An asymmetric nickel oxide superconducting thin film, a superconducting diode and a preparation method thereof

Abstract

This invention relates to an asymmetric nickel oxide superconducting thin film, a superconducting diode, and a method for fabricating the same. The asymmetric nickel oxide superconducting thin film comprises at least three nickel oxide layers with an RP structure, stacked sequentially along the film growth direction. Each nickel oxide layer is composed of alternating rare-earth oxide layers and nickel-oxygen facets. The number of nickel-oxygen facets in a single nickel oxide layer is n, and the n values ​​of all nickel oxide layers are asymmetrically distributed along the film growth direction to break spatial inversion symmetry. The intermediate layer region, excluding the bottom and outermost layers, is defined as the intermediate layer region, which includes at least one nickel oxide layer with an n value of 2. The advantages are: simple structure, high device stability, easy application in superconducting electronic devices, enhanced freedom and programmability in device design, and realization of high-performance superconducting non-reciprocal effects defined by the structure.

Description

Technical Field

[0001] This invention relates to the field of superconducting materials technology, and in particular to an asymmetric nickel oxide superconducting thin film, a superconducting diode, and a method for preparing the same. Background Technology

[0002] The superconducting diode effect refers to the phenomenon where, in the superconducting state, a device exhibits different critical currents under forward and reverse currents, resulting in direction-dependent superconducting transport behavior. This effect is considered closely related to the breaking of spatial and temporal inversion symmetries and has significant application prospects in ultra-low-power electronic devices and novel quantum functional devices.

[0003] Existing superconducting diode structures typically rely on the following methods: (1) Utilizing intrinsic non-centrosymmetric crystal structures to achieve non-reciprocal superconducting transport; such naturally non-centrosymmetric superconducting materials are very rare, and many materials have extremely low superconducting critical temperatures, or the materials themselves are extremely difficult to grow into high-quality single crystals. (2) By constructing heterojunction interfaces, introducing Rashba-type spin-orbit coupling, or by combining magnetism and superconductivity in two-dimensional materials or interface systems to generate non-reciprocal behavior; the superconducting diode effect depends entirely on whether the interface between the two materials is perfect. If the interface is rough or contains impurities, the diode effect will be greatly affected; because the effect only exists in the extremely thin interface layer, the overall device resistance change is very small, making it difficult to detect and apply; although introducing magnetic materials can break the symmetry, the magnetic field often destroys the superconducting state itself, leading to a significant drop in the superconducting critical temperature, which is a huge trade-off problem.

[0004] In summary, the current realization path of superconducting diode materials usually depends on the intrinsic crystal symmetry or complex interface structure of the material, which restricts the choice of system and limits the degree of freedom in structural design. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides an asymmetric nickel oxide superconducting thin film, a superconducting diode and its preparation method, which solves the technical problems of limited material selection and low degree of freedom in structural design of existing superconducting diodes.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0009] In a first aspect, embodiments of the present invention provide an asymmetric nickel oxide superconducting thin film, comprising at least three nickel oxide layers having an RP structure stacked sequentially along the film growth direction; wherein, the nickel oxide layer is composed of alternating stacks of rare earth oxide layers and nickel-oxygen facets, the number of nickel-oxygen facets in a single nickel oxide layer is n, and the n values ​​of all nickel oxide layers are asymmetrically distributed along the film growth direction to break the spatial inversion symmetry; the intermediate layer segment in the film excluding the bottom and outermost layers is defined as the intermediate layer region, and the intermediate layer region includes at least one nickel oxide layer with an n value of 2.

[0010] The above-mentioned characteristic terms are explained as follows:

[0011] RP structure: Specifically, it is a Ruddlesden-Popper phase, which is a layered crystal formed by alternating stacks of nickel-oxygen layers and rare earth oxide layers;

[0012] Number of nickel oxide layers n: n conductive nickel oxide layers sandwiched between each nickel oxide layer;

[0013] Intermediate layer region: In asymmetric nickel oxide superconducting thin films, the nickel oxide layer is multilayered, with the intermediate nickel oxide layer including layer 1, layer 2, or layer 3, etc., located in the intermediate region.

[0014] In a preferred embodiment of the present invention, the asymmetric nickel oxide superconducting thin film has three nickel oxide layers, and the n value of each nickel oxide layer along the film growth direction is 123 or 321 respectively.

