1, from 0.8 to 1, from 0.85 to 1, from 0.9 to 1, from 0.95 to 1, from 0.97 to 1, from 0.98 to 1, or from 0.99 to 1). In some embodiments, x in formula (I) is 1. Without wishing to be bound by theory, it is believed that increasing the value of x can increase the critical temperature (Tc) of a superconducting compound of formula (I) as an increasing amount of the B’ ion (i.e., alkaline earth metal ions or rare earth metal ions) in the crystal structure in the compound of formula (I) is replaced by the B ion (i.e., an alkali metal ion). In some embodiments, t in formula (I) ranges from 0.1 to 1 (e.g., from 0.2 to 1, from 0.3 to 1, from 0.4 to 1, from 0.5 to 1, from 0.6 to 1, from 0.7 to 1, from 0.8 to 1, from 0.9 to 1, from 0.95 to 1, from 0.98 to 1, or from 0.99 to 1). In some embodiments, t in formula (I) is 1. Without wishing to be bound by theory, it is believed that increasing the value of t (e.g., when t is above 0.5) can increase Tc of a superconducting compound of formula (I) as an increasing amount of the Z’ ion (i.e., alkaline earth metal ions or rare earth metal ions) in the crystal structure in the compound of formula (I) is replaced by the Z ion (i.e., an alkali metal ion). In some embodiments, n in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 3. For example, n can be any number from 0.1 to 2.9 (e.g., from 0.2 to 2.8, from 0.3 to 2.7, from 0.4 to 2.6, from 0.5 to 2.5, from 0.6 to 2.4, from 0.7 to 2.3, from 0.8 to 2.2, from 0.9 to 2.1, from 1 to 2, from 1.1 to 1.9, from 1.2 to 1.8, from 1.3 to 1.7, from 1.4 to 1.6, or 1.5). Attorney Docket No.: 40218-0028WO1 In some embodiments, m in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 6. For example, m can be any number from 0.1 to 5.9 (e.g., from 0.2 to 5.8, from 0.3 to 5.7, from 0.4 to 5.6, from 0.5 to 5.5, from 0.6 to 5.4, from 0.7 to 5.3, from 0.8 to 5.2, from 0.9 to 5.1, from 1 to 5, from 1.1 to 4.9, from 1.2 to 4.8, from 1.3 to 4.7, from 1.4 to 4.6, from 1.5 to 4.5, from 1.6 to 4.4, from 1.7 to 4.3, from 1.8 to 4.2, from 1.9 to 4.1, from 2 to 4, from 2.1 to 3.9, from 2.2 to 3.8, from 2.3 to 3.7, from 2.4 to 3.6, from 2.5 to 3.5, from 2.6 to 3.4, from 2.7 to 3.3, from 2.8 to 3.2, from 2.9 to 3.1, or 3). In some embodiments, the sum of n and m is an integer. In some embodiments, r in formula (I) can be any number (e.g., an integer or a non-integer) from 1 to 8. For example, r can be any number from 1.1 to 7.9 (e.g., from 1.2 to 7.8, from 1.3 to 7.7, from 1.4 to 7.6, from 1.5 to 7.5, from 1.6 to 7.4, from 1.7 to 7.3, from 1.8 to 7.2, from 1.9 to 7.1, from 2 to 7, from 2.1 to 6.9, from 2.2 to 6.8, from 2.3 to 6.7, from 2.4 to 6.6, from 2.5 to 6.5, from 2.6 to 6.4, from 2.7 to 6.3, from 2.8 to 6.2, from 2.9 to 6.1, from 3 to 6, from 3.1 to 5.9, from 3.2 to 5.8, from 3.3 to 5.7, from 3.4 to 5.6, from 3.5 to 5.5, from 3.6 to 5.4, from 3.7 to 5.3, from 3.8 to 5.2, from 3.9 to 5.1, from 4 to 5, from 4.1 to 4.9, from 4.2 to 4.8, from 4.3 to 4.7, from 4.4 to 4.6, or 4.5). In some embodiments, q in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 6. For example, q can be any number from 0.1 to 5.9 (e.g., from 0.2 to 5.8, from 0.4 to 5.6, from 0.6 to 5.4, from 0.8 to 5.2, from 1 to 5, from 1.2 to 4.8, from 1.4 to 4.6, from 1.6 to 4.4, from 1.8 to 4.2, from 2 to 4, from 2.2 to 3.8, from 2.4 to 3.6, from 2.6 to 3.4, or from 2.8 to 3.2). In some embodiments, p in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 7. For example, p can be any number from 0.1 to 6.9 (e.g., from 0.2 to 6.8, from 0.4 to 6.6, from 0.6 to 6.4, from 0.8 to 6.2, from 1 to 6, from 1.2 to 5.8, from 1.4 to 5.6, from 1.6 to 5.4, from 1.8 to 5.2, from 2 to 5, from 2.2 to 4.8, from 2.4 to 4.6, from 2.6 to 4.4, from 2.8 to 4.2, from 3 to 4, from 3.2 to 3.8, from 3.4 to 3.6, or 3.5). In some embodiments, s in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 20. For example, s can be any number from 1 to 19 (e.g., from 1.5 to 18.5, from 2 to 18, from 2.5 to 17.5, from 3 to 17, from 3.5 to 16.5, from 4 to 16, from Attorney Docket No.: 40218-0028WO1 4.5 to 15.5, from 5 to 15, from 5.5 to 14.5, from 6 to 14, from 6.5 to 13.5, from 7 to 13, from 7.5 to 12.5, from 8 to 12, from 8.5 to 11.5, from 9 to 11, from 9.5 to 10.5, or 10). In some embodiments, y in formula (I) can be any number (e.g., an integer or a non-integer) from 0 to 20. For example, y can be any number from 1 to 19 (e.g., from 1.5 to 18.5, from 2 to 18, from 2.5 to 17.5, from 3 to 17, from 3.5 to 16.5, from 4 to 16, from 4.5 to 15.5, from 5 to 15, from 5.5 to 14.5, from 6 to 14, from 6.5 to 13.5, from 7 to 13, from 7.5 to 12.5, from 8 to 12, from 8.5 to 11.5, from 9 to 11, from 9.5 to 10.5, or 10). In some embodiments, the compounds of formula (I) can satisfy the following equation: x*r + t*q = s. In such embodiments, the number of the alkali metal ions (or the valence charges of the alkali metal ions) is identical to the number of the halogen anions (or the valence charges of the halogen ions). Without wishing to be bound by theory, it is believed that such a compound of formula (Ia) can form a structure that is more stable thermodynamically due to the formation of an alkali metal halogen salt layer. In some embodiments, the compounds of formula (Ia) can satisfy the following equation: x*r = s. In such embodiments, the compounds of formula (Ia) can also satisfy the following equation: t*q = u. In such embodiments, the number of the alkali metal ions (or the valence charges of the alkali metal ions) is identical to the number of the halogen anions (or the valence charges of the halogen ions). Without wishing to be bound by theory, it is believed that such a compound of formula (Ia) can form a structure that is more stable thermodynamically due to the formation of an alkali metal halogen salt layer. In some embodiments, a superconducting compound described herein is a crystalline metal oxide containing at least one transition metal ion (e.g., a Cu ion), at least one alkaline earth metal ion (e.g., a Sr or Ba ion) or at least one rare earth metal ion, and at least one chalcogen anion (e.g., O anion), in which from 10% to 100% of the at least one alkaline earth metal ion or at least one rare earth metal ion (i.e., in the crystal structure) is replaced by an alkali metal ion (e.g., an ion of Li, Na, K, Rb, or Cs) from 10% to 100% of the at least one chalcogen anion is replaced by a halogen anion (e.g., F or Cl). Examples of the crystalline metal oxides before modification include Bi2Sr2CaCu2Oy (Bi2212), Bi2Sr2Ca2Cu3Oy (Bi2223), and YBa2Cu3O7 (YBCO). In some embodiments, the superconducting compound is a crystalline metal oxide described Attorney Docket No.: 40218-0028WO1 above in which from 20% to 100% (e.g., from 30% to 100%, from 40% to 100%, from 50% to 100%, from 60% to 100%, from 70% to 100%, from 80% to 100%, from 90% to 100%, from 95% to 100%, from 99% to 100%, or 100%) of the at least one alkaline earth metal ion or at least one rare earth metal ion in the crystal structure is replaced by an alkali metal ion. Without wishing to be bound by theory, it is believed that a superconducting metal oxide in which a higher amount (e.g., more than 50%) of an alkaline earth metal ion in its crystal structure is replaced by an alkali metal ion would exhibit a higher Tc based on the model described below. In some embodiments, the superconducting compound is a crystalline metal oxide described above in which from 10% to 90% (e.g., from 15% to 85%, from 20% to 80%, from 25% to 75%, from 30% to 70%, from 35% to 65%, from 40% to 60%, from 45% to 55%, or 50%) of the at least one chalcogen anion in the crystal structure is replaced by a halogen anion. Without wishing to be bound by theory, it is believed that a certain amount of chalcogen anion is replaced by a halogen anion can form a structure that is more stable thermodynamically due to the formation of an alkali metal halogen salt layer. In some embodiments, the crystalline metal oxide described above can further include a post-transition metal ion (e.g., an ion of Bi or Tl) or a transition metal ion (e.g., a Hg ion), such as those described above. In some embodiments, the crystalline metal oxide described above can include a rare earth metal ion (e.g., Y), such as those described above. In some embodiments, the crystalline metal oxide described above can include two or more (e.g., three or four) alkaline earth metal ions (e.g., Sr, Ba, and / or Ca ions). In such embodiments, only one of the alkaline earth metal ions can be replaced by an alkali metal ion or two or more of the alkaline earth metal ions can be replaced by alkali metal ions. In some embodiments, when two or more alkaline earth metal ions in a crystalline metal oxide are replaced by two or more alkali metal ions, each alkaline earth metal ion can be replaced by any one of the two or more alkali metal ions. Attorney Docket No.: 40218-0028WO1 In some embodiments, the crystalline metal oxide described above can include two or more (e.g., three or four) halogen anions (e.g., F, Cl, Br, and / or I anions). For example, in a compound of formula (Ia), A’ and A’’ can be two different halogen anions. In some embodiments, a superconducting compound described herein (e.g., a compound of formula (I)) is a compound having a crystal structure, where the crystal structure includes a plurality of cell units, at least 10% of the cell units include a cluster (e.g., a sub cell unit); the cluster includes a plurality of anions (e.g., O anions and / or halogen anions), a plurality of transition metal ions (e.g., Cu ions), and at least one alkali metal ion (e.g., ions of Li, Na, K, Rb, and Cs); each transition metal ion forms a covalent bond with at least one anion; the plurality of anions define a plane; the at least one alkali metal ion is located approximate to the plane; the distance between the at least one alkali metal ion and the plane is less than twice of the radius of the at least one alkali metal ion; and at least two of the plurality of anions have a distance of from 3.8 Å to 4.2 Å. In some embodiments, the at least two of the plurality of anions can have a distance of at least 3.8 Å (e.g., at least 3.85 Å, or at least 3.9 Å) and / or at most 4.2 Å (e.g., at most 4.15 Å, at most 4.1 Å, at most 4.05 Å, or at most 4 Å). In some embodiments, at least 20% (e.g., at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or at least 99%) of the cell units in the crystal structure include the cluster described above (which contains at least one alkali metal ion). In some embodiments, other anions and metal ions described above can be used in addition to the cluster to form a superconducting compound. For example, a charge reservoir layer or a doping mechanism (e.g., interstitial ions) can be included in addition to the cluster to form a superconducting compound. FIG.1 illustrates a crystal structure that includes an exemplary cluster (i.e., an octahedral cluster) described above that includes four in-plane ions and two buffer ions (e.g., an alkali metal ion and an alkaline earth metal ion or a transition metal ion). As shown in FIG.1, the cluster includes anions 21, 22, 23, and 24 (e.g., O anions), transition metal ions 11, 12, 13, and 14 (e.g., Cu ions), and two buffer ions 31 and 32, at least one of which is an alkaline metal ion (e.g., ions of Li, Na, K, Rb, and Cs). Note that halogen anions in the superconducting compounds described herein are not shown in FIG.1 and Attorney Docket No.: 40218-0028WO1 the atoms drawn in FIG.1 are for illustration purpose only and are not drawn to scale. Each of transition metal ions 11, 12, 13, and 14 forms a covalent bond with a neighboring anion. Transition metal ions 11, 12, 13, and 14 and anions 21, 22, 23, and 24 form a plane in which metal ions 11, 12, 13, and 14 are located at the vertices of the plane, and anions 21, 22, 23 and 24 are located at the edges of the plane. The distance 34 or 35 between the buffer ion 31 or 32 and the plane is less than twice of the radius of the buffer ion. In some embodiments, when alkali metal ion 31 is the same as the alkali metal ion 32, the distance 34 is substantially similar to the distance 35. The distance between two anions facing each other (i.e., the distance between anions 21 and 23 or the distance between anions 22 and 24) in the plane is from 3.8 Å to 4.2 Å. In some embodiments, ion 31 is an alkali metal ion and ion 32 is a different ion (e.g., an alkaline earth metal ion or a transition metal ion). FIG.5 illustrates a representative crystal structure of a superconducting compound containing halogen anions. Specifically, the crystal structure shown in FIG.5 includes alternating MA (e.g., CuO) layers and BA’ (e.g., NaCl) layers, in which B, M, A, and A’ are defined above in connection with formula (I). As shown in FIG.5, the crystal structure includes anions 21, 22, 23, and 24 (e.g., O anions and / or halogen anions), transition metal transition metal ions 11, 12, 13, and 14 (e.g., Cu ions), two alkaline metal ions 31 and 32 (e.g., ions of Li, Na, K, Rb, and Cs), and halogen anions 41, 42, 43, 51, 53, and 54. As shown in FIG.5, at least one apical chalcogen anion (A) is replaced by the halogen anion (A’). In some embodiments, a superconducting compound of formula (I) can include a cluster (e.g., a sub cell unit in the crystal structure of the compound) having a formula of BZMA2, BZMA2A’, B2MA2A’, BZ’MA2, or BZ’MA2A’, in which B, Z, Z’, M, A, and A’ are defined above. Without wishing to be bound by theory, it is believed that the cluster described herein (e.g., a cluster having a structure of