Method for measuring superconducting energy gap under high voltage

Abstract

The invention relates to a method for measuring a superconducting energy gap under high pressure, which belongs to the technical field of high-temperature superconducting material testing, and comprises the following steps of: firstly, determining a target sample, then removing a metal electrode on the surface of a diamond anvil cell, cutting a platinum or gold electrode with the tip smaller than 1mu m by manual cutting or laser cutting, and arranging the platinum or gold electrode on the surface of the diamond anvil cell, adjusting the position of the electrode so that the contact area of the electrode and the sample is less than 1 [mu] m * 1 [mu] m to obtain a point contact junction under high voltage; and finally, testing a variation curve of the differential conductance of the sample along with the voltage by using a test system consisting of a lock-in amplifier, a current source and a nanovoltmeter, so as to obtain a signal capable of displaying the superconducting energy gap of the sample. The method is simple, and a practical and effective method is provided for testing the energy gap value of the superconducting material under high voltage.

Description

Technical Field

[0001] The invention belongs to the technical field of high-temperature superconducting material testing, and in particular relates to a method for measuring a superconducting energy gap under high pressure. Background Art

[0002] Superconductors have zero resistance effect and Meissner effect. In theory, they can transmit electric current without loss and are completely antimagnetic under certain conditions. Therefore, they have broad application potential in the fields of electricity, transportation, medical imaging (such as MRI), etc. According to their superconducting mechanism, superconductors can be divided into two categories: conventional superconductors and non-conventional superconductors. The superconductivity of conventional superconductors can be explained by the BCS theory, that is, electrons form Cooper pairs through acoustic-acoustic interactions, which leads to the disappearance of resistance. However, the superconductivity mechanism of non-conventional superconductors, such as high-temperature copper-based superconductors and iron-based superconductors, has not been fully understood and cannot be explained by the BCS theory. The discovery of these materials has stimulated widespread research interest because their superconducting transition temperatures are much higher than those of conventional superconductors, and it is possible to achieve superconductivity at conditions close to room temperature.

[0003] Recently, scientists have discovered a new type of superconductor - nickel-based superconductor, La3Ni2O 7-δ and Nd 0.8 Sr 0.2 NiO2 is the representative. Nd 0.8 Sr 0.2 NiO2 has a superconducting temperature of 15K at normal pressure, and its superconducting energy gap and symmetry can be measured at normal pressure by scanning tunneling microscopy (STM) and other methods. 7-δ External pressure is required to achieve high-temperature superconductivity. Current experimental reports show that the superconducting transition temperature reaches 80K at 20GPa. This discovery has set off a wave of research on nickel-based high-temperature superconductors. The many controversies between theoretical studies and the lack of experimental results emphasize the need for further experimental exploration of pressure-induced high-temperature superconductors La3Ni2O 7-δ Therefore, studying the size and structure of the superconducting gap under high pressure is crucial to revealing the superconducting mechanism of nickelate superconductors. Summary of the invention

[0004] The technical problem to be solved by the present invention is: by preparing a stable point contact junction under high pressure, it is intended to measure the superconducting energy gap and symmetry of a nickel-based superconductor under high pressure, with the purpose of obtaining a superconducting energy gap measurement method and corresponding data of a superconductor under high pressure. The method coats diamond by direct current magnetron sputtering, and photolithography is used to prepare an electrode shape to perform electrical measurement on the sample at low temperature. After determining that the sample is a target sample, the metal coating on the surface of the diamond is removed, platinum or gold electrodes are manually prepared, and the electrodes are arranged on the surface of the diamond, ensuring that the contact area between the tip of the electrode and the sample is small to ensure that a good signal can be obtained, and after controlling the pressure to make the superconducting transition temperature the maximum value, the point contact spectrum of the sample is measured.

[0005] The specific technical solutions of the present invention are as follows:

[0006] A method for measuring a superconducting energy gap under high pressure, the steps comprising preparing a point contact junction and testing the superconducting energy gap;

[0007] First, a 1 μm molybdenum film is plated on the surface of a diamond anvil at 700°C by a direct current magnetron sputtering method, and then four electrodes are photoetched by a photolithography device. The sample is electrically tested by the Van der Pauw method to determine the target sample, and the gasket with the sample is removed. The metal electrode on the surface of the diamond anvil is removed by a molybdenum etching solution, and a platinum or gold electrode with a tip less than 1 μm is manually cut or laser cut and arranged on the surface of the diamond anvil. The electrode position is adjusted so that the contact area between the electrode and the sample is less than 1 μm×1 μm, so as to obtain a point contact junction under high pressure;

[0008] The superconducting energy gap is tested using a test system consisting of a phase-locked amplifier, a current source and a nanovoltmeter. The current source is set to change in steps of μA and below, and the initial setting of the automatic adjustment range of the equipment is canceled to prevent signal jumps. A phase-locked amplifier is used to apply an AC signal of μA level to the sample through a 48kΩ resistor, and the differential conductance of the sample is tested at the input end. The change curve of the voltage can obtain a signal that can show the superconducting energy gap of the sample.

