Orbital Fulde-Ferrell pairing state in moir\'e Ising superconductors

TL;DR

This paper predicts an orbital Fulde-Ferrell superconducting state in moiré Ising superconductors under large in-plane magnetic fields, revealing a novel finite-momentum pairing mechanism driven by orbital effects rather than Zeeman splitting.

Contribution

It introduces the concept of an orbital FF state in moiré Ising superconductors, highlighting its distinct origin and potential for giant diode effects, expanding understanding of unconventional superconductivity.

Findings

Orbital FF state appears when in-plane field exceeds conventional orbital $B_{c2}$.

Giant superconducting diode effect can be induced by electric gating.

Upturn of $B_{c2}$ with decreasing temperature signals the orbital FF state.

Abstract

In this work, we study superconducting moir\'e homobilayer transition metal dichalcogenides where the Ising spin-orbit coupling (SOC) is much larger than the moir\'e bandwidth. We call such noncentrosymmetric superconductors, moir\'e Ising superconductors. Due to the large Ising SOC, the depairing effect caused by the Zeeman field is negligible and the in-plane upper critical field ($B_{c2}$) is determined by the orbital effects. This allows us to study the effect of large orbital fields. Interestingly, when the applied in-plane field is larger than the conventional orbital $B_{c2}$, a finite-momentum pairing phase would appear which we call the orbital Fulde-Ferrell (FF) state. In this state, the Cooper pairs acquire a net momentum of $2q_B$ where $2q_B=eBd$ is the momentum shift caused by the magnetic field $B$ and $d$ denotes the layer separation. This orbital field-driven FF state is different from the conventional FF state driven by Zeeman effects in Rashba superconductors. Remarkably, we predict that the FF pairing would result in a giant superconducting diode effect under electric gating when layer asymmetry is induced. An upturn of the $B_{c2}$ as the temperature is lowered, coupled with the giant superconducting diode effect, would allow the detection of the orbital FF state.