BZMA2, BZMA2A’, B2MA2A’, BZ’MA2, or BZ’MA2A’) is primarily responsible for the high Tc and superconducting activities / properties at a high temperature (e.g., at least about 150K). Thus, without wishing to be bound by theory, it is believed that all crystalline compounds (e.g., metal Attorney Docket No.: 40218-0028WO1 oxide crystalline compounds) having such a cluster would exhibit high Tc and superconducting activities / properties at a high temperature. In some embodiments, a superconducting compound described herein includes at least 15% (e.g., at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99%, or 100%) of cell units that have the cluster described above (e.g., such as that shown in FIG. 1) in its crystal structure. Without wishing to be bound by theory, it is believed that a superconducting compound containing a higher amount (e.g., more than 50%) of the cluster described above would exhibit a higher Tc based on the model described below. In some embodiments, a superconducting compound described herein further contains one or more clusters similar to that shown in FIG.1 except that the alkali metal ion is replaced by an alkaline earth metal ion (e.g., Ca, Sr, or Ba) or a rare earth metal ion (e.g., La). In some embodiments, a superconducting compound containing the cluster described above can further include a transition metal ion or a post-transition metal ion, such as the L ion in formula (I). Without wishing to be bound by theory, it is believed that additional anions attached to the L ion can be considered as doping ions for the cluster described above so as to render the plane formed by anions 21, 22, 23, and 24 conducting. Further, without wishing to be bound by theory, it is believed that such a doping effect can facilitate the formation of the superconductivity of the compound. In some embodiments, the cluster described above can include only two anions, which have a distance of from 3.8 Å to 4.2 Å. In such embodiments, the other metal ions in the cluster can be located at any locations in space so as to keep the two anions at the above distance. Any reference to the plane formed by anions 21, 22, 23, and 24 defined above can now be replaced by the line connecting these two anions. In some embodiments, a superconducting compound having such a cluster (e.g., a sub cell unit in the crystal structure of the compound) can have a formula of BMA2, in which B, M, and A are defined above. Attorney Docket No.: 40218-0028WO1 In some embodiments, the superconducting compounds described herein are substantially pure. For example, the superconducting compounds can have a purity of at least 50% (e.g., at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 98%, at least 99%, or 100%). In general, the compounds described herein can be superconductors (e.g., capable of carrying superconductive current) at a relatively high temperature under atmospheric pressure. In some embodiments, the superconducting compounds described herein can be a superconductor at the temperature of at least 150K (e.g., at least 160K, at least 170K, at least 180K, at least 190K, at least 200K, at least 210K, at least 220K, at least 230K, at least 240K, at least 250K, at least 260K, at least 270K, at least 273K, at least 283K, at least 293K, at least 300K, at least 320K, at least 340K, at least 360K, at least 380K, or at least 400K) and / or at most about 500K (e.g., at most about 480K, at most about 460K, at most about 450K, at most about 440K, at most about 420K, or at most about 400K). In some embodiments, the superconducting compounds described herein can have Tc of at least 150K (e.g., at least 160K, at least 170K, at least 180K, at least 190K, at least 200K, at least 210K, at least 220K, at least 230K, at least 240K, at least 250K, at least 260K, at least 270K, at least 273K, at least 283K, at least 293K, at least 300K, at least 320K, at least 340K, at least 360K, at least 380K, or at least 400K) and / or at most 500K (e.g., at most 480K, at most 460K, at most 450K, at most 440K, at most 420K, or at most 400K). Without wishing to be bound by theory, it is believed that crystalline compounds having the cluster structure described above can exhibit a high Tc based on the model described below. In some embodiments, this disclosure features a composition containing a superconducting compound described herein. In such embodiments, the composition can contain at least 1% (e.g., at least 2%, at least 3%, at least 5%, at least 10%, at least 20%, at least 30%, at least 40%, or at least 50%) and / or at most about 99.9% (e.g., at most 99%, at most 98%, at most 95%, at most 90%, at most 80%, at most 70%, at most 60%, or at most 50%) of the superconducting compound. In some embodiments, this disclosure features a method of forming a superconducting compound. The method can include (1) mixing a metal oxide (e.g., a Attorney Docket No.: 40218-0028WO1 crystalline metal oxide) with an alkali metal halogen salt containing an alkali metal ion (e.g., an ion of Li, Na, K, Rb, or Cs) and a halogen anion (e.g., F, Cl, Br, or I) to form a mixture as a starting material, in which the metal oxide contains at least one transition metal ion (e.g., a Cu ion) and (2) sintering the mixture at an elevated temperature to form a crystalline compound containing the alkali metal ion and the halogen anion. Examples of metal oxides that can be used as starting materials to prepare the superconducting compounds described herein include Y2O3, CuO, Bi2O3, CaO, BaO, and SrO. Examples of suitable alkali metal halogen salts that can be used as starting materials to prepare the superconducting compounds described herein include KF, KCl, NaF, NaCl, RbF, RbCl, CsF, and CsCl. In some embodiments, the starting material can also include a metal salt other than the alkali metal halogen salt. For example, the starting material can include a metal carbonate salt (e.g., calcium carbonate (CaCO3), barium carbonate (BaCO3), or strontium carbonate (SrCO3)). In some embodiments, the components in the starting materials (e.g., the metal oxide, the metal salt, and the alkali metal halogen salt) can be in stoichiometric amounts. In some embodiments, the crystalline compound containing the alkali metal ion prepared from the methods described above does not include a halogen anion. In some embodiments, the preparation method described herein can include (1) mixing a superconductive crystalline metal oxide with an alkali metal halogen salt containing an alkali metal ion (e.g., an ion of Li, Na, K, Rb, or Cs) and a halogen anion (e.g., F, Cl, Br, or I) to form a mixture, in which the metal oxide contains at least one transition metal ion (e.g., a Cu ion) and at least one alkaline earth metal ion (e.g., a Ca, Sr, or Ba ion) and the atomic ratio between the alkali metal ion and the at least onealkaline earth metal ion is higher than 1:1; and (2) sintering the mixture at an elevatedtemperature to form a crystalline compound containing the alkali metal ion. Suitable crystalline metal oxides that can be used as starting materials to prepare the superconducting compounds described herein include for example Bi2212, YBCO, Bi2223, Tl2212, Tl2223, Hg1201, Hg1212, and Hg1223. Thus, in some embodiments, the superconducting compounds of formula (I) can be prepared by the above Attorney Docket No.: 40218-0028WO1 manufacturing method using a corresponding metal oxide and a suitable alkali metal halogen salt (e.g., KF, KCl, NaF, NaCl, RbF, RbCl, CsF, or CsCl) as starting materials. In some embodiments, when the superconducting compounds of formula (I) contain an element D, the element D can be introduced into the superconducting compounds by adding a salt (e.g., an alkali metal salt) containing element D in the mixture described in step (1) above. Suitable salts containing element D that can be used to prepare the superconducting compounds described herein include for example K2CO3, K2SiO3, K2B4O7, Rb2CO3, Rb2SiO3, Cs2CO3, Cs2SiO3, KHCO3, RbHCO3or CsHCO3.For example, to prepare the superconducting compounds of formula (I) containing an element D where D is carbon, an alkali metal salt containing carbon (e.g., K2CO3, Rb2CO3, or Cs2CO3) can be used in step (1) described above. In addition, the superconducting compounds described herein in which D is carbon can be prepared by sintering a crystalline metal oxide and an alkali metal salt under a flow of CO2 to induce incorporation of carbon in the structure. It is believed that carbon atoms, if imbedded in the crystal structure, can facilitate the incorporation of alkali metal ions in the crystal. In some embodiments, the atomic ratio (i.e., the molar ratio) between the alkali metal ion in the alkali metal salt and the at least one alkaline earth metal ion in the metal oxide is at least 1.3:1 (e.g., at least 1.5:1, at least 1.7:1, at least 2:1, at least 2.3:1, at least 2.5:1, at least 2.7:1, at least 3:1, at least 4:1, at least 5:1, at least 6:1, at least 7:1, at least 8:1, at least 9:1, at least 10:1, at least 11:1, at least 12:1, at least 13:1, at least 14:1, at least 15:1, or at least 16:1). In some embodiments, when the metal oxide starting material contains two or more alkaline earth metal ions, the atomic ratio described above can be between the alkali metal ion in the alkali metal salt and one of the two or more alkaline earth metal ions in the metal oxide. Without wishing to be bound by theory, it is believed that using an excess amount (e.g., more than 1:1 atomic ratio) of an alkali metal salt in the method described above can facilitate replacement of the alkaline earth metal ion in the crystal structure of the metal oxide compound by the alkali metal ion. Further, without wishing to be bound by theory, it is believed that a superconducting metal oxide containing a higher amount of an alkali metal ion in its crystal structure would exhibit a higher Tc based on the model described below. Attorney Docket No.: 40218-0028WO1 In general, the sintering temperature used in the method described can depend on various factors such as the structure of the compound to be synthesized and their melting temperatures. In some embodiments, the sintering temperature is at least 300ºC (e.g., at least 400ºC, at least 500ºC, at least 600ºC, at least 700ºC, at least 750ºC, or at least 800ºC) and / or at most 1200ºC (e.g., at most 1100ºC, at most 1000ºC, at most 900ºC, at most 850ºC, at most 820ºC, or at most 800ºC). The sintering time (or the dwelling time) can be at least 20 hours (e.g., at least 30 hours, at least 40 hours, at least 50 hours, at least 100 hours, or at least 150 hours) and / or at most 300 hours (e.g., at most 280 hours, at most 250 hours, at most 220 hours, at most 200 hours, or at most 150 hours). In some embodiments, the mixture of a crystalline metal oxide and an alkali metal salt can be sintered at a first temperature for a first period of time and then sintered at a second temperature different from the first temperature for a second period time. In some embodiments, the second temperature can be higher than the first temperature. The first or second temperature can be at least 750ºC (e.g., at least 760ºC, at least 770ºC, at least 780ºC, at least 790ºC, at least 800ºC, or at least 810ºC) and / or at most 850ºC (e.g., at most 840ºC, at most 830ºC, at most 820ºC, at most 810ºC, or at most 800ºC). In some embodiments, the superconducting compounds described herein, e.g., one or more layers of its crystalline structure, can be prepared by using other methods known in the art, such as pulsed laser deposition (PLD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD). Examples of the MBE methods that can be used to prepare the superconducting compounds described herein have been described in WO 2019 / 116103, the entire contents of which are incorporated herein by reference. For example, a superconducting compound described herein BMAA’ (e.g., NaCuOF) can prepared by using PLD or MBE methods known in the art. In some embodiments, this disclosure features a device that is superconductive (e.g., exhibiting superconductive properties such as the capability of carrying a superconductive current) at a temperature of at least 150K (e.g., at least 180K, at least 200K, at least 230 K, at least 250K, at least 273K, at least 278K, at least 283K, at least 288K, at least 293K, at least 298K, at least 300K, at least 305K, or at least 310K) under Attorney Docket No.: 40218-0028WO1 atmospheric pressure. Exemplary devices include cables, magnets, levitation devices, superconducting quantum interference devices (SQUIDs), bolometers, thin film devices, motors, generators, current limiters, superconducting magnetic energy storage (SMES) devices, quantum computers, communication devices, rapid single flux quantum devices, magnetic confinement fusion reactors, beam steering and confinement magnets (such as those used in particle accelerators), RF and microwave filters, and particle detectors. Without wishing to be bound by theory, the inventor believes that the high temperature superconducting compounds and methods of making such compounds are based on the principles and model described in more detail below. It is believed that superconductive behavior of charge carriers arises as a result ofnearly degenerate dispersion relation (k) of a material, at proximity to the Fermi levelthereof. Accordingly, the complete many-body Hamiltonian is simplified to a residual Hamiltonian, formally similar to the reduced Hamiltonian postulated by the well-knownBCS model [Bardeen, et. al., Phys. Rev. 108, 1175 (1957)], while maintaining aconnection