Side-gate modulation of supercurrent in InSb nanoflag-based Josephson junctions

TL;DR

This work demonstrates that lateral side-gates can effectively control both dissipative transport and superconducting transport in InSb nanoflag-based Josephson junctions, with a measured side-gate modulation efficiency of about $1/30$ relative to the back-gate. Side-gate bias not only modulates the supercurrent amplitude but also reshapes the Fraunhofer interference pattern by altering the effective current distribution and junction area. At high magnetic fields, clear Landau level–driven conductance features with an extracted effective g-factor of $g^*\approx 44$ confirm 2D transport and strong spin-orbit coupling in the platform. Overall, side-gates provide an additional, tunable electrostatic knob for designing Josephson devices and exploring spin-orbit–related phenomena, including potential tuning of the Josephson diode effect.

Abstract

InSb nanoflags, due to their intrinsic spin-orbit interactions, are an interesting platform in the study of planar Josephson junctions. Ballistic transport, combined with high transparency of the superconductor/semiconductor interfaces, was reported to lead to interesting phenomena such as the Josephson diode effect. The versatility offered by the planar geometry can be exploited to manipulate both carrier concentration and spin-orbit strength by electrical means. Here we present experimental results on InSb nanoflag-based Josephson junctions fabricated with side-gates placed in close proximity to the junction. We show that side-gates can efficiently modulate the current through the junction, both in the dissipative and in the dissipation-less regimes, similarly to what obtained with a conventional back-gate. Furthermore, the side-gates can be used to influence the Fraunhofer interference pattern induced by the presence of an external out-of-plane magnetic field.

Figures (5)

• Figure 1: Device F6 and Device F7 from sample SC20. The SEM images have been color-coded: InSb orange, Nb blue, and side-gates green.
• Figure 2: Back-gate and side-gate modulation of the conductance. (a) Source-drain conductance as a function of back-gate voltage. The arrows indicate the sweep direction. The three red dots indicate the back-gate values for which the data in (c-e) were taken. (b) Comparison between the conductance variation with respect to the back-gate voltage (left axis) and the side-gate voltage (right axis). The two quantities have analogous structures but they differ in magnitude by a factor $30$. (c-e) Conductance versus side-gate voltage for $V_{BG}$$=$$5$ V, $15$ V, and $30$ V, respectively. Grey points represent the experimental data, while the red lines are linear fitting curves. $T$$=$$4.2$ K. $B$$=$$0$. Device SC20-F6.
• Figure 3: Conductance pinch-off versus side-gate voltage. Six traces are reported, for different values of $V_{BG}$ between $9.1$ V and $9.35$ V. The conductance drops for $V_{SG}$ below a threshold voltage, which depends monotonically on $V_{BG}$. $T$$=$$250$ mK. $B$$=$$0$. Device SC20-F6.
• Figure 4: (a-c) Interference pattern of the critical current in presence of a perpendicular magnetic field. (a) Voltage drop and (b) differential resistance, obtained by numerical differentiation, are reported as a function of perpendicular magnetic field ($x$-axis) and bias current ($y$-axis). The critical current is indicated by orange dots. (c) Comparison between the extracted values of critical current $I_c$ and the models in Eq. (\ref{['eq:fit-1-F']}) (purple) and Eq. (\ref{['eq:fit-2-F']}) (light blue). A robust superconducting region is present, centered at $B$$=$$-6$ mT, due to a residual magnetization of the cryostat (see text). The first side lobes weakly appear in the experimental data. For the measurements shown in (a-c), the side-gates were grounded. (d-f) Fraunhofer pattern for different values of the side-gate voltage $V_{SG}$. (d) $V_{SG} = 10$ V, (e) $V_{SG} = 0$ V, and (f) $V_{SG} = -10$ V. The two-dimensional maps show the differential resistance versus perpendicular magnetic field and bias current. The orange dots represent the experimental values of $I_c$, while the purple lines are the best-fit curves based on the model in Eq. (\ref{['eq:fit-1-F']}). (a-c) $V_{BG} = 40$ V, and (d-f) $V_{BG} = 16$ V. $T = 250$ mK. Device SC20-F6.
• Figure 5: Conductance modulation as a function of perpendicular magnetic field and back-gate voltage. (a) Conductance as a function of $B$ ($x$-axis) and $V_{BG}$ ($y$-axis). A kink is present at $B$$=$$3.15$ T, indicated by the black arrow, due to the transition of the contacts (see text). (b) Conductance variation with respect to $V_{BG}$, versus $B$ ($x$-axis) and $V_{BG}$ ($y$-axis). The white dots represent the local minima of the traces. Two families of minima follow the light-blue lines for $B$$\leq$$7$ T, and subsequently spit up in 4 total traces, indicated by the red lines. The arrows denote the line separation at $B$$=$$10$ T. $T$$=$$250$ mK. Device SC20-F7.