[0009] Preferably, the model of the lock-in amplifier is SR830, the model of the current source is Keithley6221, and the model of the nanovoltmeter is Keithley2182A.

[0010] Beneficial effects:

[0011] The advantages of the point contact junction prepared by the present invention are: the method of the present invention is simple, and many testing methods cannot be applied to high-voltage superconducting materials due to technical conditions. The present method provides an effective method for numerically testing the energy gap of superconducting materials under high pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1The high-temperature nickel-based superconducting material La3Ni2O in the diamond anvil prepared in Example 1 and Example 2 7-δ Micrographs of samples and four electrodes.

[0013] Figure 2 It is a structural diagram of the internal point contact junction of the diamond anvil prepared by the present invention.

[0014] Figure 3 It is the test circuit schematic diagram of the test point contact spectrum application.

[0015] Figure 4 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ In situ high-voltage resistance versus temperature curves at 24.5 GPa and 23 GPa using platinum and gold electrodes, respectively.

[0016] Figure 5 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ At 24.5 GPa, the functional relationship between resistance and temperature under different magnetic fields tested using platinum electrodes.

[0017] Figure 6 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ At 24.5 GPa, the voltage-current curves at different temperatures were tested.

[0018] Figure 7 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ At 24.5 GPa, the critical current versus temperature curve fitted using the voltage-current curve data (Single Vortex model)

[0019] Figure 8 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ Experimental parameter diagram of point contact spectrum curves and d-wave fitting curves tested with platinum electrode and gold electrode at 24.5GPa and 23GPa respectively.

[0020] Fig. 9 It is a high-temperature nickel-based superconducting material La3Ni2O 7-δ At 24.5 GPa, point contact spectrum curves and d-wave fitting curves at different temperatures tested using a platinum electrode.

[0021] Fig.10 High temperature nickel-based superconducting material La3Ni2O 7-δ At 24.5 GPa, the relationship between the energy gap value Δ and temperature and the d-wave fitting curve are shown. DETAILED DESCRIPTION

[0022] Example 1 Preparation and Photolithography of Molybdenum Film

[0023] Since some samples may be non-uniform, the sample placed in the sample chamber of the diamond anvil device may not be the target sample, so the target sample needs to be determined first. First, the molybdenum target (purity 99.9%) is loaded into the magnetron sputtering device and vacuumed to 1*10 -3 Pa, using DC sputtering method (power 120W), at a temperature of 700℃, 0.5Pa argon atmosphere, sputtering on the diamond substrate for 5 minutes, a 1μm molybdenum film can be obtained. After sputtering, the diamond is fixed on a cubic boron nitride support block, and the photoresist is spin-coated at 600 revolutions per minute for the first 10 seconds and 3000 revolutions per minute for the last 20 seconds. Then, the shape of the four electrodes is photoetched using a mask and developed using the corresponding photoresist developer. After that, the excess molybdenum metal film is etched under a microscope using a molybdenum etching solution to prepare four metal molybdenum electrodes. Among them, the volume ratio of the molybdenum etching solution is nitric acid (concentration 68%): phosphoric acid (concentration 85%): acetic acid (concentration 99%): deionized water = 9:4:14:7.

[0024] Example 2 Preparation of sample chamber under high pressure

[0025] The preparation of point contact junction under high pressure is carried out in a diamond anvil cell apparatus, the schematic diagram of which is shown in Figure 1 and Figure 2 The symmetrical diamond anvil was used for pressure application. The diamond anvil surface size was 300μm and the tungsten sheet was used as the gasket. The sample assembly process of the diamond anvil chamber is as follows: First, a 15μm thick pit was made by pre-pressing 25GPa in the center of the tungsten gasket, and then a 250μm diameter hole was made in the center of the pit as a cavity for loading the insulating layer and the sample. The insulating layer made by mixing cubic boron nitride (c-BN) and magnesium oxide (MgO) powder with epoxy resin binder was filled into the cavity in turn and the insulating powder was pressed to about 10μm at about 28GPa. A 100μm diameter hole was made in the center of the insulating powder, and ammonia borane (AB) was filled in the hole as the pressure transmission medium, and then the sample was filled. The diamond with molybdenum electrode prepared in Example 1 is loaded into a diamond anvil apparatus, a 5μm platinum foil is arranged on the molybdenum electrode and connected to an external copper wire, and the electrical properties of the sample are tested using the four-electrode method. After confirming that the sample properties are correct, a sample cavity under high pressure can be prepared.