[0015] In a preferred embodiment of the present invention, the asymmetric nickel oxide superconducting thin film includes 2-6 continuously distributed nickel oxide layers with an n value of 2 in the intermediate layer region.

[0016] In a preferred embodiment of the present invention, the asymmetric nickel oxide superconducting thin film includes 2-4 continuously distributed nickel oxide layers with an n value of 2 in the intermediate layer region.

[0017] In a preferred embodiment of the present invention, the asymmetric nickel oxide superconducting thin film has 5 or 6 nickel oxide layers, and the n value of each nickel oxide layer along the film growth direction is any one of the following groups: 12233, 13221, 11223, 122233, and 122223.

[0018] In a preferred embodiment of the present invention, the asymmetric nickel oxide superconducting thin film further includes a substrate for thin film growth, wherein the substrate is selected from SrLaAlO4 or LaAlO3.

[0019] Secondly, embodiments of the present invention provide a method for preparing an asymmetric nickel oxide superconducting thin film, comprising the following steps:

[0020] S1. Heat the substrate to the preset growth temperature and introduce an oxygen-containing oxidizing atmosphere into the reaction chamber;

[0021] S2. Using strong oxide atom layer-by-layer epitaxy technology, according to the preset n value of each nickel oxide layer along the film growth direction, the deposition cycle number of the nickel oxide surface layer in each nickel oxide layer is controlled accordingly, so that the deposition cycle number is consistent with the n value. Rare earth oxide layers and nickel oxide surface layers are alternately deposited on the substrate surface to grow nickel oxide layers layer by layer, thereby obtaining an asymmetric nickel oxide superconducting thin film.

[0022] Thirdly, embodiments of the present invention provide a superconducting diode, including the aforementioned asymmetric nickel oxide superconducting thin film.

[0023] Fourthly, embodiments of the present invention provide a method for fabricating a superconducting diode, comprising the following steps:

[0024] S1. Perform micro-nano processing on the asymmetric nickel oxide superconducting thin film to form a preset electrical test pattern. The electrical test pattern includes a conductive channel defined by an etching process and multiple electrode contact areas disposed on the conductive channel to obtain a primary device sample.

[0025] S2. The primary device sample is placed in a mixed oxidizing atmosphere containing oxygen and ozone, and heated to a preset low-temperature annealing temperature for heat preservation annealing treatment to repair the surface oxygen loss and lattice disturbance caused by the etching process, thereby preparing the superconducting diode.

[0026] As a preferred embodiment of the present invention, the method for fabricating the superconducting diode,

[0027] In S1, the primary device sample is a Hall strip structure;

[0028] In S2, the preset low-temperature annealing temperature is 300-400℃, and the annealing time is 30-60 min. During the annealing process, the resistance of the primary device sample is monitored in real time, and annealing is terminated when the resistance reaches a stable value. A stable resistance value is defined as the resistance dropping to its lowest level and remaining stable for ≥5 min, at which point annealing can be terminated.

[0029] (III) Beneficial Effects

[0030] The beneficial effects of this invention are as follows: This invention provides an asymmetric nickel oxide superconducting thin film, a superconducting diode, and a method for fabricating the same. By utilizing the asymmetric distribution of the n-values ​​of each nickel oxide layer, the spatial inversion symmetry of the thin film is successfully broken within the same chemical system. This structural design does not rely on intrinsically non-centrosymmetric single-crystal materials or complex heterojunction interface engineering; structural polarity can be constructed at the atomic scale simply by controlling the layered stacking order, significantly broadening the range of materials suitable for achieving the superconducting diode effect. The intermediate layer region includes at least one nickel oxide layer with an n-value of 2. The asymmetric matching of the n-values ​​of all nickel oxide layers effectively controls the interlayer coupling strength and charge transfer distribution. Under an applied magnetic field, this specific structure can stably generate significant direction-dependent superconducting transport behavior, including observable forward and reverse critical current differences and second harmonic resistance response signals, realizing a high-performance superconducting non-reciprocal effect defined by the structure. The direction of structural polarity can be directly reversed simply by changing the stacking order of nickel oxide layers (RP phase) with different n-values ​​(i.e., the reverse growth sequence), thereby achieving controllable reversal of the superconducting diode's polarity. This "structure-as-function" characteristic greatly enhances the freedom and programmability of device design, providing a foundation for constructing logically reconfigurable superconducting circuits. Asymmetric nickel oxide superconducting thin films are entirely based on layered modulation within the same chemical system of nickel oxide, avoiding the introduction of additional magnetic doping or amorphous interface layers, and reducing scattering and instability caused by interface mismatch or impurity introduction. Compared to existing technologies, the structure is simple, the device stability is high, and it is easy to promote and apply in superconducting electronic devices, enhancing the freedom and programmability of device design and realizing high-performance superconducting non-reciprocal effects defined by the structure. Attached Figure Description

