A parallel-resistor-free low-temperature superconducting Josephson junction current measurement system and method

Abstract

The present application relates to a kind of parallel resistance-free low-temperature superconducting Josephson junction current measurement system and method, wherein the system includes test rod, the test rod is provided with voltage end and current end at one end, and the other end is connected with switcher through cable;The voltage end and the current end are connected with sample;The switcher is connected with the input end of filter through coaxial cable, and the output end of the filter is connected with ammeter and voltmeter respectively, and the ammeter and voltmeter are connected with computer through data line.The present application can solve the problem of insufficient accuracy and excessive uncertainty of existing parallel resistance-free low-temperature superconducting JJ current I c ​

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of superconducting electronics, and particularly to a low-temperature superconducting Josephson junction current measurement system and method without parallel resistance. BACKGROUND

[0002] The determination of the low-temperature superconducting Josephson junction (JJ) zero-voltage transition current (I c ) without parallel resistance is a difficult problem in the current superconducting JJ performance measurement technology, which has made it difficult to achieve I c comparison among international research institutions, and the measurement uncertainty of I c of the superconducting JJ without parallel resistance in international laboratories is too large (generally greater than 10%), which directly affects the establishment of I c measurement standards in the field of superconducting electronics and indirectly affects the research and development of superconducting devices and circuits. According to the Ambegaokar-Baratoff theory, the theoretical value I c of I c0 is I g ×π / 4, and I g is calculated by measuring I c on the I-V curve, and I g is the current value corresponding to the JJ band gap voltage V g on the IV curve. Since the influence of spatial magnetic field and electromagnetic noise is not considered, I c0 is significantly higher than the actual measured I c value.

[0003] There are two traditional methods for determining I c : 1) I c is obtained based on the empirical formula I N R c-Emp =C (C is an empirical value), and R N is obtained by measurement, where C varies with the test site and noise environment. The measurement result of this method depends on the uncertainty of R N , spatial magnetic field and electromagnetic noise, which often changes with the environment and time, and does not have universality; 2) a multiple demagnetization random measurement method is used, which does not consider the influence of environmental electromagnetic field and magnetic field, and there is no basis for setting whether the demagnetization is sufficient, and the number of demagnetization is random, so the measurement result of I c changes with the number of tests, and the uncertainty is high, and finally the measured value of I c is generally less than 0.5I c0 , which seriously affects the performance of superconducting Josephson devices and circuits. Therefore, it is necessary to directly measure the low-temperature superconducting JJ without parallel resistance, and to propose a method for testing and determining I c . SUMMARY

[0004] The application provides a parallel resistance-free low-temperature superconducting Josephson junction current measurement system and method, to solve the problems of insufficient accuracy (overall low), and excessive uncertainty. c The precision is insufficient (overall low), and the uncertainty is too large.

[0005] The application provides a parallel resistance-free low-temperature superconducting Josephson junction current measurement system, which comprises a test rod, one end of the test rod is provided with a voltage end and a current end, and the other end is connected with a switcher through a cable; the voltage end and the current end are connected with a sample; the switcher is connected with an input end of a filter through a coaxial cable, an output end of the filter is connected with an ammeter and a voltmeter respectively, and the ammeter and the voltmeter are connected with a computer through a data line.

[0006] The filter cutoff frequency of the filter is less than 10 Hz.

[0007] The switcher is a one-to-four switcher.

[0008] The precision of the ammeter is 0.01 mu A.

[0009] The precision of the voltmeter is 1 nV.

[0010] The coaxial cable comprises inner and outer conductive wires, and the inner and outer conductive wires are insulated from each other by an insulating layer; the inner conductive wire serves as a data line, and the outer conductive wire serves as a ground wire.

[0011] The application provides a parallel resistance-free low-temperature superconducting Josephson junction current measurement method, which adopts the parallel resistance-free low-temperature superconducting Josephson junction current measurement system, and comprises the following steps:

[0012] The sample to be measured is placed on a sample holder, and the sample holder is placed at the bottom of the test rod; when the test rod is vertically placed, the sample to be measured is parallel to the horizontal plane.

