Josephson diode effects in twisted nodal superconductors

Abstract

Recent Josephson tunneling experiments on twisted flakes of high-Tc cuprate superconductor Bi2Sr2CaCu2O8+x revealed a nonreciprocal behavior of the critical interlayer Josephson current, i.e., a Josephson diode effect. Motivated by these findings we study theoretically the emergence of the Josephson diode effect in twisted interfaces between nodal superconductors, and highlight a strong dependence on the twist angle θ and damping of the junction. In all cases, the theory predicts diode efficiency that vanishes exactly at θ=45∘ and has a strong peak at a twist angle close to θ=45∘, consistent with experimental observations. Near 45∘, the junction breaks time-reversal symmetry T spontaneously. We find that for underdamped junctions showing hysteretic behavior, this results in a dynamical Josephson diode effect in a part of the T-broken phase. The direction of the diode is trainable in this case by sweeping the external current bias. This effect provides a sensitive probe of spontaneous T breaking. We then show that explicit T-breaking perturbations with the symmetry of a magnetic field perpendicular to the junction plane lead to a thermodynamic diode effect that survives even in the overdamped limit. We discuss an experimental protocol to probe the double-well structure in the Josephson free energy that underlies the tendency towards spontaneous T breaking even if T is broken explicitly. Finally, we show that in-plane magnetic fields can control the diode effect in the short junction limit, and predict the signatures of explicit T breaking in Shapiro steps.

Source-reported materials — not catalogue approval

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