[0031] Figure 1 The second harmonic resistance of different diodes under different magnetic field strengths in the embodiments of the present invention R 2ω The curves show the changes, where 12233 represents the diode sample prepared in Example 1, and 1221 represents the control group. Detailed Implementation

[0032] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0034] Example 1

[0035] This embodiment provides a method for fabricating a superconducting diode, including the following steps:

[0036] (1) Substrate preparation and loading: Single crystal SrLaAlO4 (SLAO) was selected as the growth substrate. The substrate was cleaned according to standard procedures to remove organic contaminants and particles from the surface, and then loaded into the vacuum chamber of the pulsed laser deposition system.

[0037] (2) Construction of 12233 type asymmetric nickel oxide superconducting thin film: The structure utilizes strong oxide atom-by-layer epitaxy (GAE) technology. By precisely controlling the alternating deposition of rare earth oxide layers (LnO layer) and NiO2 layers, atomic-level structural control is achieved. In this embodiment, the chemical composition of the LnO layer is La. 0.45 Pr 0.55 O, other alternative materials for LnO layers include La. 0.55 Pr 0.45 O, La 0.65 Pr 0.35 O, La 0.77 Pr 0.08 Sm 0.15 O.

[0038] By artificially adjusting the number of deposition cycles to construct nickel oxide layers (RP phase unit layers) with different n values, and setting the stacking order of each n-value RP unit layer, an asymmetric stacking structure is constructed in the film growth direction (thickness direction), thereby destroying the spatial inversion symmetry.

[0039] In this embodiment, according to the target design (the number of nickel oxide layers n along the film growth direction is 1, 2, 2, 3, 3), the number of deposition cycles per layer of nickel oxide is adjusted to construct nickel oxide layers (RP phase unit layers) with different numbers of nickel oxide layers (deposition cycles are 1, 2, 2, 3, 3), and they are stacked in a preset asymmetric order.

[0040] When constructing the 12233 type asymmetric structure, its deposition sequence is: LnO-NiO2-LnO / LnO-NiO2-LnO-NiO2-LnO / LnO-NiO2-LnO-NiO2-LnO / LnO-NiO2-LnO-NiO2-LnO-NiO2-LnO / LnO-NiO2-LnO-NiO2-LnO-NiO2-LnO. By artificially setting the stacking sequence, the spatial inversion symmetry in the thickness direction is destroyed, and the polarity direction can be controlled.

[0041] The thin film was grown on a single-crystal SrLaAlO4 (SLAO) substrate. During growth, the substrate temperature was controlled at 850 °C by laser heating of the sample holder, and deposition was carried out in a strong oxidizing atmosphere of mixed oxygen and ozone (the ratio of the two gases was 1:10) to ensure that the nickel oxide system was in a stable high-oxidation state environment. The laser energy was adjusted to 22.8 mJ, and the laser spot area was approximately 1 cm². 2 After being transmitted through the optical path system, the pulsed laser is focused on the surface of the target material, ablation of the target material and producing plumes.

[0042] Following the above deposition sequence, the specific process is as follows: first, the target material (La) to be deposited as the LnO layer is placed... 0.45 Pr 0.55 O) Move the target to a position 55.5 mm directly below the substrate and rotate it at a radius of 6 mm and a speed of 5 mm / s to ensure the uniformity of the target scorching area. After the LnO layer growth is complete, rotate the target material (NiO2) for the nickel oxide layer to the same position and maintain rotation to deposit the nickel oxide layer. Repeat this process according to the set deposition sequence until the growth is complete. After completion, cool the sample at a rate of 100 °C / min and remove it at 200 °C.

[0043] During this process, the reflection high-energy electron diffraction (RHEED) oscillations are monitored in real time to ensure the growth quality and interface clarity of each layer;

[0044] After growth, a strong oxidizing atmosphere was maintained, and the sample was cooled to room temperature at a rate of 100℃ / min to obtain an asymmetric nickel oxide superconducting film with stable structure, clear interface and accurate stoichiometry.