between prediction of superconducting behavior and electronic and chemical structure of a corresponding material composition through the Schrodinger equation. More specifically, the nearly degenerate dispersion relation may be a result of little overlap between electronic states. This allows prediction of superconducting behavior as a result of calculation of electronic states in small atomic clusters providing reasonable accuracy of meV (millielectronvolts). Thus, it is believed that materials suspected as providing superconducting behavior may be identified by the use of energy state computation for energies of at least two electronic states associated with a corresponding atomic cluster. Such an atomic cluster generally includes a plurality of atoms of at least one candidate element / species being neutral atoms, cations and anions. The calculation utilizes geometrical characterization of the atomic structure including distances between the elements of the cluster. It should be noted that the computation may generally include variation of one or more distances, and which may imply that certain atoms of the cluster are to be replaced with others. The frontier molecular orbitals of the cluster should be identified by the appropriate calculation and such frontier molecular orbitals having relatively low overlap Attorney Docket No.: 40218-0028WO1 may be detected. The frontier molecular orbitals generally relate to Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). Additionally, the band structure of a similar superconducting compound can be calculated to provide an estimation of the corresponding Fermi level. The atomic cluster may be varied to provide that the Fermi level lays in proximity with the energetic level of the identified low overlap frontier molecular orbitals. The compound described by the calculated atomic cluster may be determined as having high probability to exhibit superconducting behavior if the selected low overlap frontier molecular orbitals show that bonding and anti-bonding energies are different by less than about 150 meV (e.g., less than 100 meV, or preferably less than 50 meV) and / or more than 1 meV (e.g., more than 5 meV or more than 10 meV). Usually, such atomic clusters have high separation between adjacent energy levels. It is believed that one should look for such cases where the highest levels of the cluster, preferably the ground state and the first excited state are intrinsically nearly degenerate. In some embodiments, the inventor believes that the following analysis provides a basis for identifying a superconducting material and a method of making such a superconducting material. Starting with a full Hamiltonian expression including the kinetic energy, the phonon part, the electron-phonon interaction and the electron-electron interaction: =+ ( ) + + + , + energy; and are electronic creation and destruction operators respectively; andare phononic creation and destruction operators respectively; is the phononfrequency, is an electron-phonon matrix element and ( ) is a screened Coulombicpotential. The Hamiltonian of equation (1) is simplified utilizing the standing wave assumption: k0(2) Attorney Docket No.: 40218-0028WO1This assumption states that all of the electronic k states are degenerate, i.e. having similarenergy. Additionally, based on the assumption that the second term in equation (1) is a small perturbation, the following transformation introduces renormalized phonon operators: M * A a q qqck q c k k (3) ,)0t1(q t c0tk q(t)ck(t)ck qe c ke1(q)and 2(q,t) ck q(t)c k' q(t)ck'(t)c k(t) 2( q )k, k ' retainsthe canonical relations (boson commutation relations): Aq, Aq' q, q ' , Aq,Aq' 0 Aq , A q ' (6) qA q A q q a q a q M q a q a q c k q c kkusing Using the renormalized phonon operator in the Hamiltonian of equation (1), the Hamiltonian can be diagonalized under the standing wave condition of equation (2) while neglecting the kinetic energy term as a perturbation: 2 1 M q (8)This (2)and canonical transformation (3). The kinetic energy term can be treated as a perturbation Attorney Docket No.: 40218-0028WO1 H1 k 0 ckck k (9)i.e. 2 M H0c v( )q e k kc k q2(q)in Rev. 108, . among themselves are neglected in the low quasi-particle density (low temperature) limit. Following BCS, these interactions can be considered to be similar as in the normal state. It should be noted that no pairing is assumed. It arises from the assumption of standing wave behavior. Based on the BCS theory, () = 2 | | + ( ) ( ) (11) t2 q 0(12)kformally similar to that predicted by (13) The BCS theory is therefore found to be embedded in the standing wave theory. The ground state is found to be a condensate of non-dispersing standing electronic wave functions. The excited states are dispersive quasi-particle electronic states (bogolons). It also should be noted that the electronic operators c+and c in equation (9) are understood as perturbed standing wave states. Additionally, the electrodynamics of superconducting materials can be derived from the London equations. According to the present disclosure, the London equations Attorney Docket No.: 40218-0028WO1 may provide microscopic relation between standing wave electrons and the vector potential, without requiring the rigidity of the many-body wave function. One can start with the single standing wave electron function: it(r,t) 1r ei (r) e i ( r ) e2 and are as appears diagonalized Hamiltonian H0, a single pair wave function can be obtained. This can preserve the 2e charge observed experimentally. The superconducting standing wave states at T=0, provided by an electron pair thus provides: (r1,r2,t) C (r1,t) ( r 2, t )(15)where wave function given by equation . The spatial part of (r, t ) function withrespect to a vector potential in the London Gauge, i.e., assuming A 0 , A 0 at thesurface of an isolated body. The corresponding probability current is: q J r, t ReAwhere thatthe real part of the formula is considered, and * are the electron pair wave functionand its conjugate, m is the electron mass, q is the electron charge, is Planck’s constantdivided by 2 , and i=square root of (-1). Equation (16) can be expanded to provide:CCr1,tr2,t1r1,t r2,t r1,t2r2 ,t (17) such and Attorney Docket No.: 40218-0028WO1 Jr,tqA 2mc (19) The pair and the vector potential. Thus, all electron pairs obey the London equations individually. The total current is given by a summation over these states. Therefore, the relation between the total electric current and the vector potential is given by the well- known non-local Pippard integral, giving the macroscopic London equations. Additionally, it should be noted that derivation of the London equations requires no assumption on macroscopic coherence of any kind. It should also be noted that the same derivation applies to a single standing wave electron. The pairing is not required to derivethe London relation, it is a result (by means of 2) of the same standing waveassumption. The Pippard integral now appears as a summation over standing wave states. A summation carried over the single electron probability currents to get the total current. Therefore, the coherence length is the reciprocal of the k-state summation appearing in 2 giving the non-local length scale over which the relation between the current and the vector potential is maintained. Due to Pippard, the coherence length gives an estimation of the critical temperature. Therefore, the k-space extension of the flat band region at the Fermi level gives an estimate of the critical temperature. A more accurate estimation of the critical temperature will be given by estimating the low dispersive volume in k-spaceat the proximity of the Fermi level. This will determine the parameter 2 and thereforeand therefore Tc. As additional potentials should be related to the London potential by a gauge ,transformation:A A , where (r ) may be any scalar function. The corresponding of the wave function is then: ,iqc( r )and Attorney Docket No.: 40218-0028WO1 J(r,t) Re , (r, t)qA,(r,t) ,( r, t )im mc Jr,t A( r, t)mc (22) energy gap replaced by a single-body relation that is the property of any real wave function in the London gauge. Thus, the long-range coherence, explained as the phase rigidity of the order parameter, can now be understood as merely reflecting this single-electron behavior. Without wishing to be bound by theory, it is believed that the standing waves theory provides a simple understanding of the gauge symmetry breaking observed in superconductors. This broken symmetry may arise from breaking of the periodic boundary conditions for the electronic wave function in the normal state, allowing for arbitrary phase of the wave function. The standing wave boundary condition is generally driven from the bulk, by the relaxation of the phonon cloud (or other boson for that matter) against a standing electronic wave function (equations (3)-(8)). This is the proposed physical understanding of gauge symmetry breaking in the case of superconductivity. Grain boundaries and other defects may only assist superconductivity by supporting these standing wave states. The single electronic current in the standing waves treatment is not divergence free and can be described as: J(r,t)qA(r,t) (r, t) 2being constantly created and destroyed. Again, J (r, t ) here is the probability current of a singlepair, while the total current is assumed Attorney Docket No.: 40218-0028WO1 Jdtotal0 dt (24) (t) (tdt tc cdt dtqdt k q k ) kdtck qe cke0 q , q k , q , ashould be divergence free, requiring that the wave function (r, t) 0 is null at thesurface of the superconductor (in the London gauge). The understanding of the present disclosure may also be derived from the quasi- classical point of view. In the quasi-classical picture, a standing wave is a wave-packetwith group velocity vg 0. The magnetic force acting on such wave-packet isF v g B 0. Therefore, the semi-classical standing wave state is not affected by However, it is known that vector potential acting of the wave-packet affects the phase of the single electron wave function as shown by the Aharonov-Bohm effect. A super-current phenomenon is therefore a current of all the super-electrons as appropriate wave-packets, acting not as a collective effect due to coherence of the many- body wave function, but simply as a phase current. Such super-current, therefore, does not interfere with the electron-phonon relaxation, being a uniform current for all super- electrons. The Pippard integral comes from the relation between the vector potential and the macroscopic current. In order to get the macroscopic current, one needs to integrate over all the standing wave k-states. This is affecting real space integration over a region on the order of the coherence length. As a result of the above understanding, the present disclosure provides a general rule which can identify new and improved superconducting materials. This is generally similar to the Wilson's rule for metals and insulators. According to Wilson, a simple rule differentiates between insulating and conducting material by locating the Fermi level with respect to the energetic band structure of the material. If the Fermi level cuts the Attorney Docket No.: 40218-0028WO1 energy band, the material is a metal; if it falls in the gap, the material is an insulator. Additionally, if the gap is on the order of the thermal energy, the material is a semiconductor. Based on the above understanding, the present disclosure provides the general principle that a superconductor behaves as a metal where the Fermi level is in theproximity (e.g., at most 50 meV) of a very shallow region of the energy levels (k). Thiscondition complies with the above treatment of the kinetic energy term in equation (9) as a perturbation. Additionally, this allows for the above diagonalization of the Hamiltonian H0. FIGs.2A-2G illustrate schematically the relationship among the energy band of a material, its Fermi level and its corresponding conductance according to the Wilson rule and the present disclosure. According to the above understanding, the critical temperature for superconducting effects (Tc) is believed to be determined by the size of the component 2q and therefore by the extension in k-space of the low dispersion region (k ) .Equation (5) determines 2 q as a three-dimensional sum in k-space over states that arerelevant to the treatment of equations (1) to (13). These are the k states that can be described in the normal state as perturbed standing wave states. These states constitutethe low dispersion region (k ) . This is consistent with measurements performed onknown superconductors by ARPES showing extended low dispersion region at the proximity of the Fermi level, as can be seen in FIGs.3A-3G, which show known measured electronic structure results. Thus, the general principle above can identify new materials that can act as superconductors in wide range of temperature, which can be higher than the currently available superconducting materials. Additionally, the general principle of the present disclosure provides certain superconducting materials capable of exhibiting superconductive behavior with critical temperature higher than the currently known materials. For example, certain superconducting materials described herein may provide Tc higher than 150K, higher Attorney Docket No.: 40218-0028WO1 than 200K, higher than 250K, higher than 273K, and at about room temperature (about 300K). As shown in equations (8) and (10) above, the energy gain in the superconducting state may be determined