Determining the Superconducting Transition Temperature of High Temperature Superconductor Tape

The details of an experiment for measuring the resistance and the superconducting transition temperature (Tc) of High Temperature Superconductor (HTS) tape are provided in this article.

For the experiment, the OptistatDry cryogenic system from Oxford Instruments was fitted with a demountable sample puck option and integrated with a Medium Frequency Lock-In (MFLI) amplifier from Zurich Instruments. The adaptability and controllability of the cryogenic platform and the ability of the MFLI to resolve small signals while rejecting background noise well was illustrated by the experiment.

Experimental Setup

The experimental setup consisted of the OptistatDry, the MFLI and a break-out box (Figure 1) with the SuperPower 2G YBCO HTS tape from Furukawa Electric as the sample.

Set-up of MFLI lock-in amplifier and Optistat™Dry Cryofree® cryostat.

Figure 1. Set-up of MFLI lock-in amplifier and Optistat™Dry Cryofree® cryostat.

A custom made copper bracket was used to mount 500mm of the YBCO tape, wound as a non-inductive loop, onto the OptistatDry sample puck (Figure 2).

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 applied on the 12mm wide tape at a distance of 15mm from each end, providing a 470mm gap between the voltage taps. Supply terminals were added at the end of the tape to pass the excitation current, through it. A CX 1050 SD Cernox™ sensor and a 50Ω 25 W surface-mount heater were provided in the sample puck. The end-to-end resistance of the current loop, which included the HTS sample, cabling and the wiring loom, measured at room temperature at the break-out box terminals was 149.2Ω.

A heater and a CX 1050 SD Cernox™ sensor are provided in the OptistatDry heat exchanger. The MercuryiTC control of the system enables simultaneous sweeps of the heat exchanger and sample puck temperatures at specific rates that are chosen by the user. The temperature sweep was conducted at 0.1, 0.05 and 0.01 K/min over the transition region in order to resolve the superconducting transition in the YBCO. The MFLI played a dual role in this experiment. It was a low-distortion function generator and a lock-in amplifier that recovered small demodulated responses. The input signal was monitored in real time with the help of the Scope functionality of the MFLI.

Mercury Settings

The mercury settings include:

  • Three temperature sensors, for 1st stage, sample puck and heat exchanger
  • Two control loops, one for the sample puck sensor or heater (PID 100, 0.1, 0 )and another for the heat exchanger or sensor (PID, 100, 0.2,0)
  • Temperature sweeps from 86 K to 92 K at 0.1 K/min and 89 K to 92 K at 0.05 K/min and 0.01 K/min

MFLI Settings

The MFLI settings are shown in Figure 3. A sinusoidal wave with a frequency 117Hz, an amplitude of ± 20 V, and a differential configuration was used for excitation.

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 0.62 mV.

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 0.62 mV.

The settings for measurement were as follows:

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

Experimental results

Although the OptistatDry’s puck loading system is customized for handling small device samples it can be extended to handle larger samples as well. As figure 4 shows, the superconducting transition (Tc) occurs over a temperature range because of the temperature gradient that exists across the comparatively large YBCO coil which has a 40mm diameter. The granular structure of the YBCO material is exposed by the rapid temperature sweeps. As the YBCO loop temperature increases, the domains seem to change their state in avalanche groups.

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

For more uniform control and resolution of the transition, a more gradual temperature sweep is required, which can be achieved with the accuracy and precision of the MercuryiTC controller.

Determining a material’s Tc using the 4-wire resistance measurement method is not ideal but his experiment aimed to illustrate the adaptability and measurement characteristics of the integrated OptistatDry and the MFLI system. Since it would have been challenging to resolve the small signals with a DC resistivity technique, an AC technique with a MFLI lock-in amplifier was used instead. This technique was able to reach a noise base of approximately 12µV without the need for caution during cabling. An ideal measurement frequency of 117Hz was chosen in order to minimize higher harmonic components, eliminate any cross-talk from cabling at higher frequencies and avoid any large phase shift between the excitation and measurement signals. The harmonic distortion and the MFLI input can be measured simultaneously using a multi-demodulator. As well as protection from electrostatic discharge, a precision in-line 5mΩ resistor was also provided in the break-out box. This arrangement enabled the use of the same measurement method to  establish that the excitation current through the YBCO sample was 104mA at a  temperature of 91K. The YBCO tape and backing material showed a normal resistance value of 5.48mΩ. As per the Superpower datasheet provided for this tape batch, the actual width was found to be 12.04mm and thickness was 216µm. The normal state resistivity of the 470mm length tape was found to be 3 x 10-8Ωm.

OptistatDry Cryofree Cryostat

The OptistatDry provides a temperature controlled sample in a vacuum measurement environment in a Cryofree cryostat. It has a series of compact cryostats with optical access cooled by a closed cycle refrigerator. The system can cool samples to helium temperatures without the need for liquid cryogens, which makes it easy to use and brings down running costs. It can also be used for both optical and electrical measurements.

OptistatDry Cryofree cryostat from Oxford Instruments

Figure 5. OptistatDry Cryofree cryostat from Oxford Instruments

MFLI lock-in Amplifier

The most advanced and latest hardware and software has been incorporated in the MFLI to enable lock-in amplifiers to achieve high performance digital signal processing at medium and low frequencies. The features of the MFLI include a dual-phase demodulator, a high quality signal generator that covers a frequency range of DC to 500kHz or DC to 5MHz and a differential voltage input plus a current input. The MFLI with its excellent performance and highly capable tool set is considered as a new benchmark for lock-in amplifiers.

MFLI lock-in amplifier from Zurich Instruments

Figure 6. MFLI lock-in amplifier from Zurich Instruments

Conclusion

The experiment demonstrates the superconducting transition of YBCO at different heating rates. Differential measurements of various physical properties could be performed over a broad range of temperatures and driving modulation based on the cryogenic and instrumentation configuration. In addition to this, multi-demodulator and phase information at higher harmonics or multiple frequencies can be achieved simultaneously without altering any hardware. This enables moreflexibility in designing 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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