Fig. 1: Device description and measurement setup.
From: Charge-4e supercurrent in a two-dimensional InAs-Al superconductor-semiconductor heterostructure
a Circuit schematic of a dc superconducting quantum interference device (SQUID) formed by two gate-tunable Josephson junctions with effective transmission probabilities \({\tau }_{1}^{* },{\tau }_{2}^{* }\), threaded by the external flux Φext. b False-color electron micrograph of the device and experimental setup. Each junction is fabricated by selectively removing the epi-Al layer (blue) over 150 nm long stripes. The charge carrier density in the exposed InAs two-dimensional electron gas (green) is tuned by a set of electrostatic gates (VG1, VG2, and VFG) shown in yellow and orange, which are galvanically isolated from the loop by 15 nm of HfO2 (light blue). dc and ac current bias is defined through the voltage drop over a bias resistor Rb = 1 MΩ. The SQUID is shunted to the ground with Rs = 10 Ω. We send a microwave tone to the device and also detect photon emission. The scale bar in the main figure is 1 μm, and the scale bar in the zoom-in is 300 nm. c Individual components I1 (blue) and I2 (orange) and total current (green) flowing through a symmetric SQUID as a function of the phase drop φ1 at Φext = Φ0/2. The current phase relation of both junctions is plotted using a single channel short diffusive model with an effective transparency τ* = 0.86. The current is normalized to units of the critical current Ic. The schematic of the SQUID helps visualize the requirements for a \(\sin (2\varphi )\) junction: a dominant 4e supercurrent is obtained with a symmetric SQUID biased at Φ0/2.