by the third term in the right-hand side of equation (8). The critical temperature is determined by that energy gain. With all other terms varying slowly among materials of the same chemical family, the energy gain depends highly onthe square density term 2( q ) as defined in equation (5). The magnitude of 2( q ) isdetermined by the k-space of the nearly flat band. As an example, Fig.3B shows the results of angular resolved photoemission measurements on several members of the cuprate family. As seen in the figure, the nearly flat region covers about one third of the Brillouin zone. This region can be increased. Thus, according to the present disclosure, the critical temperature may be increased by replacing buffer ions within the cuprate structure (e.g., Bi2212 or YBCO) with alkali metal ions. These buffer ionscontrol the dispersion (k ) as can be seen below.More specifically, the technique of the present disclosure utilizes cluster calculations for design of superconducting materials as shown in FIG.5. In all superconductors synthesized to date with critical temperatures above the boiling point of liquid nitrogen (77K) at ambient pressure, at least one of the metal ions 11, 12, 13, and 14 in FIG.5 is copper, anions 21, 22, 23, and 24 in FIG.5 are all oxygen, cations 31 and 32 are alkaline earth or rare earth ions and anions 41, 42, 43, and 44 (not shown in FIG.5 but at a location corresponding to anion 54) are all oxygen. It should be noted that the present invention includes other ionic species in the plane defined by cations 31 and anions 41, 42, and 43 and 44 and / or in the plan defined by cation 32 and anions 51, 52, 53, and 54 (in which 52 is not shown in FIG.5 but at a location corresponding to anion 42). In some embodiments, one or more of anions 41, 42, 43, 44, 51, 52, 53, and 54 can be group VII elements (e.g., fluorine F, chlorine Cl, bromine Br, or iodine I). Without wishing to be bound by theory, it is believed that these halogen anions can stabilize the alkali metal ions 31 and 32, which can enhance the critical temperature. In some embodiments, anions 21, 22, 23, and 24 can be group VI elements (e.g., oxygen O, sulfur S, selenium Se, or tellurium Te) or group V elements (pnictogens) (e.g., nitrogen N, Attorney Docket No.: 40218-0028WO1 phosphor P, or arsenide As). It is believed that the reason to include such elements is the ability to use their p-orbitals to create a nearly bonding MO (Molecular Orbital) as explained below. It is believed that one can use these nearly bonding MOs to create the nearly flat electronic band necessary for superconductivity as explained above. According to the present disclosure, accurate electronic state energy calculations can be performed for the octahedral structure shown in FIG.1, which is representative of the material to be synthesized. These calculations are repeated at several representative values of the distances 34 and 35 between metal ions 31 and 32 and the plane. These values are chosen to fall in the range expected for the actual layered material, and in any case are between half the ionic radius of metal ion 31 and twice the ionic radius of metal ion 31 for distance 34, and between half the ionic radius of metal ion 32 and twice the ionic radius of metal ion 32 for distance 35. If metal ions 31 and 32 are identical, distances 34 and 35 typically are equal in each electronic state calculation. One criterion for superconductivity is that at least two of the electronic states of the octahedral structure, typically the ground state and the first excited state, are close enough in energy at some of the values of distances 34 and 35, to produce nearly flat dispersion as explained above. It should also be noted that considering the structure of the highest occupied molecular orbitals is of no less importance, as explained above. If the octahedral structure satisfies the superconductivity criterion of near degeneracy (e.g., at most 50 meV between the ground state of the cluster and the first excited state) and the proximity (e.g., at most 50 meV) between the Fermi level and the corresponding energy band, the corresponding material is to be synthesized (e.g., by using the methods described herein). Without wishing to be bound by theory, the inventor believes that the results of these calculations show several clear trends for the cuprates as seen in FIGs.4A-4C. The data shown in FIGs.4A-4C are collected from Panas et al., Chem. Phys. Lett, 259, 247. The most important of which is the effect of the ionic charge of the buffer ion. The lower the charge, the lower the overlap and therefore the dispersion of the narrow band. Another trend, of lower influence is the ionic radius of the buffer ion. The higher the ionic radius, the lower the overlap. The results of the quantum chemistry calculations are reproduced in FIGs.4A-4C. These results, where pertaining to the Attorney Docket No.: 40218-0028WO1component 2 q , provide clear synthesis routes. For example, as described above, usinga precursor with an excess amount of alkali metal ions and performing ion exchange proved to be efficient for inducing room temperature superconductivity. In some embodiments, oxides and nitrates can be used as precursors. In some embodiments, when using precursors containing group VI anions other than oxygen, the corresponding chalcogenides can be used. In some embodiments, an additional step of heating the sintered mixture in an oxygen atmosphere is needed to provide interstitial oxygen for hole doping. In some embodiments, when the high temperature superconductors contain mercury or thallium, they may require special treatment to be synthesized as known in the art. Superconductors can be synthesized by laser beam ablation, sputtering, molecular beam epitaxy or other methods known in the art including thin film methods. In some embodiments, artificial structures (superlattices) containing the above cluster and required charge reservoir layer or doping source can be obtained by the synthesis methods described herein. Based on the above principle and model, the present disclosure provides the general requirements for identifying high temperature superconductors as follows: (1) the dispersion region of (k) at the proximity of the Fermi level is to be low (e.g., less than 50 meV), i.e. the energy differences between states in the cluster, should be as small as possible; (2) the states of this low dispersion regions should preferably be coupled to phonons (or other bosons); and (3) these electronic states should be itinerant. It should be noted that surface states or localized states can produce similar effects and appear non- dispersive in ARPES spectra while having no, or limited, contribution to superconductivity. In addition, a dispersive band, such as the Cu-O sigma band in thecuprates, supplies the screening of the coulombic potential ( ) in equation (1).[Deutscher et al., Chinese Journal of Physics, 31, 805, (1993)]. Thus, the present disclosure provides methods for identifying novel superconducting materials based on the following steps: (1) locating the frontier molecular orbitals which are almost non-bonding, which may be achieved by separating the anion atom centers by a proper distance (e.g.3.8-4.2 Å in cuprate compounds) and the molecular orbital composed of p-orbitals which generally extend in space in the Attorney Docket No.: 40218-0028WO1 plane; and (2) locating the frontier orbitals coupled to the vibrations of a close-by metal ion approximate to the plane. At this point, the ionic charge of the metal ion approximate to the plane is preferably selected such that the energy difference between the bonding and anti-bonding levels of the frontier orbitals is minimized. It is believed that this energy difference determines the dispersion of the very narrow band. Based on appropriate cluster calculations, the ionic charge of the metal ions approximate to the plane is preferably as small as possible. For example, in cuprate compounds (i.e., copper oxides), the preferred ionic charge for the metal ions approximate to the plane is +1 or lower. In addition to the ionic charge, it is believed that the radius of the metal ion approximate to the plane in cuprate-based materials is preferably to be the high (such as the radius of K, Rb, or Cs). This can reduce the bonding-anti-bonding energy level separation. This energy level separation determines the narrow band dispersion and therefore the size ofthe component 2 q . The size of the component 2 q determines Tc.The table in FIG.4D lists well known representatives of HTS materials. Column 1 lists the ionic charge of ions at the B site (i.e., the site corresponding to B in formula (I)). Column 2 lists the ionic radius of ions at the B site. Column 3 lists the ionic charge of ions at the Z site (i.e., the site corresponding to Z in formula (I)). Column 4 lists the ionic radius of ions at the Z site. Column 5 is the well-known name of the compound. Column 6 shows the number of CuO2 layers in the compound. Column 7 lists the Tc of the different compounds. The model described above can explain qualitatively the variety of Tc in these compounds by means of the effect of the ionic charge and the ionic radiuson 2qas follows. The last compound is an exception to the rule, to be dealt with at theend of this paragraph. The effect of the number of CuO2 layers is clear. Increased numberof layers increases 2 q by introducing more k-states into the sum (equation 5). Thisworks well as long as the doping mechanism is effective. Increasing the number of CuO2 layers also increases the distance to the charge reservoir layers. The optimum is found for three layers. Therefore, a good comparison will be for compounds of the same number of layers. The first two rows of the table compare the single layer compounds LBCO and Hg1201. The ionic charge at the B site decreases from +3 for LBCO to +2 for Hg1201. Attorney Docket No.: 40218-0028WO1The value of 2 , as an estimation of the oxygen band dispersion, decreases from about130 meV (calculated for scandium) to about 40 meV (calculated for calcium). The trend is clear and, based on the model described above, Tc increases by a factor of about 3, asthis is the factor of increase in 2 q . The next four rows compare the double layercompounds. Going from Bi2212 to YBCO, the ion at the B position increases its radius, while the ion at the Z position increases its charge. The B position is believed to be thedominant in affecting 2 q . Therefore, there is a net increase in Tc. However, based onthe model described herein, it is believed that a further increase in Tc will be obtained by replacing the +3 ion at the Z position (Y) with a +2 ion (Ca). This is what is shown in the next two rows displaying Tl2212 and Hg1212. The advantage of the Hg compounds over the Tl compounds is due to the linear coordination of Hg that relaxes structural strains. The three layers compounds are presented next. Bi2223 has 3 layers, but a smaller ion at the B position. Therefore, the increase in Tc is significant with respect to Bi2212, but not with respect to the double layer Ba compounds, Tl2212 and Hg1212. The same goes for the 3-layer Tl compound. The increase in Tc is significant with respect to the double layer compound Tl2212, but not with respect to the strain relaxed Hg2212. The last 3- layer compound Hg2223, enjoys from all the benefits and seems to have exhausted all of the benefits of having +2 ions at the B and the Z positions. The last line shows the properties of the single layer Bi2201. The relatively low Tc of this compound can be explained by the details of its Fermi surface and the Fermi landscape. Based on the model and principles established above, the next step would be touse +1 buffer ions with large ionic radius at the B site. The 2 value for using K+ insteadof Ba++ as the buffer ion, decreases from about 40 meV to below 5 meV. Therefore, the inventor believes that such a material can have a large increase in Tc due to the largeincrease in 2 q , even larger than the 3-fold increase in Tc, observed in 1987, by goingfrom the +3 buffer ion at the B position to the +2 buffer ion at the B position. The inventor believes that even higher Tc by going to +1 ions at the B and Z position, with a maximum Tc for a relaxed structure containing purely +1 ions with large ionic radius, such as HgCs2Na2Cu3O6+or HgRb2Na2Cu3O6+ Attorney Docket No.: 40218-0028WO1 Thus, without wishing to be bound by theory, the understanding of superconductivity above leads to the inventor’s belief that certain materials can exhibit superconductive behavior at a relatively high temperature (e.g., at room temperature) under atmospheric pressure. For example, such materials can have a crystal structure that includes cuprate layers (i.e., copper oxide layers) having alkali metal ions located between or proximal to the layers. In some embodiments, the fraction of alkali metal ions can be higher than 0.1 (e.g., higher than 0.2, higher than 0.3, higher than 0.4, higher than 0.5, higher than 0.6, higher than 0.7, higher than 0.8, higher than 0.9, or higher than 0.95) of the total amount of metal ions adjacent to cuprate layers in the crystal structure of the superconductor compounds described herein. Additionally, the technique of the present disclosure provides a material containing negative ions (e.g., F- or O2-) located between at least some of the alkali metal ions and at least some of the metal oxide layers (e.g., the planes defined by anions 21-24 in FIG.1). The negative ions provide further screening of the