Transition-metal nitride and production method therefor

Abstract

This transition-metal nitride is a nitride of a transition metal belonging to the fifth or sixth period, the nitride containing a rare-earth element. This transition-metal nitride exhibits superconductivity. For example, the transition metal can be rhenium (Re) and the rare-earth element can be praseodymium (Pr). The method for producing this transition-metal nitride comprises growing a nitride of a transition metal belonging to the fifth or sixth period on a substrate by molecular beam epitaxy while adding a rare-earth element.

Description

Transition metal nitride and method for producing the same

[0001] The present invention relates to a transition metal nitride exhibiting superconductivity and a method for producing the same.

[0002] Nitrides with covalent bonds, such as those of fifth- and sixth-period transition metals, are expected to be candidates for new superconductors with high superconducting transition temperatures due to their similarity to existing copper-based oxide high-temperature superconductors. One such material is rhenium nitride. There have been reports that rhenium nitride contains a phase that exhibits superconductivity at low temperatures (Non-Patent Document 1, Non-Patent Document 2).

[0003] M. Fuchigami et al., "Interstitial binary nitride ReNx phases prepared by pulsed laser deposition: Structure and superconductivity dependence on nitrogen stoichiometry", Journal of Alloys and Compounds, vol. 486, pp. 621-627, 2009.A. ul. Haq et al., "Superconducting and electrical properties of rhenium nitride and amorphous rhenium prepared by ion implantation" J. Low. Temp. Phys. vol. 50, pp. 123-133 (1983).A. Friedrich et al., "Novel Rhenium Nitrides", Physical Review Letters, vol. 105, 085504, 2010.

[0004] However, due to the low reactivity of rhenium with nitrogen, high temperature or high pressure conditions are required for the preparation of rhenium nitrides (Non-Patent Document 3). Because rhenium nitrides are difficult to synthesize and stabilize, there have been no examples of identifying the crystal structure of their superconducting phase and simultaneously observing the zero resistance and Meissner effect, which are evidence of superconductivity. Thus, conventionally, nitrides of transition metals such as rhenium have had the problem of requiring extreme environments, such as high temperature and high pressure, for their preparation due to the low reactivity of nitrogen.

[0005] The present invention has been made to solve the above problems, and aims to make it possible to produce nitrides that exhibit superconductivity using transition metals such as rhenium without requiring extreme environments.

[0006] The transition metal nitride according to the present invention is made of a nitride of a transition metal of the fifth and sixth periods to which a rare earth element is added, and exhibits superconductivity.

[0007] Furthermore, in the transition metal nitride and the method for producing the same according to the present invention, nitrides of transition metals of the fifth and sixth periods are grown on a substrate by molecular beam epitaxy with the addition of a rare earth element.

[0008] As explained above, according to the present invention, since a rare earth element is added, nitrides exhibiting superconductivity due to transition metals such as rhenium can be produced without requiring an extreme environment.

[0009] Fig. 1 is a characteristic diagram showing the X-ray diffraction results of ReN doped with Pr (a) and ReN not doped with Pr (b). Fig. 2 is a characteristic diagram showing the temperature dependence of magnetization of ReN doped with Pr (a) and ReN not doped with Pr (b).

[0010] A transition metal nitride according to an embodiment of the present invention will be described below. This transition metal nitride is a nitride of a fifth-period or sixth-period transition metal to which a rare earth element is added. This transition metal nitride exhibits superconductivity. For example, the transition metal can be rhenium (Re) and the rare earth element can be praseodymium (Pr).

[0011] Next, a method for producing a transition metal nitride according to an embodiment of the present invention will be described. In this production method, nitrides of period 5 and period 6 transition metals are grown (formed) on a substrate by molecular beam epitaxy with the addition of a rare earth element. The transition metal can be rhenium, and the rare earth element can be praseodymium. By molecular beam epitaxy, the transition metal nitride can be grown on the substrate at a temperature of, for example, 730°C or less.