[0045] (3) A positive photoresist is uniformly spin-coated on the surface of the grown asymmetric thin film and pre-baking is performed to remove the solvent; using an ultraviolet lithography machine, exposure is performed through a customized Hall strip mask to define the long strip conductive channel and electrode pad positions; development is performed to remove the photoresist in the exposed area and expose the thin film area to be etched.

[0046] The thin film areas not protected by photoresist were removed using an ion beam etching process until the insulating substrate was completely exposed, thereby forming an isolated Hall strip structure. Residual photoresist was removed using an organic solvent, and the sample was ultrasonically cleaned to ensure that there were no organic residues on the surface.

[0047] Platinum (Pt) is deposited at specific locations on both ends and sides of the Hall bar using magnetron sputtering or electron beam evaporation to form four-terminal measurement electrodes (current and voltage terminals). If necessary, a lift-off process is performed to form a clear electrode pattern.

[0048] (4) Place the processed Hall strip sample in an annealing furnace and introduce a controlled ratio of oxygen-ozone mixture to establish a strong oxidizing annealing atmosphere; set the annealing temperature at 360℃ and maintain the annealing time at 30-60 min; monitor the resistance of the sample in situ in real time throughout the annealing process. Observe the resistance change curve: the resistance value usually shows a trend of "first increase (desorption of impurities / structural rearrangement) → rapid decrease (oxygen atom backfilling / superconductivity recovery) → tending to stabilize" with the processing process; when the resistance value is monitored to reach the lowest point and stabilizes and no longer changes, immediately terminate the annealing process and turn off the heating power supply; the superconducting diode product is obtained, and the sample is taken out after naturally cooling to room temperature in an oxygen atmosphere.

[0049] After this treatment, the superconducting transition temperature (T) of the sample... c The significant recovery of the zero-resistance characteristics indicates that the thin film has regained good superconducting transport properties and can be used for subsequent superconducting diode effect testing.

[0050] Example 2

[0051] This embodiment provides a performance testing method for the superconducting diode prepared in Example 1:

[0052] After the structural construction and performance recovery were completed, the superconducting diode sample was tested for superconducting diode effect.

[0053] Following the preparation method in Example 1, a 1221-type symmetrical nickel oxide superconducting thin film was constructed as a control group.

[0054] The sample prepared in Example 1 (hereinafter referred to as the sample) and the control group sample (hereinafter referred to as the control group) were subjected to the following tests:

[0055] The sample prepared in Example 1 was subjected to low-temperature transport testing using a Power Physical Measurement System (PPMS). First, the sample was cooled to 2 K. An external magnetic field parallel to the film surface was applied in the superconducting state, ensuring that the direction of the applied magnetic field, the polarity direction of the film, and the direction of the current were all perpendicular to each other. Then, pulsed forward and reverse currents were applied, and the voltage response was measured and the current-voltage characteristic curves were recorded. When there were small fluctuations near zero voltage, 50 μV was set as the standard voltage value for determining the critical current, and the corresponding current was defined as the critical current. The forward critical currents were obtained. I c + With reverse critical current I c - Experimental results show that both the sample and the control group were tested under a limited magnetic field. I c + and I c- There are significant differences, namely I c + ≠ I c - This indicates that the system exhibits superconducting non-reciprocal transport behavior. When the direction of the applied magnetic field is reversed, the sample and the control group... I c + and I c - The magnitude of the change corresponds to the change in the pattern of the effect, further proving that the non-reciprocal effect originates from the superconducting diode effect generated by the breaking of structural inversion symmetry and magnetic field coupling.

[0056] To improve the sensitivity of non-reciprocal signal measurements, AC phase-locked loop (PLL) technology was further employed to extract the second harmonic resistance signal. An AC current was applied to both the sample and control groups. I = I 0 sin(ωt) Synchronous detection of 2 using a lock-in amplifier oh Frequency components are used to obtain the second harmonic voltage signal and calculate the result. R 2ω See also Figure 1 For the sample and control group R 2ω The relationship between the signal and the direction of the magnetic field was analyzed, and the results showed that... R 2ω The magnetic field direction exhibits an odd function relationship. The red curve represents sample 1 (12233). As the magnetic field strength increases, R 2ω The gradual decrease from a positive value to a negative value after crossing zero indicates that the second harmonic resistance of material 12233 has a strong response to the applied magnetic field. The blue curve represents the control group (1221), which shows a relatively gentle change overall. When the magnetic field strength is close to zero, R 2ω There is a small peak value, which gradually decreases and tends to stabilize under both positive and negative magnetic fields, indicating that the second harmonic resistance of material 1221 has a weak response to the applied magnetic field. This verifies the crucial role of artificially designed structural asymmetry in superconducting non-reciprocal transport.

