NMR studies of CDW and superconducting gap structure in layered 2H-NbSe2

Single crystals of layered transition metal dichalcogenide 2H-NbSe₂ were probed to investigate the charge density wave CDW (T_CDW = 33.5 K) and superconducting states (T_c = 7.2 K) using ⁹³Nb and ⁷⁷Se nuclear magnetic resonance (NMR). The low temperature quadrupolar ⁹³Nb above H_c2 shows two broadened Gaussians of unequal amplitude per transition, which is believed to be evidence of a discommensurate CDW phase. In orientations parallel and perpendicular to the applied field H₀, the spin-lattice relaxation (T₁) data of both nuclei reveal Korringa behavior above T_c, an absence of coherence peak, and a linear crossover for T < T_c. In both orientations, 1/T₁ is accurately fit by a two-gap function for relaxation. A field-independent suppression in 1/T₁ was also observed in both orientations, with deviations from Korringa behavior occurring above T_c and far above H_c2, implying the presence of a pseudogap. The field-dependence of T₁ in the SC state shows 1/T₁ ∝ H₀. This behavior most likely originates from the Volovik effect, which suggests 2H-NbSe₂ is a two-gap s-wave superconductor.