Spectroscopic evidence for a first-order transition to the orbital Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This study provides spectroscopic evidence for a first-order phase transition to an orbital FFLO state in multilayer NbSe2, revealing a sudden superconducting gap enhancement and hysteresis, and establishing the phase diagram with theoretical support.

Contribution

First direct spectroscopic demonstration of a first-order transition to the orbital FFLO state in spin-orbit coupled superconductors, supported by phase diagram and vortex rearrangement analysis.

Findings

Observation of a sudden superconducting gap increase at specific magnetic fields.

Detection of hysteresis indicating a first-order phase transition.

Agreement between experimental phase diagram and theoretical calculations.

Abstract

A conventional superconducting state may be replaced by another dissipationless state hosting Cooper pairs with a finite momentum, leaving thermodynamic footprints for such a phase transition. Recently, a novel type of finite momentum pairing, so-called orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, has been proposed to occur in spin-orbit coupled superconductors such as bilayer $2\mathrm{H-NbSe_{2}}$. So far, a thermodynamic demonstration, which is key for establishing this exotic phase, has been lacking. Here, we reveal a first-order quantum phase transition to the orbital FFLO state in tunneling spectroscopic measurements on multilayer $2\mathrm{H-NbSe_{2}}$. The phase transition manifests itself as a sudden enhancement of the superconducting gap at an in-plane magnetic field $B_{//}$ well below the upper critical field. Furthermore, this transition shows prominent hysteresis by sweeping $B_{//}$ back and forth and quickly disappears once the magnetic field is tilted away from the sample plane by less than one degree. We obtain a comprehensive phase diagram for the orbital FFLO state and compare it with the theoretical calculation that takes into account the rearrangement of Josephson vortices. Our work elucidates the microscopic mechanism for the emergence of the orbital FFLO state.