Sample device preparation method for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials

Abstract

This invention discloses a method for fabricating a sample device for measuring the in-plane anisotropy of unconventional two-dimensional superconducting materials, comprising the following steps: mechanically thinning an unconventional superconducting crystal and transferring it to a silicon wafer surface; coating a resist layer on the surface of the two-dimensional superconducting material; firstly using electron beam lithography to photolithographically etch the resist layer into a concentric ring structure, followed by development and fixing to fully expose the lithographic area; depositing metal onto the silicon wafer using thermal evaporation, removing the residual resist layer and excess metal from the silicon wafer to obtain a ring electrode; covering half of the ring electrode with an insulating layer, recoating with a resist layer, and then performing a second electron beam lithography, followed by development and fixing, and finally depositing a test electrode to obtain the target device. This invention, by designing a ring electrode structure to explore the in-plane resistivity anisotropy of materials, eliminates the influence of traditional "four-wire method" measurements on the measurement results, and can explore more intrinsic material properties.

Description

Technical Field

[0001] This invention belongs to the field of unconventional superconductor property measurement technology, specifically relating to a method for preparing a sample device for measuring the in-plane anisotropy of unconventional two-dimensional superconducting materials. Background Technology

[0002] In conventional superconductors, electrons with antiparallel spins and opposite momentum form Cooper pairs, and the superconducting order parameter exhibits isotropic s-wave symmetry. The primary driving force for electron pairing is electron-phonon interaction. While unconventional superconductors also exhibit zero resistance and the Meissner effect below the superconducting transition temperature, their Cooper pair formation mechanism and the symmetry of their superconducting wavefunction differ significantly from those of conventional superconductors. For example, in copper oxide superconductors, the superconducting order parameter is considered to be nodally d-wave symmetric, and Cooper pair formation is not attributed to electron-phonon interaction but is believed to be closely related to magnetic fluctuations. Unlike conventional superconductors, electron-electron interactions in unconventional superconductors may be a key factor influencing superconducting electron pairing, and they also bring about a series of novel physical phenomena.

[0003] For a long time, the study of two-dimensional superconductivity has been a highly anticipated frontier area of ​​superconductivity research. With continuous technological advancements, experimental techniques such as molecular beam epitaxy, mechanical exfoliation, and field-effect transistors have been successively applied to the study of two-dimensional superconductivity. Furthermore, the electronic correlations and superconducting order parameters of two-dimensional superconducting materials may change differently with variations in dimensionality. Therefore, investigating the potential interactions between superconductivity and other emerging symmetry-breaking quantum states in two-dimensional superconducting materials will be of great significance for exploring the superconducting mechanisms of unconventional superconductors.

[0004] Recently, a study used pulsed laser deposition (PLD) technology to prepare perovskite precursor Nd on SrTiO3(001) substrates. 0.8 Sr 0.2 NiO3 thin films, followed by removal of root tip oxygen via soft chemical topological reduction, resulted in a topological transformation of the nickelate film from a perovskite phase to an infinitely layered phase, thus yielding superconducting Nd2O3. 0.8 Sr 0.2 NiO2 thin films were used, and their critical magnetic field and magnetoresistance were measured under different magnetic field orientations. In this study, a Corbino-disk device was used to eliminate the influence of current flow in angle-dependent magnetoresistance measurements, which cannot be completely avoided in standard four-probe measurements. However, this device has the drawback of requiring relatively complete and large-area material, and the sample preparation is relatively complex due to the need for PLD growth followed by reduction.

[0005] Furthermore, traditional methods for fabricating electrodes to measure the anisotropy of superconductors mostly involve manually applying silver paste. This method is relatively crude and cannot guarantee a perfectly circular electrode shape, leading to potential measurement errors. Moreover, this method is often used to measure electrical signals along the c-axis of the material, making it difficult to measure signals within the ab-plane of the sample, thus hindering the exploration of in-plane anisotropy in superconductors. Summary of the Invention

[0006] To address the problems of existing technologies, this invention provides a sample device for measuring the in-plane anisotropy of unconventional two-dimensional superconducting materials by mechanically exfoliating the material and fabricating it using micro-nano processing techniques. This solves the problems of complex sample device fabrication methods and the inability of traditional electrode fabrication to achieve effective and accurate measurements.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for fabricating a sample device for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials includes the following steps: Two-dimensional superconducting materials are obtained by mechanically thinning unconventional superconducting crystals. Specifically, PVC tape is adhered to the surface of the unconventional superconducting crystal, and then it is dissociated vertically into two-dimensional superconducting materials with a thickness of less than 100 nm. Preferably, the unconventional superconducting crystal is an Fe / Cu-based superconductor or a Kagome structure superconductor, and the Kagome structure superconductor has the molecular formula AV3Sb5, where A is K, Rb, or Cs.

