Measuring the Resistance of a High Temperature Superconducting Samples

This article outlines an experiment to determine the superconducting transition temperature (Tc) of High Temperature Superconductor (HTS) tape by measuring its resistance. These measurements were performed using a combination of the MFLI (Medium Frequency Lock-In) amplifier from Zurich Instruments and the OptistatDry cryogenic system from Oxford Instruments with the demountable sample puck option.

This experiment shows the controllability and adaptability of the cryogenic system as well as the capability of the MFLI to resolve tiny signals with excellent background noise rejection.

Experimental Set-Up

Figure 1 displays the set-up of the OptistatDry with the MFLI and a break-out box. Furukawa Electric’s SuperPower 2G YBCO HTS tape was used as the sample.

A 500 mm length of HTS tape was coiled in a non-inductive loop, and using a custom made copper bracket this tape was mounted to an OptistatDry sample puck (Figure 2).

Set-up of MFLI lockin amplifier and OptistatTMDry Cryofree® cryostat.

Figure 1. Set-up of MFLI lockin amplifier and OptistatTMDry Cryofree® cryostat.

YBCO coil (marked with the arrow) mounted on the sample puck.

Figure 2. YBCO coil (marked with the arrow) mounted on the sample puck.

Voltage taps were then applied on the 12 mm wide tape, 15 mm from each end, providing 470 mm between the voltage taps. At each end of the tape, current supply terminals were added to send an excitation current via the tape. The sample puck contained a 50 Ω 25 W surface-mount heater and a CX 1050 SD Cernox™ sensor.

The current loop’s end-to-end resistance at room temperature was 149.2 Ω, as determined at the break-out box terminals. A CX 1050 SD Cernox™ sensor and a heater were also included in the OptistatDry heat exchanger.

Under MercuryiTC control, the system enables simultaneous sweeps of the sample puck and heat exchanger temperatures at precise user-defined rates. Temperature sweeps were then performed at 0.1 K/min, 0.05 K/min, and 0.01 K/min over the transition region to resolve the superconducting transition in the YBCO HTS tape.

To recover the slight demodulated response, the MFLI was used both as a lock-in amplifier and a low-distortion function generator. In addition, its Scope capability enabled real-time monitoring of the input signal.

Mercury settings

Three temperature sensors:
(1st stage, heat exchanger, sample puck)

Two control loops:

  • Heat exchanger sensor/heater: PID 100, 0.2, 0
  • Sample puck sensor/heater: PID 100, 0.1, 0

Temperature sweeps:

  • 89 K to 92 K at 0.05 K/min and 0.01 K/min
  • 86 K to 92 K at 0.1 K/min

MFLI settings (Figure 3)

Excitation:

  • Profile - Sinusoidal
  • Configuration - Differential
  • Amplitude - ± 20 V
  • Frequency - 117 Hz

Measurement:

  • Mode - Scope wave/LIA
  • Input range - 3 mV
  • Configuration - Differential
  • Scaling factor - 1
  • Transfer rate -1674 Sa/s
  • Sample rate - 469 kHz
  • PC read rate - 1 Sa/s
  • Data buffer -16384 samples

The MFLI LabOne interface from Zurich Instruments. The scope chart shows the signal across the YBCO at 100 K with a ± 20 V, 117 Hz excitation, peak-to-peak voltage of 0.62mV.

Figure 3. The MFLI LabOne interface from Zurich Instruments. The scope chart shows the signal across the YBCO at 100 K with a ± 20 V, 117 Hz excitation, peak-to-peak voltage of 0.62mV.

Experimental Results

Although the OptistatDry’s puck loading system has been optimized for small quantum device samples, it can still be adapted for larger items. In Figure 4, the data shows the occurrence of the superconducting transition (Tc) across a temperature range.

This is because there is a temperature gradient over the large YBCO coil, which measures roughly 40 mm in diameter. The granular nature of the YBCO material is revealed by the faster temperature sweeps.

As the temperature of the YBCO loop increases, the domains appear to change state in the avalanche groups. It is possible to control and resolve this transition more consistently with a slower temperature sweep. This temperature sweep is obtained using the accuracy and precision of the MercuryiTC controller.

Tc data showing the propagation of the superconducting state through the YBCO material as the coiled tape sample is warmed at different heating rates. *Note: a smaller excitation voltage was applied for the 100 mK/min.

Figure 4. Tc data showing the propagation of the superconducting state through the YBCO material as the coiled tape sample is warmed at different heating rates. *Note: a smaller excitation voltage was applied for the 100 mK/min.

However, a material’s Tc cannot be effectively measured with a 4-wire resistance measurement method. This measurement is a method to describe the measurement qualities and adaptability of the MFLI and the OptistatDry systems.

While making these measurements, the small signals that had to be resolved would have been too complicated for a DC resistivity method. However, these measurements were achieved by using an AC-technique with the MFLI lock-in amplifier.

A noise base of about 12 µV was also reached without taking excessive care with cabling. A 117 Hz measurement frequency was subsequently selected to prevent higher frequency cross-talk from the cabling, in addition to eliminating a major phase shift between the measurement and excitation signal and to reduce higher harmonic components.

Harmonic distortion can be simultaneously measured using the multi-demodulator MFLI input.

In addition to the electrostatic discharge protection, the break-out box (Figure 1) also contained a precision in-line 5 mΩ resistor. Keeping this factor in mind, the same measurement method was used to establish that when the sample was at 91 K, the excitation current passing via the YBCO sample is 104 mA.

A normal state resistance value of 5.48 mΩ was shown for the both backing material and the YBCO tape. Based on the SuperPower datasheet for this tape batch, the exact width and thickness were 12.04 mm and 216 µm, respectively. Considering this resistance measurement of the 470 mm length, the usual state resistivity of the tape (both YBCO and backing material) was estimated to be 3 x 10-8 Ωm.

Conclusion

The results demonstrate the superconducting transition of YBCO at various heating rates. With this cryogenic and instrumentation setup, many differential measurements of different physical properties, such as capacitance, current, and resistivity, can be carried out over a broad range of temperatures and driving modulation.

Phase information and multidemodulator configuration can be obtained simultaneously at higher harmonics or multiple frequencies without any hardware modification. This allows more flexibility while designing such low-temperature experiments.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Nanoscience.

For more information on this source, please visit Oxford Instruments Nanoscience.

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