Friday 03 October 2025
Orbital Fulde–Ferrell–Larkin–Ovchinnikov State and Associated Symmetry Transitions
Puhua Wan (University of Geneva)

In the Bardeen– Cooper–Schrieffer (BCS) theory, Cooper pairs are formed with zero total momentum, preserving both time and inversion symmetries. A finite momentum Cooper pairing can be induced by the Zeeman effect of magnetic fields, forming the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, characterized by a periodic spatial variation of the superconducting order parameter.

In layered superconducting transition metal dichalcogenides, such as NbSe₂, the Ising-type spin-orbit coupling (SOC) strongly suppresses the Zeeman effect of external fields, thus preventing the conventional approach to the FFLO state. Nevertheless, the orbital effect of the magnetic field can shift the Fermi surfaces of layers, leading to a transition to an orbital FFLO state.

In this talk, we present evidence for orbital FFLO in a multilayer Ising superconductor NbSe₂, and discuss its fine structures. Through transport measurements, we reveal the accompanying symmetry transitions across the Ising superconductor-orbital FF-orbital LO phase boundaries.