Continuous tuning of spin-orbit coupled superconductivity in layered NbSe₂

Ion intercalation has become a versatile and powerful tool to realize quantum phases. To fully explore the potential of lithium intercalation, we have successfully developed a lateral intercalation method via a solid state ion back-gate [1]. The substantially improved sample homogeneity is demonstrated in both low-temperature transport measurements and topographical imaging by in situ atomic force microscopy. We apply this technique to modulate superconductivity in NbSe₂ . In the few-layer case where Ising superconductivity prevails, lithium intercalation can further enhance the in-plane upper critical magnetic field by boosting the spin-splitting. In bulk-like NbSe₂, we observe a tentative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at intermediate lithium concentration and Ising pairing at higher doping. Our work helps provide a complete phase diagram of a spin-orbit coupled superconductor as a function of doping and layer separation.