[0008] Two-dimensional superconducting materials are transferred to the surface of a silicon wafer. The method is as follows: First, the two-dimensional superconducting material is transferred to a polydimethylsiloxane (PDMS) substrate. After heating the silicon wafer, the side of the PDMS substrate with the two-dimensional superconducting material is aligned with the silicon wafer. Then, the PDMS substrate is brought into contact with the silicon wafer. Finally, the PDMS substrate is lifted upwards. The two-dimensional superconducting material will adhere to the surface of the silicon wafer due to surface forces. A resist layer is coated on the surface of a two-dimensional superconducting material. Electron beam lithography (EBL) is used for the first time to prepare the resist layer into a concentric ring structure. After development and fixing, the electron beam lithography area is fully exposed. The spacing between two adjacent concentric rings in the concentric ring structure is 1-3µm. Preferably, the aperture diameter of the first electron beam lithography is 30µm, in order to make the shape of the ring electrode more precise.

[0009] Metal is deposited onto a silicon wafer using thermal evaporation technology, and the residual resist layer and excess metal on the silicon wafer are removed to obtain a ring electrode. The annular electrode is covered with an insulating layer on half of its disk area. After applying a resist layer, a second electron beam lithography is performed to form the test electrode pattern. After development and fixing, the test electrode is deposited by vapor deposition to obtain the target device. Preferably, the aperture diameter for the second electron beam lithography is 60µm, because this step does not require a very fine pattern. Using a larger aperture can increase the exposure speed and shorten the preparation time.

[0010] As a preferred technical solution, the material of the resist layer is polymethyl methacrylate (PMMA).

[0011] As a preferred technical solution, the insulating layer is made of cubic boron nitride insulating layer or Al2O3 and other insulating materials, with a thickness of less than 100nm, to avoid subsequent electrode breakage.

[0012] The present invention has the following beneficial effects: This invention investigates the in-plane resistivity anisotropy of materials by designing a ring electrode structure, eliminating the influence of the traditional "four-wire method" on the measurement results and revealing more intrinsic material properties. In the traditional "four-wire method" for measuring in-plane anisotropy, the applied current is easily affected by external magnetic fields, leading to eigenfactors in the test results. This invention can explore the in-plane anisotropy of the crystal structure and electron pairing of superconducting materials, focusing more on the in-plane contribution of the material, thus more intuitively revealing the microscopic coexistence of superconducting states with other electronic states. Traditional anisotropy measurements often focus on out-of-plane contributions, and the "silver paste" electrode preparation method is prone to interference from contact resistance.

[0013] The device electrodes in this invention are fabricated using electron beam lithography, which makes the fabrication process more precise, with the minimum linewidth controllable to 1µm, and reduces the unevenness error of traditional hand-applied silver paste.

[0014] Compared with other methods, the electrode preparation method in this invention avoids the ion beam etching step, thereby reducing the damage to the material caused by etching and making it easier to measure the intrinsic information of the material.

[0015] This invention uses mechanical exfoliation to prepare two-dimensional materials, which requires small material size, does not require large-area material preparation methods such as molecular beam epitaxy or pulsed laser deposition, and causes less damage to the material during the preparation process, resulting in higher sample quality and simpler material preparation. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the mechanical peeling and transfer process; Figure 2 A schematic diagram of the process for fabricating a ring electrode using electron beam lithography for the first time; Figure 3 for Figure 2A schematic diagram of the structure of the ring electrode prepared in the process; Figure 4 A schematic diagram of the process for fabricating test electrodes using electron beam lithography for the second time; Figure 5 A photograph of the final product; Figure 6 The present invention is used to measure the in-plane resistivity anisotropy of two-dimensional superconducting materials using the device of the present invention.

[0017] Figure reference numerals: 1-unconventional superconducting crystal, 2-tape, 3-two-dimensional superconducting material, 4-PDMS, 5-silicon wafer, 6-resist layer, 7-ring electrode, 8-insulating layer, 9-test electrode. Detailed Implementation

[0018] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments are not intended to limit the present invention. In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail.

[0019] Example refer to Figures 1 to 5 A method for fabricating a sample device for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials includes the following steps: The unconventional superconducting crystal 1 was mechanically thinned using PVC tape 2 to obtain a two-dimensional superconducting material 3, which was then transferred onto a PDMS 4 substrate. The transfer was performed using an optical microscope.

[0020] Place a clean silicon wafer 5 on a 70°C heating plate. Invert the two-dimensional superconducting material 3 on the PDMS substrate and align it with the center of the silicon wafer 5. Slowly press the PDMS to make the two-dimensional superconducting material 3 contact the silicon wafer. After 2-3 minutes, lift the PDMS. At this time, the sample will adhere to the silicon wafer due to surface forces.