[0026] Example 3 Preparation of point contact junction under high voltage and electrical property test of samples

[0027] After determining the sample properties in Example 2, the molybdenum electrode on the diamond surface was removed using a molybdenum etching solution, and then a 3μm platinum foil or gold foil was manually cut to ensure that the electrode tip width was less than 1μm. The electrode was placed on the diamond surface and the electrode position was adjusted so that the contact area between the electrode and the sample was less than 1μm×1μm. Figure 3 The circuit in the sample is used to test the signal of the point contact spectrum. After an obvious zero-bias conductivity peak is measured, the point contact junction is completed.

[0028] Four-probe resistance transmission measurement and point contact spectrum measurement were performed on the samples.

[0029] Figure 4 It is a high-temperature superconductor La3Ni2O 7-δ The in-situ high-voltage resistance versus temperature curves measured at 24.5 GPa and 23 GPa using platinum electrodes and gold electrodes respectively show that the superconducting transition temperature of the sample is 70 K and 72 K. Figure 5 It is a high-temperature superconductor La3Ni2O 7-δ A plot of the electrical resistance as a function of temperature for different magnetic fields, showing that the superconducting transition temperature decreases by applying a magnetic field. Figure 6 It is to test the high-temperature superconductor La3Ni2O at different temperatures 7-δ The voltage-current curve can be used to read the critical current of the sample and Figure 7 The critical current vs. temperature curve (Single Vortex model) is obtained, showing that the critical current J of the sample at 24.5 GPa is c It is 2.03±0.13mA.

[0030] Figure 8 The high-temperature superconductor La3Ni2O was tested using platinum electrodes and gold electrodes respectively. 7-δ The point contact spectra and fitting curves at 24.5 GPa and 23 GPa show that the platinum electrode test results are fitted with a broadening coefficient Γ of 10 meV and a junction shielding strength parameter Z of 2.96 under d-wave fitting, and the energy gap 2Δ=36 meV obtained by fitting. The gold electrode test results are fitted with a broadening coefficient Γ of 17.8 meV and a junction shielding strength parameter Z of 2.3 under d-wave fitting, and the energy gap 2Δ=36.4 meV obtained by fitting. The fitting results of the two electrodes are very close, proving the reliability of the method. Fig. 9 It is a high-temperature superconductor La3Ni2O 7-δ The point contact spectra and fitting curves tested with platinum electrodes at different temperatures at 24.5 GPa can be obtained based on the results. Fig.10 Medium- and high-temperature superconductor La3Ni2O 7-δ The curve of energy gap value changing with temperature.

[0031] Measuring the superconducting energy gap of superconducting materials under high pressure is a difficult point in current high-pressure measurement. The present invention provides a simple and effective method to measure the superconducting energy gap, which is of great significance for understanding the superconducting mechanism and optimizing the performance of superconducting materials under extreme conditions.

Claims

1. A method for measuring a superconducting energy gap under high pressure, comprising the steps of preparing a point contact junction and testing the superconducting energy gap; First, a 1 μm molybdenum film is plated on the surface of a diamond anvil at 700°C by a direct current magnetron sputtering method, and then four electrodes are photoetched by a photolithography device. The sample is electrically tested by the Van der Pauw method to determine the target sample, and the gasket with the sample is removed. The metal electrode on the surface of the diamond anvil is removed by a molybdenum etching solution, and a platinum or gold electrode with a tip less than 1 μm is manually cut or laser cut and arranged on the surface of the diamond anvil. The electrode position is adjusted so that the contact area between the electrode and the sample is less than 1 μm×1 μm, so as to obtain a point contact junction under high pressure; The superconducting energy gap is tested using a test system consisting of a phase-locked amplifier, a current source and a nanovoltmeter. The current source is set to change in steps of μA and below, and the initial setting of the automatic adjustment range of the equipment is canceled to prevent signal jumps. A phase-locked amplifier is used to apply an AC signal of μA level to the sample through a 48kΩ resistor, and the differential conductance of the sample is tested at the input end. The change curve of the voltage can obtain a signal that can show the superconducting energy gap of the sample.

2. The method for measuring superconducting energy gap under high pressure according to claim 1, characterized in that: The model of the lock-in amplifier is SR830, the model of the current source is Keithley6221, and the model of the nanovoltmeter is Keithley2182A.

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