alkali metal ion charge and thus provide ions having effective charge below +1 (e.g., at most 0.8, at most 0.6, at most 0.5, at most 0.4, at most 0.2, at most 0.1, or 0). The contents of all publications cited herein (e.g., patents, patent application publications, and articles) are hereby incorporated by reference in their entirety. The following examples are illustrative and not intended to be limiting. Synthesis of High Temperature Superconductors As mentioned herein, the chemical compositions of the compounds described in Example 1 below were measured using Energy Dispersive Spectroscopy (EDS). The Tc of the compounds described in Example 1 was measured by using the four-probe method [Low Level Measurements Handbook, 6th edition, Keithley]. The transport measurements were obtained using a Quantum Design PPMS system available from Quantum Design (San Diego, CA). The magnetic measurements were done using Quantum Design MPMS system available from Quantum Design (San Diego, CA). The following family of compounds derived from the model outlined above were synthesized and exhibited room temperature superconductivity properties: YBCO and Attorney Docket No.: 40218-0028WO1 bismuth strontium calcium copper oxide (BSCCO) modified to contain an alkali metal ion (e.g., K or Rb) and a halogen anion (e.g., F). Variety of compounds belonging to the family above were synthesized by the following general procedure. For YBCO, stoichiometric amounts of CuO, BaCO3, KF or RbF, and Y2O3were ground, pressed, and sintered at 600-780ºC for 24-72 hours to prepare Y(KxBa1-x)rCupOyFsor Y(RbxBa1-x)rCupOyFs, in which x is from 0.2 to 1, r is 1 to 5, p is from 1 to 4, y is from 3 to 7, and s is from 0.3 to 4. For BSCCO, stoichiometric amounts of CuO, Bi2O3, RbF, SrCO3, and CaCO3 were ground, pressed, and sintered at 600-820ºC for 24-96 hours to prepare Bin(RbxSr1-x)rCaqCupOyFs, in which n is from 2 to 4, x is from 0.2 to 1, r is 1 to 3, q is from 1 to 3, p is from 1 to 3, y is from 6 to 18, and s is from 0.3 to 2. Further, in some of the cases, the reaction was done in 1-5 stages of grinding, pressing, and sintering. The mixtures in most cases were grinded in a glove box filled with Ar or N2, pressed in the glove box and then sintered at 600-810ºC. Referring to FIG.6, a further method of forming a superconducting crystal having the composition discussed above includes growing a crystal 610 of a metal oxide, such as YBCO or BSCCO. The crystal 610 is placed inside a container 601 (e.g., a vial or tube) along with an alkali metal halogen salt 620, e.g., in powdered form. The container 601 is evacuated and sealed. After sealing, the container is heated using a heat source 630 to a temperature sufficient to cause ions from the salt to diffuse into and replace ions in the crystal 610, resulting in a superconducting crystalline compound having a composition described herein. Generally, the container 601 is formed from a material that is inert with respect to the crystal 610, the salt 620 and the crystalline compound, and the material can withstand the temperature to which it is heated. In some examples, container 601 is a quartz container. The alkali metal halogen salt 620 can be a fluoride salt. For example, the salt can be Rubidium Fluoride, Potassium Fluoride, or Cesium Fluoride. In some cases, the alkali Attorney Docket No.: 40218-0028WO1 metal halogen salt 620 is a chloride salt. For instance, the salt can be Rubidium Chloride, Potassium Chloride, or Cesium Chloride. The container 601 can be heated to a temperature in a range from 700 °C to1,100 °C (e.g., 750 °C or more, 760 °C or more, 770 °C or more, 780 °C or more, 790 °Cor more, 800 °C or more, 810 °C or more, 820 °C or more, 830 °C or more, 840 °C ormore, 850 °C or more, such as 1,000 °C or less, 950 °C or less, 920 °C or less, 900 °C orless, 890 °C or less, 880 °C or less, 870 °C or less, 860 °C or less, 850 °C or less).Generally, the elevated temperature is maintained until sufficient ions from the salt have replaced ions from the metal oxide. This can be for one hour or more (e.g., five hours or more, eight hours or more, 10 hours or more, 12 hours or more, 18 hours or more, 24 hours or more). In some cases, the metal oxide crystal 610 is composed of Bi2Sr2CaCu2Oy, Bi2Sr2Ca2Cu3Oy, or YBa2Cu3Oy. In an example, the metal oxide crystal is Bi2Sr2CaCu2Oy, and the alkali metal halogen salt is Rubidium Fluoride or Potassium Fluoride. In another example, the metal oxide crystal is YBa2Cu3Oy,and the alkali metal of the salt is Cesium. Synthesis of High Temperature Superconductors via Pulsed Laser Deposition (PLD) Pulsed laser deposition (PLD) can be used in the synthesis of the superconducting compounds described herein. For example, PLD can be used to form at least one (e.g., one or multiple) layers of material that ultimately form a superconducting compound. FIG.15 is a schematic diagram showing an example of a pulsed laser deposition (PLD) system 700 for forming a superconducting crystal from targets 702-1 through 702- N on a substrate 704, where the superconducting crystal is a superconducting crystalline compound having a composition described herein. As shown in FIG.15, the PLD system 700 includes a vacuum chamber 710, a rotary motor 720, a laser source 730, and an electronic controller 740 for performing PLD, e.g., layer-by-layer PLD, of the targets 702-1 through 702-N onto the substrate 704. Attorney Docket No.: 40218-0028WO1 Although three targets are illustrated, more generally, the PLD system 700 can use one or more targets, e.g., two targets, three targets, four targets, five targets, six targets, seven targets, eight targets, nine targets, ten targets, eleven targets, twelve targets, thirteen targets, fourteen targets, fifteen targets, sixteen targets, or more, depending on the superconducting crystal being formed. For example, the superconducting crystal can be formed from a single target that is composed of an amorphous composition of the superconducting compound, e.g., formed via another method described herein (e.g., the method described above with reference to FIG.6). As another example, the superconducting crystal can be formed from two or more targets that are each composed of a crystalline compound that includes elements of the resulting superconducting compound. In general, amorphous, crystalline, and / or polycrystalline targets can be used. Examples of suitable targets that can be used with the PLD system 700 are described below. The targets 702 can include one or more targets composed of a metal oxide (e.g., a crystalline metal oxide) containing at least one transition metal ion (e.g., a Cu ion). Examples of suitable metal oxides include Y2O3, CuO, Bi2O3, CaO, BaO, and SrO. In some embodiments, the metal oxide is a superconductive crystalline metal oxide containing at least one transition metal ion (e.g., a Cu ion) and at least one alkaline earth metal ion (e.g., a Ca, Sr, or Ba ion). Examples of suitable superconductive crystalline metal oxides include Bi2212, YBCO, Bi2223, Tl2212, Tl2223, Hg1201, Hg1212, and Hg1223. In some examples, targets 702 can include one or more targets composed of a metal nitrates. Examples of suitable metal nitrates include – Cu(NO3)2), Y(NO3)3, Ba(NO3)2, Ca(NO3)2, Sr(NO3)2 …. The targets 702 can include one or more targets composed of an alkali metal halogen salt containing an alkali metal ion (e.g., an ion of Li, Na, K, Rb, or Cs) and a halogen anion (e.g., F, Cl, Br, or I). Examples of suitable alkali metal halogen salts include KF, KCl, NaF, NaCl, RbF, RbCl, CsF, and CsCl. The targets 702 can include one or more targets composed of a metal salt other than an alkali metal halogen salt, such as a metal carbonate salt. Examples of suitable metal carbonate salts include CaCO3, BaCO3, and SrCO3. Attorney Docket No.: 40218-0028WO1 When the superconducting crystal is a compound of formula (I) containing an element D, the targets 702 can include one or more targets composed of a salt (e.g., an alkali metal salt) containing the element D. Examples of suitable salts containing the element D include K2CO3, K2SiO3, K2B4O7, Rb2CO3, Rb2SiO3, Cs2CO3, Cs2SiO3, KHCO3, RbHCO3, and CsHCO3. For example, to prepare the superconducting compounds of formula (I) containing an element D where D is carbon, a target 702 can be composed of an alkali metal salt containing carbon (e.g., K2CO3, Rb2CO3, or Cs2CO3). The targets 702 can include one or more targets composed of a starting material (e.g., a crystalline compound) that has been prepared by sintering a mixture at an elevated temperature, where the mixture includes two or more of: (i) one or more metal oxides each containing at least one transition metal ion, (ii) one or more alkali metal halogen salts each containing an alkali metal ion and a halogen anion, and (iii) one or more metal salts other than an alkali metal halogen salt. For example, the starting material can have an atomic ratio between an alkali metal ion of an alkali metal halogen salt and an alkaline earth metal ion of a superconductive crystalline metal oxide higher than 1:1. When the superconducting crystal is a compound of formula (I) containing an element D, one or more salts (e.g., alkali metal salts) containing the element D can be introduced in the mixture. In addition, the superconducting compounds described herein in which D is carbon can be prepared by sintering a mixture including a crystalline metal oxide and an alkali metal salt under a flow of CO2 to induce incorporation of carbon into the target 702. As one example, see Example 2 below, the PLD system 700 can use four targets702-1, 702-2, 702-3, and 702-4, including: a Bi2101 target (e.g., with a nominal composition of Bi2SrCuOx), a Ca2CuO3target, a KF target, and a CuO target, to form a superconducting crystal having a composition of Bi2SrK3CaCu2FO10. The targets 702-1, 702-2, 702-3, and 702-4 can be prepared from high purity Bi2O3, KF, SrCO3, CaCO3, and CuO. For example, the Bi2101 target can be prepared by sintering a mixture including Bi2O3, SrCO3, and CuO, the Ca2CuO3target can be prepared by sintering a mixture including CaCO3 and CuO, the KF target can be prepared by sintering KF, and the CuO target can be prepared by sintering CuO. Attorney Docket No.: 40218-0028WO1 The substrate 704 can include one or more layers of different materials stacked on one another. Examples of suitable materials include silicon (Si), sapphire (Al2O3), strontium titanate (SrTiO3 or STO), magnesium oxide (MgO), lanthanum aluminate (LaAlO3or LAO), quartz (SiO2), zinc oxide (ZnO), gallium nitride (GaN), calcium fluoride (CaF2), yttria-stabilized zirconia (YSZ), lanthanum strontium aluminate (LSAT), lanthanum strontium Manganite (LSMO), neodymium gallate (NdGaO3 or NGO), rutile titanium dioxide (TiO2), gallium arsenide (GaAs), zirconium oxide (ZrO2), cerium oxide (CeO2), perovskite oxides (e.g., gadolinium scandate (GdScO3), dysprosium scandate (DyScO3), etc.), magnesium aluminate spinel (MgAl2O4), muscovite mica, glasses (e.g., borosilicate, soda-line, etc.), single crystal oxides (e.g., yttrium aluminum garnet (YAG)), polymers (e.g., polyethylene terephthalate (PET), polyimides (e.g., Kapton), etc.), metals (e.g., stainless steel, platinum, gold, silver, copper, nickel, or alloys thereof), and lanthanum strontium cobalt ferrite (LSCF). In general, substrate 704 can be one of a variety of different form factors. For example, the substrate 704 can be a tape substrate. As an example, substrate 704 can include a stainless steel tape that includes one or more layers of the following materials: Al2O3, Y2O3, MgO, and / or LSMO. In certain examples, the substrate 704 is a wafer substrate, such as a Si wafer. As noted previously, the substrate 704 can include one or more layers of different material stacked on one another. For example, a Si wafer substrate can include one or more layers of other materials on a surface of the wafer which the superconducting crystal is formed. Such interfacial layers may be thin, such as less than 200 nm thick., e.g., 100 nm, 50 nm, 20 nm, 10 nm, 9 nm, 8 nm, 7 nm, 6 nm, 5 nm, or less. The interfacial layers can include a layer of Strontium (Sr). The interfacial layers can include a layer of Strontium Titanate (SrTi). The interfacial layers can include a layer of Lanthanum Oxide (LaO). In an example, a Si wafer includes, first, a layer of Sr, then a layer of SrTi, then a layer of LaO. Interfacial layers can be grown using a variety of different deposition methods, including epitaxial growth techniques. In some examples, atomic layer deposition or molecular beam epitaxy can be used to deposit the interfacial layers. Attorney Docket No.: 40218-0028WO1 The chamber 710 is a rigid, airtight enclosure that includes multiple ports 712-1 through 712-7, in this case seven ports 712, disposed on its periphery for performing various functions. The chamber 710 is configured to support a low-pressure environment, referred to as a vacuum. For example, when evacuated of air and other gases, the chamber 710 can have a base pressure in range from about 10-6Torr to 10-8Torr, or less (e.g., as low as 10-9Torr in ultra-high vacuum (UHV) implementations). In general, each port 712-1 through 712-7 includes a vacuum flange for installation of instruments (or windows) in the walls of the chamber 710 or allowing instruments to be positioned within the chamber 710 through the port 712. Port 712-1 provides access for a stage 714 on which the substrate 704 is mounted. The stage 714 extends through the port 712-1 such that the substrate 704 is positioned