[0012] By molecular beam epitaxy, transition metals and nitrogen (constituent elements) are supplied separately in an ultra-high vacuum, and a thin film is grown on a suitably heated single crystal substrate. A small amount of a rare earth element such as praseodymium is added, and film formation is performed at a low substrate temperature of 730° C. or less, thereby crystallizing the transition metal nitride and exhibiting superconductivity. In molecular beam epitaxy, if the substrate temperature is set to 700° C. or higher, adding a small amount of a rare earth element such as praseodymium makes it possible to obtain a transition metal nitride that exhibits superconductivity.

[0013] The present invention will be described in more detail below using examples. In the following, a KTaO substrate with a (001) plane as the crystal substrate was used. Rhenium was used as the transition metal, and praseodymium was used as the rare earth element.

[0014] First, a KTaO3 substrate is placed (carried) into the processing chamber of a well-known molecular beam epitaxy apparatus, and the processing chamber is sealed under ultra-high vacuum (for example, 10 -8 Next, activated nitrogen (for example, nitrogen plasma) is introduced into the processing chamber, and the pressure in the processing chamber is increased to 133.32×10 -6 Next, the substrate temperature is set to 730° C., and atomic beams of Pr and Re are supplied so that the molar ratio of Pr is 1.5%, thereby forming a film of ReN doped with Pr on the KTaO substrate.

[0015] Figure 1(a) shows the X-ray diffraction results of the Pr-doped ReN film formed as described above. Figure 1(b) shows the X-ray diffraction results of the Pr-free ReN film. The arrows in the figure indicate the crystal planes of the ReN. As shown in Figure 1, it can be seen that the addition of Pr is effective in crystallizing rhenium nitride.

[0016] Figure 2 shows the temperature dependence of magnetization of ReN with and without Pr addition (a) and (b). As shown in Figure 2(a), the diamagnetism observed in Pr-added ReN during field cooling (FC) is due to the Meissner effect, which is unique to superconductors, and is important evidence of superconductivity. The difference in the temperature change of diamagnetism between field cooling and zero-field cooling (ZFC) is due to the Meissner effect and the additional effect of the zero-field cooling, in which the supercurrent flowing with zero resistance induces a shielding current in the direction away from the magnetic field. On the other hand, as shown in Figure 2(b), in Pr-free ReN, no diamagnetism is observed and the superconducting state is not realized.

[0017] As described above, according to the present invention, since a rare earth element (e.g., praseodymium) is added, nitrides of transition metals such as rhenium that exhibit superconductivity can be produced without requiring an extreme environment. According to the present invention, since nitrides of period 5 and period 6 transition metals are formed by adding a rare earth element (e.g., praseodymium) by molecular beam epitaxy, nitrides of transition metals that exhibit superconductivity can be synthesized without using high temperatures and high pressures.

[0018] For example, pulsed laser deposition (PLD) and ion implantation have been reported as typical growth and formation methods for rhenium nitride, but there have been no reports of film formation in low energy states using molecular beam epitaxy, nor have there been any reports of superconductivity. As a result of extensive research by the inventors, it was confirmed that the addition of rare earth elements such as Pr partially crystallizes rhenium nitride, which is difficult to form into a film, and thereby exhibits superconductivity.

[0019] Incidentally, in the above-described examples, nitrides of Re by adding Pr have been described as an example, but the present invention is not limited to this. It can be easily inferred that nitrides of transition metals of the fifth and sixth periods, which have high melting points similar to those of Re, can be synthesized in a stable state by molecular beam epitaxy by adding Pr or a rare earth element having similar properties (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc).

[0020] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

Claims

1. Superconducting transition metal nitrides consisting of nitrides of period 5 and period 6 transition metals with the addition of rare earth elements.

2. The transition metal nitride according to claim 1, wherein said transition metal is rhenium and said rare earth element is praseodymium.

3. A method for producing transition metal nitrides, in which nitrides of period 5 and period 6 transition metals are grown on a substrate by molecular beam epitaxy with the addition of rare earth elements.

4. A method for producing a transition metal nitride according to claim 3, wherein the transition metal is rhenium and the rare earth element is praseodymium.

5. A method for producing a transition metal nitride according to claim 3 or 4, wherein the nitride of the transition metal is grown on the substrate at a temperature of 730° C. or less.

Images