[Paper Review] A Josephson junction supercurrent diode

Christian Baumgärtner, Lorenz Fuchs+11 more|arXiv (Cornell University)|Mar 11, 2021

Physics of Superconductivity and Magnetism44 references6 citations

TL;DR

This paper demonstrates a supercurrent diode effect in fully superconducting Josephson junctions fabricated on InAs quantum wells, leveraging strong spin-orbit coupling and in-plane magnetic fields to break both inversion and time-reversal symmetries. The key result is the experimental observation of nonreciprocal supercurrent flow with a quantified supercurrent magnetochiral anisotropy coefficient $\gamma_L$, confirmed via Josephson inductance measurements and a semi-quantitative theoretical model.

ABSTRACT

Transport is called nonreciprocal when not only the sign, but also the absolute value of the current, depends on the polarity of the applied voltage. It requires simultaneously broken inversion and time-reversal symmetries, e.g., by the interplay of spin-orbit coupling and magnetic field. So far, observation of nonreciprocity was always tied to resistivity, and dissipationless nonreciprocal circuit elements were elusive. Here, we engineer fully superconducting nonreciprocal devices based on highly-transparent Josephson junctions fabricated on InAs quantum wells. We demonstrate supercurrent rectification far below the transition temperature. By measuring Josephson inductance, we can link nonreciprocal supercurrent to the asymmetry of the current-phase relation, and directly derive the supercurrent magnetochiral anisotropy coefficient for the first time. A semi-quantitative model well explains the main features of our experimental data. Nonreciprocal Josephson junctions have the potential to become for superconducting circuits what $pn$-junctions are for traditional electronics, opening the way to novel nondissipative circuit elements.

Motivation & Objective

• To realize a dissipationless, nonreciprocal supercurrent device in a fully superconducting platform.
• To demonstrate supercurrent rectification far below the superconducting transition temperature.
• To experimentally measure and quantify the supercurrent magnetochiral anisotropy coefficient $\gamma_L$.
• To establish a link between nonreciprocal supercurrent and asymmetry in the current-phase relation through Josephson inductance.
• To develop a semi-quantitative theoretical model that reproduces the observed nonreciprocal behavior.

Proposed method

• Fabrication of highly transparent Josephson junctions on InAs quantum wells with a 2250-island array geometry.
• Application of in-plane magnetic fields to break time-reversal and inversion symmetries via interplay with Rashba spin-orbit coupling.
• Measurement of Josephson inductance $L(I)$ as a function of current and magnetic field orientation to probe asymmetries in the current-phase relation.
• Expansion of $L(I)$ up to second order to extract the supercurrent magnetochiral anisotropy coefficient $\gamma_L$ via $-2L_0'/ (L_0 B_{\mathrm{ip}}) = \gamma_L \sin\theta$.
• Use of Kwant-based numerical simulations to model the junctions, including finite-size effects and a confining potential along $\hat{z}$.
• Comparison of experimental $L(I)$ curves with theoretical predictions to validate the origin of nonreciprocity.

Experimental results

Research questions

• RQ1 Can nonreciprocal supercurrent flow be achieved in a fully superconducting device without dissipation?
• RQ2 What is the magnitude and origin of the supercurrent magnetochiral anisotropy coefficient $\gamma_L$ in the superconducting state?
• RQ3 How does the asymmetry in the current-phase relation manifest in Josephson inductance measurements?
• RQ4 Why is the magnetochiral anisotropy in supercurrents comparable in magnitude to that in resistive transport near $T_c$, despite different energy scales?
• RQ5 What role does the 3D confinement potential play in stabilizing the observed nonreciprocal behavior?

Key findings

• The supercurrent diode effect is experimentally observed at temperatures far below $T_c$, demonstrating dissipationless nonreciprocal supercurrent flow.
• The supercurrent magnetochiral anisotropy coefficient $\gamma_L$ is directly extracted for the first time from Josephson inductance measurements.
• The Josephson inductance $L(I)$ exhibits clear asymmetry around zero current when the in-plane magnetic field is applied along $\hat{y}$, confirming nonreciprocal supercurrents.
• Theoretical modeling with a finite confinement potential $V_{\mathrm{conf}}$ is essential to reproduce the experimental $L(I)$ trends, including the decrease of $L_0$ with increasing $\theta$.
• The magnitude of $\gamma_L$ is found to be comparable to the resistive magnetochiral anisotropy $\gamma_S$ near $T_c$, suggesting a non-trivial interplay beyond spin-orbit coupling alone.
• The observed nonreciprocity is linked to a $\varphi_0$-shift in the current-phase relation induced by the interplay of Rashba spin-orbit coupling and in-plane magnetic field.