Evidence for orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2

Abstract

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an unusual superconducting phase that survives beyond the Pauli paramagnetic limit through spatial modulation of the order parameter. An even more exotic variant-the orbital FFLO state-was recently reported in thin flakes of 2H-NbSe2, involving the interplay of Ising spin-orbit coupling and orbital pair breaking. Here, we report thermodynamic signatures consistent with an orbital FFLO state in bulk 2H-NbSe2, based on high-resolution magnetization and torque measurements under strictly parallel to the NbSe2 basal plane. In the magnetic phase diagram, a crossover to a first-order transition appears above 3 T and disappears with slight field misalignment, indicating field-angle dependent Pauli-limited behavior. Additionally, we observe a reversible step-like anomaly within the superconducting state, and a pronounced six-fold in-plane modulation of the upper critical field above this phase transition. These results suggest that the orbital FFLO state is likely realized even in the bulk limit of 2H-NbSe2.