An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe2
Time reversal and spatial inversion are two key symmetries for conventional Bardeen–Cooper–Schrieffer (BCS) superconductivity1. Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties1–5. Two-dimensional (2D) NbSe2 has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin–orbit coupling (SOC)6–9. Here we report the observation of an unusual continuous paramagnetic-limited superconductor–normal metal transition in 2D NbSe2. Using tunelling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin-film superconductors10–12. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe2. Our findings establish 2D NbSe2 as a promising platform to explore novel spin-dependent superconducting phenomena and device concepts1, such as equal-spin Andreev reflection13 and topological superconductivity14–16.Tunnelling spectroscopy reveals a continuous closing of the superconducting gap at low temperature and high in-plane magnetic field in few-layer NbSe2, due to the Ising spin–orbit coupling of these materials.