MEASUREMENT
Application note

Topological insulator Josephson junctions
O. Breunig, A. Taskin and Y. Ando – Physics Institute II, University of Cologne, Germany

Introduction
In a topological insulator (TI), the electronic structure of thebulk reveals a unique topology that leads to the formation ofconducting surface states with intriguing properties.
The topological protection of the surface states and thepossible emergence of Majorana fermions in superconductor/
TI hybrid devices make these materials a leading candidate
for use as a robust platform for future fault-tolerant
quantum computation.
The main challenge in creating these structures lies in thefabrication of an electrically transparent interface between
the conventional superconductor and the topologicalinsulator. A Josephson junction, formed by two closely
spaced superconducting electrodes separated by a gapof less than 100 nm, can be used as a measure of thequality of the electrical interface. At low temperatures thesuperconducting electrodes induce superconductivity in the
topological insulator by the proximity effect, with a finite
Josephson current observed across a sufficiently narrow gap.

Equipment used
Oxford Instruments Triton 200 dilution refrigeratorNanonis Tramea with lock-in module
Keithley 2450 Source meter
NF LI-75A preamplifier
Tabor 9200 high-voltage amplifier

Triton and Nanonis Tramea

MEASUREMENT
Application note

Experimental set-up
For our experiment, Josephson junctions are fabricated using electron beam lithography and the deposition of highquality aluminium on an ultra-thin flake of the topological insulator BiSbTeSe2 (BSTS2), which has bulk-insulating
properties.
The device was installed on the mixing chamber of an Oxford Instruments Triton 200 dilution refrigerator andmeasured at a temperature of less than 8 mK. Using a superconducting solenoid magnet installed on the Oxford
Instruments Triton, magnetic fields of up to 6.8 mT were applied during these experiments. The magnetic field wasdriven with a Keithley 2450 current source via the Nanonis Tramea external devices module. A maximum currentof 100 mA was applied to the superconducting magnet of the dilution refrigerator. The current was changed slowly
with a maximum rate of 0.5 mA/s. DC bias sweeps were performed every mA whilst keeping the current fixed.
In order to distinguish between a supercurrent through the bulk and that through the surface, we perform gatingof the device by using a back-gate voltage applied to the back side of the Si/SiO2 substrate.
The size of the critical current of the fabricated Josephson junction is obtained from the IV-characteristics of thedevice. A four terminal resistance measurement is performed by applying a biased AC current (I = IDC + IAC) andmeasuring the voltage drop with the lock-in amplifier module of Nanonis Tramea.

NF LI-75AATramea PC

Tabor 9200

40dB gain

A-B

x15 gain

BOutTCP
interfaceInTramea
SoftwareExternal VI
interfacecustom software toautomate runs ofmultiple sweeps
LabVIEW driverNanonis Tramea
GPIB interface
Keithley 2450

1 MΩ
I- I+
Oxford Triton

IDC+IAC

V+ VAl/Au

SiO2

BSTS2

Si (n++)

VBG

superconducting magnet

low-pass & Cu-powder filters

Figure 1. Schematic of the measurement setup.

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Application note

Experimental results
The differential resistance of the device was measured whilst stepping the DC current biases from 0 to 0.3 µA in 121
steps. As the magnitude of the voltage to be measured using the Nanonis Tramea was in the range of 10 nV up to afew µV, a preamplifier was used. Each bias sweep took 9 seconds and the magnetic field was driven with a Keithley
2450 current source via the Nanonis Tramea external devices module.
LabVIEW software was written to control the Tramea via the TCP interface to run several sweeps automatically tocheck for reproducibility. A gate voltage of up to 80 V was applied to the device using a Tabor 9200 high-voltage
amplifier with a fixed gain (x 15) connected to one of the ± 10V outputs of the Nanonis Tramea.
The measurements were repeated with both positive and negative bias current and with opposite magnetic field
directions. The results were fully symmetric (Figure 2) and the shape of the observed oscillations gives insight into themicroscopic details of the Josephson junction.
The critical current changes as a function of an applied magnetic field and a Fraunhofer pattern is observed. Thispattern was measured using Nanonis Tramea in the setup shown schematically in Figure 1.

Figure 2: Fraunhofer pattern of a topological insulator
Josephson junction measured using Nanonis Tramea.

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Application note

Conclusion
In summary, topological-insulator-based superconducting devices have been fabricated and at cryogenic
temperatures a Fraunhofer pattern of the Josephson junction measured, providing insight into the microscopic
details of the junction.
The full data set for either a positive or negative magnetic field and bias current was acquired in 20 minutes withNanonis Tramea. Previously the same data taken with conventional lock-in amplifiers and LabVIEW controlled
instrumentation typically took several hours to obtain. Using Nanonis Tramea, faster data acquisition was possible
due to the rapid processing rate and the precise control of settling and averaging times.

Further reading“Anomalous Fraunhofer patterns in gated Josephson junctions based on the bulk-insulating topological insulator BiSbTeSe2”, Subhamoy Ghatak,
Oliver Breunig, Fan Yang, Zhiwei Wang, Alexey A. Taskin, Yoichi Ando - Nano Letters 2018 18 (8), 5124-5131,
https://pubs.acs.org/doi/full/10.1021/acs.nanolett.8b02029.

About Triton and Nanonis Tramea
The ultra low temperatures andhigh magnetic fields provided by theTriton dilution refrigerator make ita key research tool in revealing thequantum properties of many materials
of interest. The Triton systems alreadylead the way in experiment-readinesswith high-density RF and DC wiring
capability, unique sample exchangemechanisms, and unbeatablesuperconducting magnet integration.
SPECS’ Nanonis Tramea QTMS is anatural complementary partner to theTriton, with its fast, multi-channel,multi-functional capability. The systemenables quantum measurements to becarried out on a variety of samples, asshown in this application note.

Nanonis Trameamultifunctional,
low noise, low drift
and high resolution
electronics.

The latest Triton dilution refrigerator with increased
experimental space and cooling power.

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September 2018, Issue 1