Fulde-Ferrell-Larkin-Ovchinnikov state in spin-orbit-coupled superconductors

TL;DR

This paper investigates the emergence and characteristics of FFLO and drift-BCS superconducting states in spin-orbit-coupled Li₂Pd₃B under magnetic fields, revealing phase transitions, enhanced Pauli limits, and triplet correlations.

Contribution

It demonstrates the coexistence of drift-BCS and FFLO states in spin-orbit-coupled superconductors and details their distinct magnetic-field-dependent properties.

Findings

Identification of drift-BCS and FFLO states in Li₂Pd₃B.

Observation of first-order phase transition between states.

Enhanced Pauli limit due to spin-orbit coupling.

Abstract

We show that in the presence of magnetic field, two superconducting phases with the center-of-mass momentum of Cooper pair parallel to the magnetic field are induced in spin-orbit-coupled superconductor Li$_2$Pd$_3$B. Specifically, at small magnetic field, the center-of-mass momentum is induced due to the energy-spectrum distortion and no unpairing region with vanishing singlet correlation appears. We refer to this superconducting state as the drift-BCS state. By further increasing the magnetic field, the superconducting state falls into the Fulde-Ferrell-Larkin-Ovchinnikov state with the emergence of the unpairing regions. The observed abrupt enhancement of the center-of-mass momenta and suppression on the order parameters during the crossover indicate the first-order phase transition. Enhanced Pauli limit and hence enlarged magnetic-field regime of the Fulde-Ferrell-Larkin-Ovchinnikov state, due to the spin-flip terms of the spin-orbit coupling, are revealed. We also address the triplet correlations induced by the spin-orbit coupling, and show that the Cooper-pair spin polarizations, generated by the magnetic field and center-of-mass momentum with the triplet correlations, exhibit totally different magnetic-field dependences between the drift-BCS and Fulde-Ferrell-Larkin-Ovchinnikov states.