Orbital-driven Fulde–Ferrell–Larkin–Ovchinnikov state in a layer-locked 3D Ising superconductor

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, characterized by Cooper pairs with non-zero center of mass momentum, can emerge from the interplay between the Zeeman effect induced by an external magnetic field and spin–orbit coupling (SOC). Beyond these conventional mechanisms, the FFLO state driven by the orbital effect of the in-plane field has been revealed in the few-layer 2D transition metal dichalcogenides with strong Ising-type SOC. However, the clear and comprehensive identification of the unconventional FFLO state in 3D bulk materials remains a significant challenge. Here we report the experimental observation of the orbital FFLO state in a bulk superconductor 4Ha-NbSe2. Transport measurements show that with increasing in-plane magnetic field, the bulk 4Ha-NbSe2 undergoes a superconducting transition from an isotropic Ising superconductor to an orbital FFLO state with broken in-plane rotational symmetry. Remarkably, our angle-resolved photoemission spectroscopy measurements (ARPES) and first-principles calculations reveal that bulk 4Ha-NbSe2 hosts novel electronic structures arising from monolayer-like and bilayer-like structures, which allows us to determine the physical configuration of the two-fold symmetric orbital FFLO state. Combining with the phase diagram revealed by the nonreciprocal transport measurements, our observations provide the hallmark signatures of an orbital driven FFLO state in 3D Ising superconductors. Orbital-driven FFLO superconductivity has so far been established mainly in few-layer systems. Here, the authors observe an orbital FFLO state in bulk 4Ha-NbSe₂, with an in-plane field driving a transition from isotropic Ising superconductivity to a two-fold-symmetric paired state.