[0013] The test rod is inserted into a dewar filled with liquid helium.

[0014] The dewar is placed in a magnetic shielding room, and the door of the magnetic shielding room is closed during testing.

[0015] The temperature of the sample to be measured is detected by a temperature detector, and the testing is started when the temperature of the sample to be measured is lower than 4.3K.

[0016] The I-V curve of the sample to be measured is tested, and the zero-voltage transition current I c and the hysteresis zero-voltage current I r If I r > 5% I cIf I r <5%I c , then go to the next step;

[0017] Start the test I cij , test m times, take the average of the m measured values to obtain I ci-ave ; wherein i represents the round of testing, and j represents the number of times of testing per round;

[0018] Determine whether the current round reaches the set number of rounds n, when it does not reach, return to the previous step for the next round of testing, when it reaches, take the maximum I ci-ave of the n rounds of testing as the test result output.

[0019] When the test rod is inserted into the helium-filled Dewar, the sample to be tested is placed more than 10 cm deep below the liquid surface.

[0020] The demagnetization operation is realized by rotating the test rod by 90°, self-heating, or using the pull rod to heat and then cool.

[0021] Advantages

[0022] Compared with the prior art, the present application has the following advantages and positive effects: by introducing a low-pass filter with a cutoff frequency less than 10 Hz to filter out electromagnetic noise, and introducing I r (hysteresis zero voltage current) as a basis for determining whether the demagnetization is sufficient (less than 5% (I c is sufficient), m repeated tests obtain an average I c , n 90° rotation demagnetization and then repeated measurement obtains n average I c , take the maximum value of the n average I c as the best I c measurement value. The test method of the present application overcomes the shortcomings of random measurement results, poor repeatability, and low I c measurement value of the conventional method, and has the advantages of simple process, good measurement repeatability, I c measurement value generally greater than 0.5I c0 , and uncertainty less than 2% (current step less than 1uA). BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a schematic diagram of a parallel resistance-free low-temperature superconducting Josephson junction current measurement system of the embodiment;

[0024] Figure 2 is a schematic diagram of four-wire method for measuring superconducting JJ connection in the embodiment;

[0025] Figure 3This is a flowchart of the method for measuring the current of a low-temperature superconducting Josephson junction without parallel resistance in this embodiment;

[0026] Figure 4 It is sample I of s0712 c Repeatability test curve;

[0027] Figure 5 This is the IV curve of sample S0712;

[0028] Figure 6 It is sample I of s0506 c Repeatability test curve;

[0029] Figure 7 It is sample I of the S0906 JUR series. c Graph showing the variation of knot area. Detailed Implementation

[0030] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0031] Embodiments of the present invention relate to a low-temperature superconducting Josephson junction current measurement system without parallel resistance, such as... Figure 1 As shown, the system comprises: a test rod 3, an ammeter 8, a voltmeter 9, a cable 4, a data cable 10, a coaxial cable 6, a filter 7, and a switch 5. The sample to be tested is placed on a sample holder 2, which is located at the bottom of the test rod 3. Figure 2 As shown, the sample to be tested is connected to the test rod using a four-lead method. In the figure, A is the superconducting JJ, B is the lead connected to the positive current terminal of the test rod, C is the lead connected to the negative current terminal of the test rod, D is the lead connected to the positive voltage terminal of the test rod, and E is the lead connected to the negative voltage terminal of the test rod. The test rod 3 is located in Dewar 1, which is filled with liquid helium. The sample to be tested is located 10 cm below the liquid surface. The test rod 3 is connected to the switch 5 via cable 4. The switch 5 is connected to the filter 7 via four coaxial cables 6. The filter 7 is connected to the ammeter 8 and voltmeter 9 via coaxial cables 6. The ammeter 8 is connected to the computer host 11 via data cable 10. The voltmeter 9 is connected to the computer host 11 via data cable 10. The computer host 11 is connected to the monitor 12 via data cable 10 to display and save data. The computer host 11 controls the output current of the ammeter through a LabVIEW program and reads the voltage value through the voltmeter.