[0057] In summary, by constructing an artificial asymmetric RP phase stacked structure through layer-by-layer epitaxy using strong oxide atoms, and combining this with oxygen-controlled recovery treatment after microstructure fabrication and transport measurement methods under specific geometries, the superconducting diode effect in nickel oxide thin films can be stably realized and characterized. The above implementation method achieves a complete technical path from structural design and device fabrication to non-reciprocal transport measurement, providing a repeatable and tunable solution for constructing structurally controllable superconducting diode devices.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An asymmetric nickel oxide superconducting thin film, characterized in that, The film includes at least three nickel oxide layers with RP structures stacked sequentially along the film growth direction; wherein the nickel oxide layers are composed of alternating stacks of rare earth oxide layers and nickel-oxygen facets, the number of nickel-oxygen facets in a single nickel oxide layer is n, and the n values ​​of all nickel oxide layers are asymmetrically distributed along the film growth direction to break the spatial inversion symmetry; the intermediate layer segment in the film excluding the bottom and outermost layers is defined as the intermediate layer region, and the intermediate layer region includes at least one nickel oxide layer with an n value of 2.

2. The asymmetric nickel oxide superconducting thin film as described in claim 1, characterized in that, The nickel oxide layer has 3 layers, and the n value of each nickel oxide layer along the film growth direction is 123 or 321 respectively.

3. The asymmetric nickel oxide superconducting thin film as described in claim 1, characterized in that, The intermediate layer region comprises 2-6 consecutively distributed nickel oxide layers with an n value of 2.

4. The asymmetric nickel oxide superconducting thin film as described in claim 3, characterized in that, The intermediate layer region comprises 2-4 continuously distributed nickel oxide layers with an n value of 2.

5. The asymmetric nickel oxide superconducting thin film as described in claim 4, characterized in that, The nickel oxide layer has 5 or 6 layers, and the n value of each nickel oxide layer along the film growth direction is any one of the following groups: 12233, 13221, 11223, 122233, and 122223.

6. The asymmetric nickel oxide superconducting thin film according to any one of claims 1 to 5, characterized in that, It also includes a substrate for thin film growth, the substrate being selected from SrLaAlO4 or LaAlO3.

7. A method for preparing an asymmetric nickel oxide superconducting thin film according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Heat the substrate to the preset growth temperature and introduce an oxygen-containing oxidizing atmosphere into the reaction chamber; S2. Using strong oxide atom layer-by-layer epitaxy technology, according to the preset n value of each nickel oxide layer along the film growth direction, the deposition cycle number of the nickel oxide surface layer in each nickel oxide layer is controlled accordingly, so that the deposition cycle number is consistent with the n value. Rare earth oxide layers and nickel oxide surface layers are alternately deposited on the substrate surface to grow nickel oxide layers layer by layer, thereby obtaining an asymmetric nickel oxide superconducting thin film.

8. A superconducting diode, characterized in that, Includes the asymmetric nickel oxide superconducting thin film according to any one of claims 1 to 6.

9. A method for fabricating a superconducting diode according to claim 8, characterized in that, Includes the following steps: S1. Perform micro-nano processing on the asymmetric nickel oxide superconducting thin film to form a preset electrical test pattern. The electrical test pattern includes a conductive channel defined by an etching process and multiple electrode contact areas disposed on the conductive channel to obtain a primary device sample. S2. The primary device sample is placed in a mixed oxidizing atmosphere containing oxygen and ozone, and heated to a preset low-temperature annealing temperature for heat preservation annealing treatment to repair the surface oxygen loss and lattice disturbance caused by the etching process, thereby preparing the superconducting diode.

10. The method for fabricating a superconducting diode as described in claim 9, characterized in that, In S1, the primary device sample is a Hall strip structure; In S2, the preset low-temperature annealing temperature is 300-400℃ and the annealing time is 30-60 min. During the annealing process, the resistance of the primary device sample is monitored in real time, and the annealing is terminated when the resistance reaches a stable value.

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