Magnetic Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene

TL;DR

This paper demonstrates gate-tunable magnetic Josephson junctions in magic angle twisted bilayer graphene, revealing unconventional Fraunhofer patterns and enabling a programmable superconducting diode, advancing quantum electronic device development.

Contribution

It reports the creation of gate-defined magnetic Josephson junctions in MATBG with unconventional effects explained by valley polarization and orbital magnetization, introducing a new platform for superconducting electronics.

Findings

Unconventional, phase-shifted Fraunhofer patterns observed.

Magnetic hysteresis persists up to 3.5K, with pronounced effects below 800mK.

Realization of a programmable zero-field superconducting diode.

Abstract

The simultaneous co-existence and gate-tuneability of the superconducting (SC), magnetic and topological orders in magic angle twisted bilayer graphene (MATBG) open up entirely new possibilities for the creation of complex hybrid Josephson junctions (JJ). Here we report on the creation of gate-defined, magnetic Josephson junctions in MATBG, where the weak link is gate-tuned close to the correlated state at a moir\'e filling factor of {\nu}=-2. A highly unconventional Fraunhofer pattern emerges, which is phase-shifted and asymmetric with respect to the current and magnetic field directions, and shows a pronounced magnetic hysteresis. Interestingly, our theoretical calculations of the JJ with a valley polarized {\nu}=-2 with orbital magnetization as the weak link explain most of these unconventional features without fine tuning the parameters. While these unconventional Josephson effects persist up to the critical temperature Tc ~ 3.5K of the superconducting state, at temperatures below T < 800mK, we observed a pronounced magnetic hysteresis possibly due to further spin-polarization of the {\nu}=-2 state. We demonstrate how the combination of magnetization and its current induced magnetization switching in the MATBG JJ allows us to realize a programmable zero field superconducting diode, which represents a major building block for a new generation of superconducting quantum electronics.