Symmetry-broken Josephson junctions and superconducting diodes in magic-angle twisted bilayer graphene

Abstract

The coexistence of gate-tunable superconducting, magnetic and topological orders in magic-angle twisted bilayer graphene provides opportunities for the creation of hybrid Josephson junctions. Here we report the fabrication of gate-defined symmetry-broken Josephson junctions in magic-angle twisted bilayer graphene, where the weak link is gate-tuned close to the correlated insulator state with a moiré filling factor of υ = −2. We observe a phase-shifted and asymmetric Fraunhofer pattern with a pronounced magnetic hysteresis. Our theoretical calculations of the junction weak link—with valley polarization and orbital magnetization—explain most of these unconventional features. The effects persist up to the critical temperature of 3.5 K, with magnetic hysteresis observed below 800 mK. We show how the combination of magnetization and its current-induced magnetization switching allows us to realise a programmable zero-field superconducting diode. Our results represent a major advance towards the creation of future superconducting quantum electronic devices.

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

“…This interaction also mediates the pairing interaction in Bardeen-Cooper-Schrieffer superconductors. Understanding the electron-phonon coupling could give important insights into the origin of superconductivity in MATBG ( 25 , 60 ), with direct implications for superconducting qubits and circuits ( 13 – 15 ). For metals, electron-electron Umklapp scattering gives rise to finite electrical resistance at low temperatures.…”

“…In transition metal dichalcogenides, correlated insulating ( 7 , 8 ) and ferromagnetic states ( 9 ) are observed over a broad range of angles, with moiré excitons ( 10 , 11 ) providing a testbed for exploring Hubbard model physics and quantum computation ( 8 , 12 ). The technological implications of moiré materials are substantial, with applications envisaged in superconducting circuits ( 13 – 15 ), energy harvesting ( 16 ), nonlinear optics ( 17 , 18 ), and optical sensing ( 19 – 21 ).…”

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

“…Based on the observed need of both electron and hole pockets for superconductivity, the authors propose that an interband pairing interaction involving nearly nested electron and hole Fermi pockets could be responsible for the polarization-dependent superconductivity. Electrical control of superconductivity has also been demonstrated in magic-angle twisted bilayer graphene (tBLG) sandwiched between aligned BN layers, where similar switching off and on of the superconducting state due to an external field is observed, showing great promise for control of electronic states in “magic-angle” tBLG based Josephson junction configurations. − Although a ferroelectric polarization of the magic-angle tBLG intercalated BN may be important for the observed bistability, − , the origin of the gate hysteresis and the pairing mechanism coupling the superconductivity and ferroelectric switching remains unclear, requiring further investigations, and may likely be completely different from the mechanism proposed for bilayer T d -MoTe 2 . Ferroelectric switching has previously been observed in bilayer graphene sandwiched between BN layers, where the origin of the ferroelectricity is debated, possibly combining sliding ferroelectricity across layers and interlayer charge transfer driven by electron correlations. − , …”

Phase diagrams of the diode effect in superconducting heterostructures

“…The inversion symmetry can be broken by introducing the spin-orbit coupling. Experimentally it can be achieved in hybrid structures by proximity to a three-dimensional topological insulator (TI), in superconductors with Rashba spin-orbit coupling (like in polar SrTiO 3 films [40], few-layer MoTe 2 in the T d phase or MoS 2 [6,41,42], and twisted bilayer graphene [43,44]), or, in some cases, by the asymmetry of the device geometry. On the other hand, the time-reversal symmetry can be broken by the in-plane magnetic field, or alternatively in hybrid structures by proximity to a ferromagnetic insulator with the in-plane exchange field.…”