NMR Investigation of the Layered Superconductor NbSe2

This dissertation details the use of 93Nb (γ = 10.405 MHz/T, I = 9/2) and 77Se (γ = 8.13 MHz/T, I = 1/2) nuclear magnetic resonance (NMR) to investigate the phase transitions of the layered transition metal dichalcogenide (TMD) niobium diselenide (2H-NbSe2). 2H-NbSe2 has a trigonal prismatic structure and exhibits a charge density wave (CDW) transition at TCDW = 33.5 K and a superconducting (SC) transition with Tc = 7.2 K. The present experiments were undertaken with the external field (H0) first parallel, and then perpendicular to the crystallographic axis (c-axis). Single crystals of 2H-NbSe2 were probed in the temperature range of 0.35 K – 100 K and at frequencies and fields ranging from 19 MHz – 135 MHz and 1.8 T – 17.5 T respectively to investigate the normal, CDW and superconducting states. A value of Tc = 7.04 K was measured in situ by cooling the sample below Tc and measuring detuning. A value of Tc = 6.2 K was measured in a sample grown from the same batch using a SQUID magnetometer. 93Nb NMR spectral line shape and Knight shift were used to detect the CDW phase. The full 93Nb spectrum was field-swept and observed to possess line broadening and asymmetry which gain intensity the further the respective transition is from the central line. Pre-transitional broadening was detected in both orientations, beginning at a temperature of 60 K for the central transition and as high as 80 K for the first lower-frequency satellite transition. The broadening continues below TCDW and ends below 15 K. These results are believed to be evidence of a discommensurate CDW phase. Spin-lattice relaxation rate (T1) measurements are used to directly probe the electronic density of states (DOS) and investigate the SC gap. The T1 data of both nuclei reveal Korringa behavior above Tc, no visible Hebel-Slichter peak just below Tc, and a linear crossover to further Korringa behavior for T << Tc in both orientations. Both orientations were accurately fit to a two-gap function and 1/T1T in the H0⊥c direction displays a two-step transition, suggesting two superconducting gaps are present. A field-dependence of T1 in the SC state was detected in both sample orientations. In both the H0∥c and H0⊥c directions, 〖1/T〗_1∝H due to the Zeeman contribution of the Volovik effect. The Doppler effect contribution from the vortex supercurrents was absent from the relaxation, which suggests 2H-NbSe2 is a two-gap s-wave superconductor.