within the chamber 710. In some embodiments, the stage 714 is a rotatable stage to enhance uniformity of PLD on the substrate 704. For example, the stage 714 can have a rotational speed in a range from about 1 rotation per minute (RPM) to 20 RPM. In some embodiments, the stage 714 is thermally conductive to allow the temperature of the substrate 704 to be controlled during PLD. In some embodiments, the temperature of the substrate 704 can be in a range from about 25 °C (room temperature) to 850°C. For example, the stage 714 can include an embedded heat source (e.g., a heating system) to allow heating of the substrate 704. Alternatively, or in addition, the stage 714 can include an embedded heat sink (e.g., a cooling system) to allow cooling of the substrate 704. For example, the stage 704 can be a water-cooled stage or a cryogenically cooled stage. Port 713-2 provides access for a target carousel 722 on which each of the targets 702-1 through 702-N is mounted. Particularly, each target 702-1 through 702-N is mounted on a circumference of the target carousel 722 and faces the substrate 704. An axle 723 of the target carousel 722 extends through the port 712-2 and is mechanically coupled to the motor 720 which provides controlled rotation of the carousel 722. The target carousel 722 rotates about its axle 723 to position one of the targets 702-1 through 702-N within the field of the laser source 730, allowing sequential and periodic ablation of each target 702-1 through 702-N onto the substrate 704. Attorney Docket No.: 40218-0028WO1 Port 713-3 provides access for the laser source 730 that is configured to emit a laser beam 731 that is transmitted through a window (e.g., a quartz window) of the port 713-3. In some embodiments, the laser source 730, the window of the port 713-3, or both can include a controllable shutter for blocking the laser beam 731. The laser beam 731 is incident upon one of the targets 702-1 through 702-N positioned in the field of the laser source 730. A converging (e.g., biconvex) lens 732 is positioned between the port 713-3 and the laser source 730 to focus the laser beam 730 into a smaller spot size on the target 702. For example, the laser beam 731 can have a spot size in a range from about 0.1 millimeters-squared (mm2) to 10 mm2, e.g., about 1 mm2to 10 mm2for an excimer laser, about 0.5 mm2to 5 mm2for a Nd:YAG laser, and about 0.1 mm2to 1 mm2for a femtosecond laser. The laser beam 731 vaporizes material of the target 702 in a plasma plume 734 which deposits as a thin film on the substrate 704. In some embodiments, the deposition rate of material from the target 702 onto the substrate 704 is in a range from about 0.01 nm / s to 1 nm / s. As shown in FIG.15, the target 702 being ablated is positioned at a target-to-substrate distance ( ) from the substrate 704. The target-to-substrate distance can be in arange from about 1 centimeter (cm) to 10 cm. In some embodiments, the PL system 700 can use magnetic fields or gas flow dynamics to control the shape and direction of the plasma plume 734, e.g., in higher-pressure conditions with a background gas dispersed in the chamber 710. The laser source 730 is configured to generate the laser beam 731 in a pulsed form. Each laser pulse can have a pulse energy in arrange from about 50 millijoules (mJ) to 1 Joule (J). Each pulse can have pulse duration of about 10 nanoseconds (ns) to 30 ns. For a femtosecond laser source 730, the pulse duration can be about 100 femtoseconds (fs) to 1 picosecond (ps). The laser source 730 can have a pulse repetition rate in a range from about 1 Hertz (Hz) to 100 Hz, or more (e.g., as high as 10 kilohertz (kHz) in high repetition implementations). In some embodiments, the laser source 730 is an excimer laser, an Nd:YAG(neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) laser, or a femtosecondlaser. Examples of excimer lasers include krypton fluoride (KrF) excimer lasers having Attorney Docket No.: 40218-0028WO1 wavelengths of 248 nanometers (nm) and argon fluoride (ArF) excimer lasers having wavelengths of 193 nm. Examples of Nd:YAG lasers can have wavelengths of 1064 nm, 532 nm, 355 nm, or 266 nm. Examples of femtosecond lasers include titanium-sapphire (Ti:sapphire) femtosecond lasers having tunable wavelengths around 800 nm and Ytterbium-doped (Yb-doped) femtosecond lasers having wavelengths of about 1030 nm. Port 712-4 is configured as a vacuum port from which air and other gases can be evacuated from the chamber 710 via a vacuum pump, e.g., for preparation of the chamber 710 before performing PLD of the targets 702-1 through 702-N. The vacuum pump can establish the base pressure of the chamber 710 before a gas, if used, is introduced into the chamber 710. Port 712-5 provides access for a gas inlet 716 that is configured to inject a gas into the chamber 710 through the port 712-5. For example, the gas inlet 716 can be a mass flow controller (MFC), a leak valve (or needle valve), a gas showerhead, a gas manifold, a direct gas inlet (or gas port), an electronically controlled valve, a gas bubbler, or a pneumatic valve. In some embodiments, the gas is a reactive gas that reacts with material ablated from one or more of the targets 702-1 through 702-N and is deposited therewith on the substrate 704. Examples of reactive gases can include oxygen, nitrogen, argon, hydrogen, and ammonia, which can have a pressure within the chamber 710 in a range from about 10-4Torr to 1 Torr. In some embodiments, the gas is an inert gas or a rare gas that is inert with respect to the material ablated from each of the targets 702-1 through 702-N and / or provides specific characteristics of the plasma plume 734. Examples of inert gases can include helium and neon. Inert gases can have a pressure with the chamber 710 in a range from about 10-4Torr to 1 Torr. Rare gases can include krypton and xenon, which can have a pressure within the chamber 710 in a range from about 10-4Torr to 10-1Torr. In some embodiments, the gas is a specialty (e.g., dopant) gas that acts as a doping agent, an oxidizing agent, or for deposition of a target 702 composed of a carbon-containing material. Examples of specialty gases can include sulfur hexafluoride, carbon dioxide, and methane, which can have a pressure within the chamber 710 in a range from about 10-4Torr to 10-2Torr. Attorney Docket No.: 40218-0028WO1 Port 712-6 provides access for a pressure gauge 718 that is configured to monitor the pressure of the chamber 710 through the port 712-6, e.g., during evacuation of the chamber 710 of air and other gases and / or injection of a gas within the chamber 710. For example, the pressure gauge 718 can be an ionization gauge (e.g., a cold cathode gauge (Penning Gauge) or a hot cathode gauge (Bayard-Alpert gauge), a capacitance manometer, a Pirani gauge, a thermocouple gauge, a piezoelectric gauge, or a residual gas analyzer (RGA). Port 712-7 is configured as a viewing port that includes a transparent window for inspecting inside the chamber 710, either manually or with an instrument. For example, a camera can be attached to the port 712-7 or directed at the port 712-7 to record the PLD performed by the PLD system 700. The controller 740 is communicatively coupled with the motor 720 and the laser source 730, e.g., via wired communication channels (e.g., electrical cable or fiber-optic communication channels) and / or wireless communication channels (e.g., Wi-Fi, Bluetooth, 5G, infrared, or microwave communication channels). The controller 740 includes processing circuitry and associated memory for storing, transmitting, receiving, and processing data to execute a PLD protocol using at least the motor 720 and the laser source 730. In general, the PLD protocol can include a number of cycles where each of the targets 702-1 through 702-N is ablated for a respective number of laser pulses within each cycle. For example, the PLD protocol can include 10, 20, 30, 40, 50, 100, 200, 300, 400, 500, 1000, or more cycles, where a target 702 is ablated with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 50, 100, or more laser pulses per cycle. To execute the PLD protocol, the controller 740 can instruct the motor 720 to rotate the target carousel 722 to sequentially position the targets 702-1 through 702-N within the field of the laser source 730, e.g., where one full 360-degree rotation of the target carousel 722 corresponds to a cycle. The controller 740 can then instruct the laser source 730 to generate the laser beam 731 pulsed with the particular number of laser pulses for the particular target 702. The controller 740 can control the timing, energy, and repetition rate of the laser pulses emitted by the laser source 730, as well as any controllable shutters integrated in the port 712-3 and / or the laser source 730. Attorney Docket No.: 40218-0028WO1 The controller 740 can also be configured to control other components of the PLD system 700. The controller 740 can be integrated with a synchronization system to coordinate laser firing with the other components of the PLD system 700, e.g., for synchronization with a Q-switched or excimer laser source 730. For example, the controller 740 can be communicatively with actuators or motors mechanically coupled to the stage 714 to control rotation and / or translation of the stage 714. The controller 740 can be communicatively coupled with a heating or cooling system of the stage 714 to control the particular temperature of the substrate 704 during deposition. The controller 740 can be communicatively with vacuum systems such as a vacuum pump, the gas inlet 716, and the pressure gauge 718, to control the particular pressure of the vacuum within the chamber 710 or a gas within the chamber 710. Forming the compounds can include further treating the deposited material. For example, additional material can be deposited on top of the one or more layers of material. For example, PLD or another technique can be used to deposit the additional material. Alternatively, or additionally, the deposited material can be annealed (e.g., heated to a target temperature(s) for a certain amount of time(s)). As mentioned herein, the chemical composition of the compound described in Example 2 below was measured using EDS. The compound, derived from the model outlined above, was synthesized via PLD and was composed of bismuth strontium calcium copper oxide (BSCCO) modified to contain an alkali metal ion (e.g., K) and ahalogen anion (e.g., F). Stoichiometric amounts of Bi2O3, KF, SrCO3, CaCO3, and CuOwere ground, pressed, and sintered at temperatures ranging between 750-950ºC for sintering times ranging between 24-72 hours to prepare four targets for a layer-by-layer PLD process. The targets included a Bi2101 target with a nominal composition of Bi2SrCuOx, a Ca2CuO3target, a KF target, and a CuO target. The targets were periodically ablated with groups of laser pulses onto a substrate composed of a LAO buffer layer disposed on an IBAD-MgO layer, resulting in a superconductor having an approximate composition of Bi2SrK3CaCu2FO10, which has a similar composition as Bi2212. Further details relating to the PLD procedure are described below in Example 2. Attorney Docket No.: 40218-0028WO1 EXAMPLE 1 Room temperature superconductivity was observed in granular alkali cuprates at ambient pressure. The materials were prepared by solid state chemistry. Transportmeasurements showed a very low residual resistance (e.g., as low as 200 ) with acritical current up to 1A. The critical current (IC) was temperature dependent, with increasing ICwith temperature up to about 400K. The critical temperature was calculated to be higher than 400K. Experimental Details A mixture of Bi2O3, CuO, RbF, SrCO3, CaCO3was weighed according to the nominal composition of Bi2Rb1.4Sr0.6CaCu2OxF1.4. The powder was pelletized at a pressure of 5000 kg / cm2at room temperature and grinded in a ball mill machine at 750 RPM for 3 hours. This procedure took place inside a glove box with inert atmosphere of Nitrogen. Sintering procedure included 6 cycles of thermal treatment at 710 °C. Sintering time was 16, 8, 68, 24, 10, 50 hours for cycles 1-6, respectively. Heating rate was defined as 5 °C / minute. At the end of each cycle, samples were cooled to 200 °C inside the furnace. The cooling took several hours without control of the cooling rate. Each batch of pellets was reground and pelletized inside the glove box. Results FIGs.7A-7E shows the microstructure of the sample, consisting of a mixture of Bi2(Sr,Ca)2CuO5and other oxides. The micrographs were taken by a Phenom scanning electron microscope in a backscatter mode. Table 1 shows the relative atomic fractions as measured by EDS. While the nominal weights were calibrated to produce an alkali-halide cuprate derivative of Bi2212, the final product is mainly a derivative of Bi2201 and only a small fraction is a derivative Attorney Docket No.: 40218-0028WO1 of Bi2212 as can be seen by X-ray Diffraction (XRD) below. Bi at% Sr at% Rb at% Ca at% Cu at% O at%1 14.47 5.14 4.43 4.93 6.22 64.82asured by EDS at the marked spots in FIGs.7A-7E. FIG.8 is the result of X-ray diffraction of the sample of FIGs.7A-7E including Rietveld analysis. The granular samples showed very low resistance with negativedependence of resistance on temperature. The lowest resistance observed was 200 .This behavior was observed even above 400K. To check for superconductivity, the current was scanned in the expectation of observing the rise of the resistance above the critical current. The measurements were done in the linear 