Two-fold anisotropic superconducting properties of few-layer NbSe₂

Transition metal dichalcogenides (TMDs) are a class of layered van der Waals materials that have recently attracted considerable interest due to a wide range of properties in the two-dimensional limit, including superconductivity, topological phases, and spin-valley-locked bands due to strong orbit coupling and inversion-symmetry breaking. Here, we present transport properties of few-layer niobium diselenide (NbSe₂), a TMD that exhibits signatures of an unusual Ising superconducting state as its thickness approaches the 2D limit. We perform magneto-transport experiments on high-quality few-layer NbSe₂ samples, which are fully encapsulated by hexagonal boron nitride in order to protect the NbSe₂ from the deleterious effects of exposure to oxygen and moisture. We find that, despite the three-fold symmetry of the lattice, the magneto-transport exhibits a two-fold in-plane anisotropy for in-plane magnetic fields up to 8 Tesla. This anisotropy is restricted to the temperature region between the T_c onset and T_c offset, and disappears in the normal state. We propose a phenomenological model to explain this unusual observation, discussing its implications to the elucidation of Ising superconductivity.