Fig. 1: Measurement of CPR using asymmetric SQUID.
a A scanning electron microscope (SEM) image of an asymmetric superconducting quantum interference device (SQUID) used to measure the current–phase relation (CPR), the supercurrent I vs. φ (the phase difference between the superconductors), in the topological insulator (TI)-based Josephson junction (JJ). The asymmetric SQUID is formed between a TI-based JJ with superconducting Nb contacts and a reference (REF) junction in parallel. The REF junction is a conventional S–S′–S Josephson junction with the supercurrent IREF(φR) = ICREF sin(φR), where ICREF and φR are the critical current and the phase difference across the REF junction, respectively. Scale bar is 1 μm. b Schematic of the CPR measurement setup. We use a low-frequency (~17 Hz) square-wave pulsed current (ISQUID) with 50% duty cycle to bias the SQUID. The voltage VS across the SQUID is monitored with a lock-in amplifier. A perpendicular magnetic field B is applied to control the phase difference inside the SQUID loop (with area S). i.e. φ − φR = 2πΦB/Φ0, where ΦB = B·S is the magnetic flux and Φ0 = h/2e is the superconducting flux quantum. c Color map of VS as functions of ISQUID and B. The solid white curve marks the critical current ICSQUID of the SQUID and the dashed red line is the critical current ICREF of the REF junction. d The current–phase relation (symbols) represented by the normalized current (I/IC) of the TI-based JJ vs. the phase φ measured in sample A at temperature T = 20 mK. Dashed blue curve depicts sin(φ). Since the absolute value of the flux inside the SQUID is unknown, we shift the experimental curve in the horizontal axis so that φ = 2πΦB/Φ0.