4-point method, where current was injected from the outer leads and voltage was measured on the inside leads. FIG.9 shows the dependence of the resistance on current at 298K. A similar behavior was observed up to 401K. Above that temperature the electric contacts were degraded. An improved setup capable of measurements at higher temperatures is being built. Improving the synthesis process yielded critical currents up to 1A. A similar behavior was observed using a standard Quantum Design PPMS system. It was observed that the critical current increases with temperature in contrast to common knowledge. Without wishing to be bound by theory, this is a property of granular superconductors as explained in the discussion part below. FIG.10 shows the dependence of the magnetic moment on temperature. The measurement was done on a Quantum Design MPMS XL SQUID system. The magnetic field was 1 Tesla. The sample was cooled down to 10K at zero field (no measurement). Then the magnetic field was turned on and the temperature was scanned from 10K to 400K. Then the sample was cooled down with the magnetic field on. The difference between the FC and the ZFC measurements agrees with the XRD measurements of FIG. Attorney Docket No.: 40218-0028WO1 8, indicating a large fraction of the superconducting material (more than 50% as received from Rietveld analysis). FIG.11 is a detailed picture of the microstructure of the granular material. The picture was taken in the backscatter detector mode where the contrast depends on the atomic mass. The superconducting crystallites include the heavy element Bi and appear bright in the figure. The gray material is Rb2OHx. This material is a large gap semiconductor with an exponential temperature dependent resistivity: RN~exp(A / T) where A=-Eg / kB. Several factors are coming to play in this granular material. While the critical current of the superconducting grains decreases with temperature according to Ginzburg – Landau theory as (1-T / Tc)3 / 2, the barrier resistance decreases with temperature exponentially. As a first approximation, it was assumed that these two behaviors can be factored into a simple product. critical current depicted in FIG.12 which shows the temperature dependence of the critical current in a granular superconductor. The superconducting grains are separated by a large gap semiconductor material. TP is the temperature of the peak in the critical current of the granular superconductor. TC is the critical temperature of the superconducting grains. Using equation (26), one can derive the relation between the peak temperature (TP) in FIG.12 above and the critical temperature of the clean superconductor. current with respect to temperature. The peak in the temperature dependence of the critical current is around 403K. Attorney Docket No.: 40218-0028WO1 The energy gap of the barrier material can be obtained from the slope of the temperature dependent resistivity in FIG.14. Putting together these parameters one obtains a TC higher than 400 K. This corresponds with the magnetic measurements (FIG. 10) that indicate a TChigher than 400K. EXAMPLE 2 A room temperature superconductor was formed via pulsed laser deposition. The deposition materials used for the targets were prepared by solid state chemistry. Experimental Details Target Preparation Four targets were prepared for the layer-by-layer pulsed laser deposition (PLD) process. High purity Bi2O3, KF, SrCO3, CaCO3 and CuO, were weighed to yield the four targets including: (i) a Bi2101 target with nominal composition of Bi2SrCuOx, (ii) a Ca2CuO3target, (iii) a KF target, and (iv) a CuO target. Each weighed batch of powder was ground in a ball mill machine at 750 rotations per minute (RPM) for 5 hours and pelletized at a pressure of 7800 kilograms per square- centimeter (kg / cm2) at room temperature to approximately 1 inch diameter pellets. This procedure took place inside a glove box under an inert atmosphere of nitrogen (Ni). Final press of the Bi2101, Ca2CuO3, and CuO targets was performed at a pressure of 20 Tons in ambient environment. The sintering procedure for the Bi2101 target included 3 cycles of thermal treatments. The first and second thermal treatments were performed at 750 degrees Celsius (°C), and the third thermal treatment was performed at 800 °C. Each thermal treatment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. The sintering procedure for the Ca2CuO3 target included 3 cycles of thermal treatments. The first thermal treatment was performed at 800°C for 48 hours, and the second and third thermal treatments were performed at 850 °C. Each thermal treatment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. Attorney Docket No.: 40218-0028WO1 The sintering procedure for the KF target included 2 cycles of thermal treatments at 700 °C in an inert environment of argon (Ar) which lasted 24 hours for the 2 cycles. After each cycle, the pellets were reground and pelletized inside the glove box. The sintering procedure for the CuO target included 2 cycles of thermal treatments at 950 °C, where each thermal treatment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. Post oxidation treatment was performed at 950 °C for 12 hours. Pulsed Laser Deposition (PLD) Procedure A modified bismuth strontium calcium copper oxide (BSCCO) layer was deposited via pulsed laser deposition (PLD, Neocera LLC). The PLD system (see FIG.15 for example) included a krypton fluoride (KrF) excimer laser with a 248 nm emission wavelength (Coherent Corp., COMPex (Pro) 102). The modified BSCCO layer was deposited on a substrate having an area of 1x1 centimeters-squared (cm2). The substrate included: (i) a Hastelloy IBAD-MgO layer, i.e., a magnesium oxide (MgO) thin film deposited using the ion-beam-assisted-deposition (IBAD) technique on a Hastelloy (nickel-based alloy) substrate, and (ii) a buffer layer of lanthanum aluminate (LaAlO3) deposited via epitaxy on the IBAD-MgO layer. The PLD was carried out at room temperature (~25°C) under an oxygen pressure of ~89 millitorr (mTorr). Each of the four targets described above were used, including the Bi2101 target, the KF target, the Ca2CuO3target, and the CuO target, with a target-to- substrate distance of 6 cm. The laser fluence inside the chamber was 60 millijoules (mJ), the laser spot size was 5.4 mm2and the laser repetition rate was 8 Hertz (Hz). The PLD was carried out as layer-by-layer deposition; the Bi-2101 was ablated with 40 laser pulses, followed by rotation to the KF target, which was ablated with 10 laser pulses, then the Ca2CuO3 was ablated with 20 laser pulses, and finally the CuO target was ablated with 10 laser pulses. This procedure was repeated for 500 cycles. The sample was then post-annealed at 600°C for 24 hours under an Ar environment. Results Attorney Docket No.: 40218-0028WO1 FIGs.16A and 16B are scanning electron microscope (SEM) photographs of the modified BSCCO layer. The modified BSCCO layer was relatively homogenous and contained grey particles (e.g., areas 1 and 2) with larger bright crystals (e.g., area 3). The chemical composition of the grey particles is tabulated in Table 2 below and is approximately the composition of Bi2SrK3CaCu2FO10. The composition of the brighter areas was similar to the grey areas, but with excess of Sr. This result shows that the inventor has succeeded in designing a uniform layer with chemical stoichiometry which is very close to Bi2212, exchanging K for Sr and F for O (with some excess of K). Area BiSr K Ca Cu F O (at%) (at%) (at%) (at%) (at%) (at%) (at%) 13 measured by S at t e mar ed spots n Gs. 6 and 6 . FIGs.17A-17D are SEM photographs the modified BSCCO layer after annealing at 600°C in an argon (Ar) environment. A uniform layer was observed, including three main compositions: CuO (e.g., area 3), Bi2CuO4(e.g., area 1), and Bi2Sr1.3K0.7CaCu2O8(e.g., area 2), which are tabulated in Table 3 below. The last composition is similar to the Bi2212 composition, replacing ~35% of Sr by K. Further annealing experiments were also conducted. Area BiSr K Ca Cu F O ( t%) ( t%) ( t%) ( t%) ( t%) ( t%) ( t%) EXAMPLE 3 Four targets were prepared for the layer-by-layer PLD process. High purity Bi2O3, KF, SrCO3, CaCO3and CuO, were weighed to yield a Bi-2101 target with nominal Attorney Docket No.: 40218-0028WO1 composition of Bi2SrCuOx, Ca2CuO3target and KF target. Industrial CuO (99.7%-99.9% pure) target was used. Each weighed batch of powder was ground in a ball mill machine at 1,000 RPM for 20 hours and pelletized at a pressure of 4,530 kg / cm2at room temperature to approximately 15 mm diameter pellets. This procedure took place inside a glove box under an inert atmosphere of nitrogen. Final compression of the Bi-2101, Ca2CuO3 and KF to 1-inch targets was performed at a pressure of 12 Ton per inch. The sintering procedure for Bi-2101 target included three cycles of thermal treatments, the first and the second at 750°C and the third at 800°C, each treatment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. The sintering procedure for Ca2CuO3target included three cycles of thermal treatments, the first at 900°C for 48 hr and the second third at 950°C, each of them lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. The sintering procedure for KF target included one cycle of thermal treatment at 700°C in Ar environment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. The sintering procedure for CuO target included two cycles of thermal treatments at 950°C, each treatment lasted 24 hours. After each cycle, the pellets were reground and pelletized inside the glove box. Post oxidation treatment was performed at 950°C for 12 hours. Back-scattered electron (BSE) mode SEM images from the Bi-2101 target and from the Ca2CuO3 target are shown in FIGs.18A and 18B, respectively. EDS analysis from the SEMs is shown in Table 4. Area Bi at% Sr at% Ca at% Cu at% O at% respectively. Example 3-1: (tape) Attorney Docket No.: 40218-0028WO1 The modified BSCCO layer was deposited by pulsed laser deposition (PLD, Neocera) consisting of a KrF excimer laser with a 248 nm emission wavelength (Coherent, CompexPro102).10 mm x 10 mm Hastelloy tape coated with IBAD buffer layers (Metox Inc), was used as a substrate. The parameters for the deposition include a laser energy inside the vacuum chamber of 60 mJ, target-to-substrate distance of 60 mm, spot size of 5.4 mm2and repetition rate of 8 Hz. The first step of the deposition was solely of the Bi-2101 target and was carri ed out at 800°C under an oxygen pressure of 400 mTorr and with 15,000 laser pulses. The sample was then annealed in-situ at the same oxygen pressure for one hour at 600°C. Afterwards, a layer-by-layer deposition of all four targets was conducted, at 600°C under an oxygen pressure of 90 mTorr. (40,000 pulses in total). Back-scattered electron (BSE) mode of the modified BSCCO layer are shown in FIGs.19A and 19B, respectively. EDS analysis from the SEMs is shown in Table 5. Area BiSr K Ca Cu F O Referring to FIG.20A and 20B, the characteristics of the tape, including measured resistance, were as follows: R=2×10 5 (measured resistance at 300K)L=0.8 mm (distance between contacts)t<250 nm (film thickness)w=2 mm (width of the sample) Attorney Docket No.: 40218-0028WO1 <2 × 10 2.5 × 10 2 × 108 × 10 =< 1.25 × 10 A modified BSCCO layer was deposited by pulsed laser deposition (PLD, Neocera) that included a Nd-YAG solid state laser with a 266 nm emission wavelength. Sapphire 5 mm x 5 mm was used as substrates. Deposition was carried out at room temperature (~25°C) under an oxygen pressure of ~90 mTorr. Three targets were alternately used: Bi-2101, KF, and Ca2CuO3. with repetition rate of 2 Hz. SEM images are shown in FIGs.21A and 21B. EDS results of the layer (summarized in Table 6) revealed uniform composition that is similar to the chemical composition, expected by the theoretical model. Area BiSr K Ca Cu F O (at%) (at%) (at%) (at%) (at%) (at%) (at%) respectively. XRD in situ measurements were conducted in dry air atmosphere, see FIG.22. Crystalline phase started to form at 400ºC. Magnetic measurements were conducted on the PLD layer after heat treatment. Results of the magnetic measurements are shown in the plot in FIG.23 and the plots in FIGs.24A-24F. FIG.25 shows a plot showing the change in resistance as a function of temperature for the modified BSCCO layer grown on the Sapphire substrate. Resistance remains negligible up to 350 K. Attorney Docket No.: 40218-0028WO1 Example 3-3: Si substrate A modified BSCCO layer was deposited by pulsed laser deposition (PLD, Neocera) consisting of a KrF excimer laser with a 248 nm emission wavelength (Coherent, CompexPro102).5 mm x 5 mm Si wafer with buffer layer of STO was used as a substrate. The parameters for the deposition include a laser energy inside the vacuum chamber of 60 mJ, target-to-substrate distance of 60 mm, spot size of 5.4 mm2and repetition rate of 8 Hz. Layer-by-layer deposition of all four targets Bi-2101, KF, Ca2CuO3 and CuO was conducted, at room temperature under an oxygen pressure of 90 mTorr. (40,000 pulses in total). Area BiSr K Ca Cu F O (at%) (at%) (at%) (at%) (at%) (at%) (at%) respectively. Back-scattered electron (BSE) mode SEM images from the sample are shown in FIGs.26A and 26B, respectively. EDS analysis from the SEMs is shown in Table 7. The atomic precent of Sr is low due to the overlap of the Si K (1.740 keV) peak (originatefrom the substrate and deconvoluted from the analysis) and the Sr L (1.806 keV) peak.A number of embodiments are described. Other embodiments are in the claims.

