Strong Correlation Drives Zero-Field Josephson Diode Effect

TL;DR

This paper demonstrates that strong electron-electron interactions in a Josephson junction can spontaneously break symmetries and induce a zero-field Josephson diode effect, with controllability enhanced by a small Zeeman field.

Contribution

It reveals that strong correlations alone can cause zero-field JDE through spontaneous symmetry breaking, independent of magnetic order or spin-orbit coupling.

Findings

Strong correlations induce spontaneous symmetry breaking leading to zero-field JDE.

Applying a tiny Zeeman field enhances JDE efficiency and causes a level-crossing transition.

Spin-orbit coupling breaks SU(2) symmetry but does not determine diode polarity.

Abstract

The supercurrent diode effect (SDE), characterized by unequal critical currents in opposite directions, has been observed with or without magnetic fields, yet mechanisms enabling zero-field SDE without explicit symmetry breaking remain underexplored. Here we investigate a Josephson junction with strong electron-electron interaction modeled by a Hubbard $U$ term and an odd number of electrons. We find that strong correlations induce spontaneous breaking of time-reversal and mirror symmetries, forming a $\varphi$-junction with degenerate energy minima at $\pm\varphi$, resulting in zero-field Josephson diode effect (JDE) without magnetic order. Spin-orbit coupling breaks SU(2) symmetry but does not determine diode polarity, contrasting with magneto-chiral mechanisms. We further show that applying a tiny Zeeman field enables controllable JDE with sizable efficiency due to the enhancement by the strong magnetic correlation, and the JDE strength peaks when the field induces a level-crossing transition. These findings establish strong electron correlation as a distinct mechanism for nonreciprocal superconducting transport, broadening the understanding of SDE origins.