[0032] In this embodiment, the filter cutoff frequency of filter 7 is less than 10Hz. Switch 5 is a one-to-four switch, which converts the currents I+ and I- and voltages V+ and V- at both ends of the sample under test on the sample holder into four channels for easy testing. The ammeter 8 has an accuracy of 0.01μA. The voltmeter 9 has an accuracy of 1nV. The coaxial cable 6 includes an inner conductive wire and an outer conductive wire, which are insulated from each other by an insulating layer. The inner conductive wire serves as a data line, and the outer conductive wire serves as a ground wire, thus preventing external radio frequency signals from affecting the measurement.

[0033] Ammeter 8 is controlled by a LabVIEW program in computer host 11. The current increases in set steps, increases to the set maximum value, and then decreases to 0. Simultaneously, voltmeter 9 measures the voltage difference between V+ and V-, plots the IV curve on the computer, and stores the IV curve and I. c value.

[0034] When using the above-mentioned low-temperature superconducting Josephson junction current measurement system without parallel resistance for measurement, if Figure 3 As shown, it includes the following steps:

[0035] S1, Place the sample to be tested on the sample holder, and place the sample holder at the bottom of the test rod. When the test rod is placed vertically, the sample is parallel to the horizontal plane.

[0036] S2, insert the test rod into the Dewar filled with liquid helium so that the sample to be tested is located more than 10 cm below the liquid surface;

[0037] S3, Place the Dewar inside the magnetic shielding room, and close the door of the magnetic shielding room during the test;

[0038] S4, Connecting Devices: Connect the test probe to the switch via cables; connect the filter to the channel switch via four coaxial cables; connect the filter to the ammeter and voltmeter via another four coaxial cables. The voltmeter and ammeter are connected to the computer host via data cables.

[0039] S5, the temperature of the sample to be tested is detected by a temperature detector, and the test is started when the temperature of the sample to be tested is below 4.3K;

[0040] S6, Test the IV curve of the sample under test to obtain the zero voltage transition current I. c Hysteresis zero voltage current I during directional scanning r If I r >5%I c If I r <5% I cIf the magnetic flux is removed, proceed to S7. During demagnetization, the test rod is rotated 90°, self-heated, or heated and then cooled by pulling the rod. After each demagnetization, the test rod is rotated 90° before the next demagnetization.

[0041] S7, Start Testing I cij The test is performed m times, and the average of the m measured values ​​is taken to obtain I. ci-ave Where i represents the test round, and j represents the number of tests in each round;

[0042] S8, determine if the current round has reached the set round number n. If not, return to the previous step to perform the next round of testing. If it has reached the set round number n, then use the maximum I obtained from n rounds of testing. ci-ave As the test result output.

[0043] The effectiveness of the above testing method will be verified through several examples below.

[0044] Example 1: Sample S0712, I c Validation of test methods

[0045] To verify the effectiveness of the method, this embodiment selects Nb / AlO x The / Nb low-temperature superconductivity JJ was tested, and the test steps are as follows:

[0046] S1, Place sample s0712 at the bottom of the test rod. When the rod is placed vertically, the sample is parallel to the horizontal plane.

[0047] S2, insert the test rod into the Dewar filled with liquid helium, with the sample located more than 10 cm below the liquid surface;

[0048] S3. Place the Dewar inside the magnetic shielding room. During the test, close the door of the magnetic shielding room.

[0049] S4, Connecting Equipment: Cables connect the test probe to the channel switch; four coaxial cables connect the filter to the channel switch; four coaxial cables connect the filter to the ammeter and voltmeter. The voltage and current sources are connected to the computer via data cables. The ammeter model is Kethley 6221, and the voltmeter model is Kethley 2182.

