[Paper Review] Josephson Diode Effect in High Mobility InSb Nanoflags

Bianca Turini, Sedighe Salimian+7 more|arXiv (Cornell University)|Jul 18, 2022

Quantum and electron transport phenomena4 citations

TL;DR

This study demonstrates the Josephson diode effect (JDE) in single ballistic InSb nanoflag Josephson junctions, where in-plane magnetic fields break time-reversal and inversion symmetries, inducing non-reciprocal supercurrent flow due to Rashba spin-orbit coupling. The key result is a tunable, dissipationless rectification of supercurrent, maximized at perpendicular field orientation and suppressed by temperature, establishing InSb as a promising platform for low-dissipation superconducting electronics.

ABSTRACT

We report evidence of non-reciprocal dissipation-less transport in single ballistic InSb nanoflag Josephson junctions, owing to a strong spin-orbit coupling. Applying an in-plane magnetic field, we observe an inequality in supercurrent for the two opposite current propagation directions. This demonstrates that these devices can work as Josephson diodes, with dissipation-less current flowing in only one direction. For small fields, the supercurrent asymmetry increases linearly with the external field, then it saturates as the Zeeman energy becomes relevant, before it finally decreases to zero at higher fields. We show that the effect is maximum when the in-plane field is perpendicular to the current vector, which identifies Rashba spin-orbit coupling as the main symmetry-breaking mechanism. While a variation in carrier concentration in these high-quality InSb nanoflags does not significantly influence the diode effect, it is instead strongly suppressed by an increase in temperature. Our experimental findings are consistent with a model for ballistic short junctions and show that the diode effect is intrinsic to this material. Our results establish InSb Josephson diodes as a useful element in superconducting electronics.

Motivation & Objective

• To demonstrate intrinsic non-reciprocal supercurrent transport in single planar Josephson junctions based on high-quality InSb nanoflags.
• To identify the role of Rashba spin-orbit coupling as the primary symmetry-breaking mechanism for the Josephson diode effect.
• To investigate the dependence of the diode effect on in-plane magnetic field orientation, strength, and temperature.
• To establish the intrinsic nature of the effect by ruling out extrinsic influences such as carrier concentration variations.
• To provide experimental validation of the Josephson diode effect in a ballistic, short-junction regime, consistent with theoretical models.

Proposed method

• Low-temperature magneto-transport measurements were performed on single ballistic InSb nanoflag Josephson junctions with superconducting contacts.
• The in-plane magnetic field was applied at variable angles relative to the current direction to probe angular dependence of the supercurrent asymmetry.
• Switching current measurements were used to extract the rectification coefficient η = ΔI_sw / B_ip, with ΔI_sw = |I_sw^+ - I_sw^-|.
• The proportionality factor m = η / B_ip was fitted as a function of sin(θ), where θ is the angle between the current and the in-plane field.
• Temperature-dependent measurements were conducted from 30 mK to 200 mK to assess thermal suppression of the diode effect.
• Theoretical modeling based on ballistic SNS junctions with anomalous phase shifts was used to interpret the field and temperature dependence of the supercurrent asymmetry.

Experimental results

Research questions

• RQ1 What is the origin of non-reciprocal supercurrent transport in high-mobility InSb nanoflags under in-plane magnetic fields?
• RQ2 How does the Josephson diode effect depend on the orientation of the in-plane magnetic field relative to the current direction?
• RQ3 To what extent is the diode effect suppressed by increasing temperature, and why?
• RQ4 Is the effect intrinsic to the material, or influenced by extrinsic factors such as carrier concentration?
• RQ5 Can the observed supercurrent asymmetry be explained by a model based on Rashba spin-orbit coupling and anomalous phase shifts in short junctions?

Key findings

• The Josephson diode effect is observed in single planar InSb nanoflag Josephson junctions, with non-reciprocal supercurrent flow induced by in-plane magnetic fields.
• The supercurrent asymmetry increases linearly with the in-plane field up to ~58 mT, then saturates and eventually decreases to zero at higher fields.
• The maximum rectification occurs when the in-plane magnetic field is perpendicular to the current direction, confirming Rashba spin-orbit coupling as the dominant symmetry-breaking mechanism.
• The rectification coefficient α = -2.9 ± 0.2 T⁻¹ is extracted from linear fits of the asymmetry vs. B_ip, with a characteristic field B₀ = 345 mT.
• The diode effect is strongly suppressed with increasing temperature, with |α| decreasing significantly from 30 mK to 200 mK, due to the faster decay of higher harmonics in the current-phase relation.
• The effect is robust against variations in carrier concentration but vanishes at high temperatures, indicating its dependence on quantum coherence and higher harmonic contributions in the current-phase relation.