Enhanced superconducting diode effect in hybrid Josephson junctions

TL;DR

The paper investigates enhancement of the superconducting diode effect (SDE) in planar Josephson junctions fabricated from InAs heterostructures with epitaxial Al by patterning periodic hole arrays on the superconducting leads. Using a double-gate architecture (junction gate $JG$ and top gate $TG$) to electrostatically deplete the hole regions, they observe an enhanced SDE at low in-plane field $B_y$ while the overall supercurrent remains largely intact. The enhancement cannot be explained solely by changes in chemical potential or spin-orbit coupling; numerical simulations indicate it arises from an increased difference in transparency between different Andreev bound-state channels, captured by a two-channel model with parameters $ au_1$, $ au_2= au_1+ abla au$, and $ abla heta$. A simple expansion of the two-channel model yields $\\eta \\propto \\nabla au \\sin( abla heta/2)$, and the observed nonmonotonic dependence of the diode efficiency on the hole-region gate $\\mu_h$ is reproduced by the model and supported by a 1D Andreev-reflection argument, indicating a novel mechanism for SDE control in hybrid JJs.

Abstract

The superconducting diode effect (SDE) has recently been observed in various systems, sparking interest in novel superconducting devices and offering a new platform to probe intrinsic material properties. Josephson junctions with strong Rashba spin-orbit coupling have exhibited nonreciprocal critical currents under applied magnetic fields. In this work, we investigate the SDE in Josephson junctions incorporating periodic hole arrays patterned into the superconducting leads on InAs heterostructures with epitaxial aluminum. We observe an enhanced diode effect when a top gate depletes the 2DEG in the region of the hole arrays, while preserving the overall supercurrent. Theoretical analysis shows that the physics behind this phenomenon is the increased difference of transparency between different bands in the junction. These results highlight a new pathway for engineering and controlling nonreciprocal superconducting transport in hybrid systems.

Figures (5)

• Figure 1: (a) False-color scanning electron micrograph of the measured device and schematic of the device and the material stacks. (b) Differential resistance as a function of the bias current and $B_z$. (c) Differential resistance as a function of the bias current and TG voltages at $B_z$ = 0 and $B_y$ = 50 mT. (d) Differential resistance as a function of the bias current and JG voltages at $B_z$ = 0 and $B_y$ = 50 mT. (e) Diode effect efficiency $\eta$ as a function of JG voltages. (f) Diode effect efficiency $\eta$ as a function of $B_y$.
• Figure 2: (a) The diode effect efficiency $\eta$ as a function of $B_y$ for different TG voltages at JG = 0. (b) Differential resistance as a function of the bias current and TG voltages at $B_y$ = 40 mT. (c) Differential resistance as a function of the bias current and TG voltages at $B_y$ = 110 mT. (d) Absolute values of the critical currents as a function of $B_y$ for JG = 0 and TG = -0.6 V. (e) Absolute values of the critical currents as a function of $B_y$ for JG = 0 and TG = -3 V. (f) The diode effect efficiency $\eta$ as a function of TG voltages for different $B_y$ at JG = 0.
• Figure 3: (a) Schematic of the punched hole setup in the simulations. The orange strip denotes the normal region; the blue region represents the superconducting area and the green squares represent the depleted holes. (b) Diode factor $\eta$ as a function of the normal region chemical potential $\mu$ at $E_Z=1$ meV. (c) Diode effect as a function of the Zeeman term. (d) Diode effect vs the chemical potential $\mu_h$ in the holes, denoted by the red circles. The fitted $\eta$ using the two-channel model is shown by the red triangles and $\Delta\tau=\tau_2-\tau_1$ is denoted by the blue circles.
• Figure 4: Spectrum at (a) $N_1$ point, (b) $N_2$, and (c) $N_3$. Corresponding CPR (dots) and two-channel fitting (line) is presented in (d)-(f). In the fitting, $\theta_1=0.77, \Delta\theta=-1.5,\tau_1=0.975$ are fixed, while $I_1= 0.376,0.386,0.385$, and $\Delta\tau=-0.013,-0.023,-0.020$ in (d)-(f). Comparison of CPR at $\phi$ (blue) and $-\phi$ (red) where $\phi>0$, for (g) $N_1$ only, (h) $N_1$ (dashed lines), $N_2$ (dash-dotted lines), and $N_3$ (solid lines). (i). The difference of CPR at $\phi$ and $-\phi$ for $N_1$ (dashed lines), $N_2$ (dash-dotted lines), and $N_3$ (solid lines). (j). Energy spectrum of the two effective modes (red for the first Andreev bound state and blue for the second) used in the fitting for $N_1$ (dashed lines), $N_2$ (dash-dotted lines), and $N_3$ (solid lines). (k). Energy difference between $N_1$ and $N_2$ (dash-dotted line) and between $N_1$ and $N_3$ (solid line), of the first mode (red) and the second mode (blue). (l). Difference between $N_1$ and $N_2$ (dash-dotted line) or $N_3$ (solid line), of the energy sum from the two modes.
• Figure 5: (a) Schematic of a simplified 1D-model for calculating the reflection. The color code is the same as Fig. \ref{['fig:hole']}. The green region represents the depleted area without superconductivity, the orange area represents the semi-infinite normal region, and the blue denotes the superconducting area. The two separate superconducting areas have the same phase, and the left one is semi-infinite. (b) The difference in Andreev reflection coefficients between two channels differing only by their Fermi momentum. $r_1, r_2$ are the Andreev reflection coefficients of each channel, respectively.