[0021] A concentric ring structure was formed on a silicon wafer with a spin-coated PMMA resist layer 6 using electron beam lithography (EBL). The spacing between adjacent concentric rings was 1-3 µm. A 30 µm aperture was used for the first EBL to ensure a more precise ring electrode shape. Then, the area lithographically patterned was fully exposed by 30 seconds of development and fixing. Subsequently, a 30 nm Au electrode was evaporated onto the silicon substrate using thermal evaporation at a rate controlled at 0.2 Å / s to achieve a denser and more uniform gold electrode. Finally, excess resist and metal were removed by acetone stripping to reveal the desired ring electrode 7. Figure 3As shown, the resulting annular electrode consists of a central circle and three concentric rings, wherein the width of each ring is 2-4µm and the spacing between two adjacent rings is 1-3µm.

[0022] Based on this, cubic boron nitride (h-BN) is transferred again as an insulating layer 8 using an optical transfer platform, covering half of the circular electrode area (observed under a microscope). The h-BN thickness should be less than 100 nm to avoid subsequent electrode breakage. The test electrode pattern is then etched again using EBL. A 60 µm aperture can be used for the second EBL, as this step does not require a very fine pattern. After development and fixing, 10 nm of Cr and 90 nm of Au are deposited by evaporation to form the test electrode 9. The evaporation rate can be 0.4 Å / s. The desired device is then obtained by peeling off the electrode. Figure 5 As shown. The test electrode formed by vapor deposition covers the uninsulated area of ​​the ring electrode and the insulating layer. The test electrode is connected to the ring electrode formed in the previous steps. The insulating layer prevents short circuits between the ring electrodes. Figure 5 As shown.

[0023] Figure 6 To measure the in-plane resistivity anisotropy of a two-dimensional superconducting material, specifically CsV3Sb5, using the device of this invention, it can be seen from the figure that the in-plane resistivity exhibits a uniform and symmetrical distribution as the magnetic field angle changes. Compared with other methods, this method has better results in measuring the in-plane anisotropy of materials.

[0024] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for fabricating a sample device for measuring the in-plane anisotropy of unconventional two-dimensional superconducting materials, characterized in that, Includes the following steps: Two-dimensional superconducting materials are obtained by mechanically thinning unconventional superconducting crystals. Transferring two-dimensional superconducting materials onto the surface of a silicon wafer; A resist layer is coated on the surface of a two-dimensional superconducting material. For the first time, electron beam lithography is used to prepare the resist layer into a concentric ring structure. After development and fixing, the electron beam lithography area is fully exposed. The spacing between two adjacent concentric rings in the concentric ring structure is 1-3µm. Metal is deposited onto a silicon wafer using thermal evaporation technology, and the residual resist layer and excess metal on the silicon wafer are removed to obtain a ring electrode. The ring electrode is covered with an insulating layer to cover half of the disk area. After coating with a resist layer, a second electron beam lithography is used to form the test electrode pattern. After development and fixing, the test electrode is deposited by vapor deposition to obtain the target device.

2. The method for fabricating a sample device for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials according to claim 1, characterized in that, The unconventional superconducting crystal is an Fe / Cu-based superconductor or a Kagome-structured superconductor.

3. The method for preparing a sample device for measuring in-plane anisotropy of a unconventional two-dimensional superconductor material according to claim 1, wherein The mechanical thinning method is as follows: adhesive tape is adhered to the surface of an unconventional superconducting crystal, and then it is dissociated into a two-dimensional superconducting material with a thickness of less than 100 nm in the vertical direction.

4. The method for preparing a sample device for measuring in-plane anisotropy of a unconventional two-dimensional superconductor material according to claim 1, wherein The method for transferring two-dimensional superconducting material to the surface of a silicon wafer is as follows: First, the two-dimensional superconducting material is transferred to a polydimethylsiloxane substrate. After heating the silicon wafer, the side of the polydimethylsiloxane substrate with the two-dimensional superconducting material is aligned with the silicon wafer. Then, the polydimethylsiloxane substrate is brought into contact with the silicon wafer. Finally, the polydimethylsiloxane substrate is lifted upwards. Due to surface forces, the two-dimensional superconducting material will adhere to the surface of the silicon wafer.

5. The method for preparing a sample device for measuring in-plane anisotropy of a unconventional two-dimensional superconductor material of claim 1, wherein, The resist layer is made of polymethyl methacrylate.

6. The method for preparing a sample device for measuring in-plane anisotropy of a unconventional two-dimensional superconductor material of claim 1, wherein, The aperture diameter used in the first application of electron beam exposure technology was 30µm.

7. The method for fabricating a sample device for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials according to claim 1, characterized in that, The insulating layer is made of cubic boron nitride or Al2O3, and its thickness is less than 100 nm.

8. The method for fabricating a sample device for measuring in-plane anisotropy of unconventional two-dimensional superconducting materials according to claim 1, characterized in that, The aperture diameter for the second electron beam exposure technique is 60µm.

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