Claims

Attorney Docket No.: 40218-0028WO1 WHAT IS CLAIMED IS:

1. A method, comprising:forming a compound of formula (I): LnDm(BxB’1-x)r(ZtZ’1-t)qMpAyA’s(I), wherein n is a number from 0 to 3; m is a number from 0 to 6; x is a number from 0.1 to 1; r is a number from 1 to 8; t is a number from 0 to 1; q is a number from 0 to 6; p is a number from 1 to 7; s is a number from 0 to 20; y is a number from 0 to 20; L comprises at least one metal ion selected from the group consisting of transition metal ions and post-transition metal ions; D comprises at least one element selected from the group consisting of the elements in Groups IIIA and IVA in Periodic Table; B comprises at least one first alkali metal ion; B’ comprises at least one first ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; Z comprises at least one second alkali metal ion; Z’ comprises at least one second ion selected from the group consisting of alkaline earth metal ions and rare earth metal ions; M comprises at least one transition metal ion; A comprises at least one chalcogen anion; and A’ comprises at least one halogen anion; wherein the compound is a crystalline compound, andAttorney Docket No.: 40218-0028WO1 wherein forming the compound comprises using pulsed laser deposition to deposit material constituting the compound.

2. The method of claim 1, wherein the pulsed laser deposition deposits one ormore layers of the material, the layers forming at least a portion of the compound.

3. The method of claim 1, wherein forming the compound further comprisestreating the material deposited by pulsed laser deposition.

4. The method of claim 3, wherein the material is treated by annealing thematerial.

5. The method of claim 1, wherein using the pulsed laser deposition comprisessequentially directing a laser at more than one target to generate a plume of a material from the target that forms a corresponding layer of material on a substrate.

6. The method of claim 1, wherein L comprises Bi, Tl, Cu, or Hg.

7. The method of claim 1, wherein D comprises C, Si, Ge, Sn, Pb, or Al.

8. The method of claim 1, wherein B comprises Li, Na, K, Rb, or Cs.

9. The method of claim 4, wherein B comprises K, Rb, or Cs.

10. The method of claim 1, wherein B’ comprises La, Mg, Ca, Sr, or Ba.

11. The method of claim 10, wherein B’ comprises La, Ca, Sr, or Ba.

12. The method of claim 1, wherein Z comprises Li, Na, K, Rb, or Cs.Attorney Docket No.: 40218-0028WO1 13. The method of claim 1, wherein Z’ comprises Ca or Y.

14. The method of claim 1, wherein M comprises Cu or Fe.

15. The method of claim 1, wherein A comprise O, S, or Se.

16. The method of claim 1, wherein A’ comprises F, Cl, Br, or I.

17. The method of claim 1, wherein the compound satisfies the following equation: x*r + t*q = s.

18. The method of claim 1, wherein the compound is of formula (Ia): LnDm(BxB’1-x)r(ZtZ’1-t)qMpAyA’sA’’u (Ia), in which A’’ is a halogen anion and u is a number from 0 to 20.

19. The method of claim 18, wherein the compound satisfies the following equation: x*r = s.

20. The method of claim 19, wherein the compound further satisfies the following equation: t*q = u.

21. The method of claim 18, wherein the compound is of formula (II): LnDm(BxB’1-x)r(ZtZ’1-t)qCupOyA’sA’’u (II).

22. The method of claim 21, wherein p is a number from 1 to 3 and A’’ comprises F, Cl, Br, or I.

23. The method of claim 21, wherein the compound is of formula (III): LnDm(BxB’1-x)r(ZtZ’1-t)qCu2OyA’sA’’u (III).Attorney Docket No.: 40218-0028WO1 24. The method of claim 23, wherein q is a number from 1 to 2 and r is a number from 2 to 4.

25. The method of claim 23, wherein L comprises Bi, Tl, Cu, Pb, or Hg and n is 0, 1, 2, or 3.

26. The method of claim 23, where D comprises carbon and m is a number from 0 to 4.

27. The method of claim 23, wherein B comprises K, Rb, or Cs.

28. The method of claim 23, wherein B’ comprises Sr.

29. The method of claim 23, wherein t is 0 and Z’ comprises Ca.

30. The method of claim 23, wherein t is a number greater than 0 and Z’ comprises Ca.

31. The method of claim 1, wherein B’ is a metal ion having a first atomic number, Z’ is a metal ion having a second atomic number, and the second atomic number is smaller than the first atomic number.

32. The method of claim 1, wherein the compound is a superconductor at the temperature of at least 150K.

33. The method of claim 32, wherein the compound is a superconductor at the temperature of at least 200K.

34. The method of claim 1, wherein the compound has a tetragonal or orthorhombic crystal structure.Attorney Docket No.: 40218-0028WO1 35. A method of forming a compound, comprising: using pulsed laser deposition to deposit a material on a substrate, the compound constituting the material, wherein the compound is a crystalline metal oxide comprising at least one transition metal ion, at least one alkaline earth metal ion or at least one rare earth metal ion, and at least one chalcogen anion, in which from 10% to 100% of the at least one alkaline earth metal ion or at least one rare earth metal ion is replaced by an alkali metal ion and from 10% to 100% of the at least one chalcogen anion is replaced by a halogen anion.

36. The method of claim 35, wherein the crystalline metal oxide before modification is Bi2Sr2CaCu2Oy, Bi2Sr2Ca2Cu3Oy, or YBa2Cu3Oy 37. The method of claim 35, wherein the alkali metal ion comprises Li, Na, K, Rb, or Cs.

38. The method of claim 35, wherein from 50% to 100% of the at least one alkaline earth metal ion or at least one rare earth metal ion is replaced by the alkali metal ion.

39. The method of claim 35, wherein the halogen anion comprises F, Cl, Br, or I.

40. The method of claim 35, wherein at least one apical chalcogen anion is replaced by the halogen anion.

41. The method of claim 35, wherein the substrate comprises a metal, a metal alloy, silicon, or sapphire.Attorney Docket No.: 40218-0028WO1 42. The method of claim 41, wherein the substrate is a multilayer substrate.

43. The method of claim 35, wherein the substrate is a tape substate.

44. The method of claim 35, wherein the substrate is a wafer substrate.

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