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

In the well-known Bardeen-Cooper-Schrieffer (BCS) theory, the total momentum of Cooper pairs is zero. However, Fulde and Ferrell, as well as Larkin and Ovchinnikov, purposed a finite momentum pairing mechanism due to time-reversal breaking caused by Zeeman effect. Recently, experimental results suggest that multilayer 2H-NbSe2 and gated bilayer MoS2 can host a new type of FFLO state, so-called the orbital FFLO state. However, the direct thermodynamic evidence for the orbital FFLO phase transition is still lacking. We construct the superconductor (multilayer NbSe2)-insulator (WS2)-metal (Graphite) tunneling junctions, and measure the corresponding tunneling spectra. We observe a sudden enhancement of the superconducting gap when increasing the in-plane magnetic field, occurring well below the upper critical field. This provides solid evidence for the first-order orbital FFLO phase transition. This sudden enhancement fades quickly when the magnetic field is tilted away from the sample plane. To strengthen our findings, we measure four multilayer NbSe2 samples with different thickness. Furthermore, we provide the microscopic mechanism for the emergence of the orbital FFLO state.