[0050] S5, the test begins when the sample temperature is detected to be below 4.3K by a temperature detector;

[0051] S6, test the IV curve of the superconducting JJ sample without parallel resistance, and obtain I c and zero voltage hysteresis current I r If I r Greater than 5% × I cThe test rod is rotated 90° to remove the magnetic flux. The removal of the magnetic flux is achieved by pulling down the rod, thus regulating the sample temperature. Here, the current step is 8μA, I r The flux is 36 μA, which is less than 520 × 10% μA (52 μA), so there is no need to remove the rod again to demagnetize.

[0052] S7, if I r Less than 5% × I c Then start testing I cij J changes from 1 to m. When j = m, m I values ​​are measured. c Values, take the average, and obtain I. ci-ave ; Here I ci-ave =514.88μA.

[0053] S8, i changes from 1 to n. When i is less than 3, repeat step 7), i = i + 1. When i equals 3, the result of the 3 measurements is I. c =514.88μA, 484.5μA, and 494.4μA. Output the maximum I for the three tests. c The value is 514.88μA, and the output result is 514.88μA.

[0054] By employing the method of this embodiment, the maximum I can be obtained. c The experimental results (514.88 μA) and uncertainty U = 8 / 514.88 / 8 = 0.2% validated the effectiveness of the method.

[0055] like Figure 4 The I of sample s0712 was shown. c Repeatability test curves. s0712-Juf3.1 indicates that the diameter of JJ is 3.1μm. Test results show that U is 6.6% under a current step size of 8μA, and U is less than 0.83% when the step size is 1μA, verifying the effectiveness of this test method.

[0056] Figure 5 The IV curve of sample juf3.1 is shown, and I can be seen. g The value is 815 μA, and the AB theory predicts I. c0 The value was 639.78 μA (3.14 / 4*815 μA). I ​​was obtained through direct measurement. c It is 514.88 μA (=0.8I) c0 ), much greater than 0.5I c0 (319.89μA), verifying the effectiveness of the test procedure.

[0057] Example 2: Verification of the demagnetization effect of rotating the rod 90 degrees on sample S0506.

[0058] The testing steps are similar to those in Example 1, except that in this example, in step S6, Ir Greater than 5% × I c At this point, the demagnetization operation is performed by rotating the test rod 90°.

[0059] Figure 6 The Ic repeatability test curves of sample S0506 are shown. The Ic repeatability measurement results were obtained by using different methods of lowering the rod to remove magnetic flux. It can be seen that rotating the lower rod by 90° significantly improves Ic repeatability. c Average value. This verifies the necessity of rotating the test rod 90° to remove the magnetic flux.

[0060] Example 3: Devices of different sizes I c Validation of test results

[0061] Figure 7 The test sqrt(I) is displayed. c The variation of junction radius with respect to the junction radius. The junction radii are 1.55 μm, 1.4 μm, 1.1 μm, 0.95 μm, 0.8 μm, and 0.7 μm, respectively. sqrt(I c )-r exhibits a good linear relationship, and as the area increases, I c Linear increase. c-Emp . represents empirical formula I c R N =C (C is taken as the empirical value of 1.7mV from the AIST of the National Institute of Advanced Industrial Science and Technology, Japan). It can be seen that the slopes of the tested and theoretical values ​​are almost the same, while the slopes of the curves differ from those of the empirical values. As shown in Table 1, for all tested samples, I... c / I c0 All values ​​were greater than 0.5, and the effectiveness of the measurement method in this embodiment was verified based on samples of different sizes.

