CRYOFREE

Application note

Measuring resistance of a high temperature superconducting sample
using MFLI from Zurich Instruments and an OptistatTMDry from Oxford Instruments

ZurichInstruments

Introduction
This application note describes an experiment to measure the resistanceof High Temperature Superconductor (HTS) tape and determine thesuperconducting transition temperature, Tc. The measurements werecarried out using Oxford Instruments’ OptistatDry cryogenic systemwith the demountable sample puck option, combined with a Zurich
Instruments’ MFLI (Medium Frequency Lock-in) amplifier. The experimentFigure 1. SampleProtect measurement
demonstrates the adaptability and controllability of the cryogenic
system set-up including OptistatTMDryplatform as well as the ability of the MFLI to resolve small signals withcryostat, MFLI Lock-in Amplifier and an ESD
excellent background noise rejection.
break-out box – used for this application.

Experimental set-up
The OptistatDry was set-up with the MFLI and a ESD break-out box (seeFigure 1). SuperPower 2G YBCO HTS tape, from Furukawa Electric wasused as the sample. A 500 mm length of this tape was coiled in a noninductive loop and mounted to an OptistatDry sample puck using a custom
made copper bracket (see Figure 2). The 12 mm wide tape had voltage
taps applied 15 mm from each end, giving 470 mm between the voltage
taps. Current supply terminals were added at each end of the tape toprovide an excitation current through the tape. The sample puck included aCX 1050 SD CernoxTM sensor and a 50 Ω 25 W surface-mount heater. The
room-temperature end-to-end resistance of the current loop (HTS sample
plus wiring loom plus cabling) was 149.2 Ω measured at the break-out
box terminals. The OptistatDry heat exchanger also has a CX 1050 SD
CernoxTM sensor and a heater. The system, under MercuryiTC control,
allows for simultaneous sweeps of the heat exchanger and sample pucktemperatures at precise user selected rates. To resolve the superconducting
transition in the YBCO, temperature sweeps were conducted at 0.1 K/min,
0.05 K/min and 0.01 K/min over the transition region. The MFLI was used,both as a low-distortion function generator and as a Lock-in Amplifier torecover the small demodulated response. Its Scope capability also allowed
monitoring of the input signal in real time.

Figure 2. YBCO coil (markedwith the arrow) mounted onthe sample puck

CRYOFREE C
Application note

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

Mercury settings

MFLI settings (see Figure 3)

Three temperature sensors:

Excitation:(1st stage, heat exchanger, sample puck)

Profile				Sinusoidal
Frequency			117 Hz
Amplitude			 ± 20 V
Configuration
Differential

Two control loops:
Sample puck sensor/heater:
PID 100, 0.1, 0
Heat exchanger sensor/heater:
PID 100, 0.2, 0Temperature sweeps:
86 K to 92 K at 0.1 K/min
89 K to 92 K at 0.05 K/min and 0.01 K/min

Measurement:
Mode				Configuration		
Input range		Scaling factor		
Sample rate		Transfer rate		
Data buffer		
PC read rate		

Scope wave / LIA
Differential
3 mV
1469 kHz
1674 Sa/s
16384 samples
1 Sa/sCRYOFREE

Application note

Experimental results

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

The puck loading system of the OptistatDry is optimised for small quantum device samples but can easily be adapted
for larger items. The data in Figure 4 shows the superconducting transition (Tc) occurs over a temperature range
because there is a temperature gradient across the relatively large coil of YBCO, which is approximately 40 mm indiameter. The faster temperature sweeps expose the granular nature of the YBCO material. The domains appear tochange state in avalanche groups as the temperature of the YBCO loop rises. The transition can be controlled andresolved more uniformly using a slower temperature sweep, which is easily achieved with the precision and accuracy of
the MercuryiTC controller.
A 4-wire resistance measurement technique is not the ideal method for determining the Tc of a material. Thismeasurement is an excercise to demonstrate the adaptability and measurement qualities of the OptistatDry system andthe MFLI. The small signals which needed to be resolved in making these measurements would have been challenging
for a DC resistivity technique. Using an AC-technique with the MFLI lock-in amplifier, makes these measurements
achievable. Without taking excessive care with cabling, it was still possible to reach a noise base of around 12 µV.
The measurement frequency of 117 Hz was chosen to avoid higher frequency cross-talk from the cabling, as well as
significant phase shift between the excitation and measurement signals and to minimise higher harmonic components.
With the multi-demodulator MFLI input, harmonic distortion can be measured simultaneously (not shown here).
The ESD break-out box (see Figure 1) provides electrostatic discharge protection, thatalso includes a precision in-line 5 mΩ resistor. Due to this, it was possible to use the samemeasurement technique to establish that the excitation current flowing through theYBCO sample was 104 mA, when the sample was at a temperature of 91 K. It showeda normal state resistance value of 5.48 mΩ for the YBCO tape and backing material.
From the SuperPower datasheet for this tape batch, the actual width is 12.04 mm witha thickness of 216 microns. From this measurement of the resistance of the 470 mm
length, we can estimate the normal state resistivity of the tape (YBCO plus backingmaterial) to be 3 x 10-8 Ωm.

CRYOFREE

Application note

Conclusion and outlook

The superconducting transition of YBCO could be clearly demonstrated
at various heating rates. With such a cryogenic and instrumentation
configuration, many differential measurements of various physical properties(resistivity, current, capacitance, etc.) can be performed over a wide rangeof temperatures and driving modulation. Phase information and multidemodulator configuration, at higher harmonics or multiple frequencies canbe acquired at the same time without any hardware modification, allowing
more flexibility in the design of such low-temperature experiments.

About the OptistatDry Cryofree® cryostat
The OptistatDry provides a temperature controlled sample in vacuum
measurement environment within a Cryofree cryostat. The OptistatDrycomprises a range of compact cryostats with optical access cooled by aclosed cycle refrigerator. The system is capable of cooling samples to heliumtemperatures without the need for liquid cryogens. This provides significant
benefits in terms of ease of use and running costs. The system enables opticaland electrical measurements to be carried out on your samples, as shown inthis application note.

OptistatDry Cryofree cryostat
from Oxford Instruments

About the MFLI lock-in amplifier
MFLI lock-in amplifier
from Zurich Instruments

Zurich Instruments’ MFLI uses the latest hardware and software technologies
to bring the benefits of high performance digital signal processing to lock-in
amplifliers at medium and low frequencies. The MFLI features a differential
voltage input as well as a current input, a dual-phase demodulator and a highquality signal generator, covering a frequency range DC to 500 kHz or DC
to 5 MHz. With its superior performance and outstanding tool-set, the MFLI
defines a new standard for lock-in amplifiers.

Contact us at:
Oxford Instruments NanoSciencenanoscience@oxinst.com
www.oxford-instruments.com

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November 2016, Issue 3