[0062] Table 1

[0063] r (pm) I c / I c0 <!-- 4 -->]]> 1.55 0.76194 1.4 0.75453 1.1 0.70003 0.95 0.65514 0.8 0.56355 0.7 0.50496

[0064] It is not difficult to see that this invention filters out electromagnetic noise by introducing a low-pass filter with a cutoff frequency of less than 10Hz, and introduces I r (Hysteresis zero voltage current) is used as a criterion for judging whether demagnetization is sufficient (less than 5% I). c (To be sufficient), m repeated tests are used to obtain 1 I. c The average value is obtained by rotating the magnetizer by 90° n times and then repeating the measurement to obtain n I values. c Average value, take n I c The maximum value of the mean is used as the optimal I. c Measured values. The testing method described in this invention overcomes the problems of random and poor repeatability in conventional methods. c The shortcomings of low measured values ​​are addressed by the simple procedure and good measurement repeatability. cThe measured values ​​are generally greater than 0.5I. c0 The uncertainty is less than 2% (current step is less than 1μA).

Claims

1. A low-temperature superconducting Josephson junction current measurement system without parallel resistance, characterized in that, The device includes a test rod, one end of which is provided with a voltage terminal and a current terminal, and the other end is connected to a switch via a cable; both the voltage terminal and the current terminal are connected to the sample; the switch is connected to the input terminal of a filter via a coaxial cable, and the output terminal of the filter is connected to an ammeter and a voltmeter respectively; the ammeter and voltmeter are connected to a computer via data cables.

2. The low-temperature superconducting Josephson junction current measurement system without parallel resistance according to claim 1, characterized in that, The filter cutoff frequency is less than 10Hz.

3. The low-temperature superconducting Josephson junction current measurement system without parallel resistance according to claim 1, characterized in that, The switcher is a one-to-four switcher.

4. The low-temperature superconducting Josephson junction current measurement system without parallel resistance according to claim 1, characterized in that, The accuracy of the ammeter is 0.01 μA.

5. The low-temperature superconducting Josephson junction current measurement system without parallel resistance according to claim 1, characterized in that, The accuracy of the voltmeter is 1nV.

6. The low-temperature superconducting Josephson junction current measurement system without parallel resistance according to claim 1, characterized in that, The coaxial cable includes an inner conductive wire and an outer conductive wire, which are insulated from each other by an insulating layer. The inner conductive wire serves as a data line, and the outer conductive wire serves as a ground wire.

7. A method for measuring the current of a low-temperature superconducting Josephson junction without parallel resistance, characterized in that, The method of measuring the current of a low-temperature superconducting Josephson junction without parallel resistance as described in any one of claims 1-6 includes the following steps: The sample to be tested is placed on the sample holder, and the sample holder is placed at the bottom of the test rod. When the test rod is placed vertically, the sample to be tested is parallel to the horizontal plane. Insert the test rod into the Dewar filled with liquid helium; Place the Dewar inside the magnetic shielding room, and close the door of the magnetic shielding room during testing; The temperature of the sample under test is detected by a temperature detector, and the test begins when the temperature of the sample is below 4.3K. The IV curve of the sample under test is measured to obtain the zero voltage transition current I. c Hysteresis zero voltage current I during directional scanning r If I r >5%I c Then a demagnetization operation is performed if I r <5% I c Then proceed to the next step; Start testing I cij The test is performed m times, and the average of the m measured values ​​is taken to obtain I. ci-ave Where i represents the test round, and j represents the number of tests in each round; Determine if the current round has reached the set round number n. If not, return to the previous step and proceed to the next round. If the round has reached, calculate the maximum I obtained from n rounds of testing. ci-ave As the test result output.

8. The method for measuring the current of a low-temperature superconducting Josephson junction without parallel resistance according to claim 7, characterized in that, When inserting the test rod into the Dewar filled with liquid helium, the sample to be tested is placed at a depth of more than 10 cm below the liquid surface.

9. The method for measuring the current of a low-temperature superconducting Josephson junction without parallel resistance according to claim 7, characterized in that, The demagnetization operation is achieved by rotating the test rod 90°, self-heating, or by pulling the rod to raise the temperature and then lowering it.

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