NMR study on a 2H-NbSe2single crystal: A microscopic investigation of the charge density waves state

Abstract

Presents a pulsed NMR study on a single crystal of 2H-NbSe₂ at atmospheric pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2-273K. Analysis of Knight shift (KS) and nuclear spin-lattice relaxation (NSLR) time data in the normal state are consistent with N(E_F)=2.4 states/eV spin and an admixture of 40% of d_xy and d_x2-y2 wavefunctions in the d_z2 band. The product T₁T=500+or-100 ms K was found constant in the temperature range 4.2-77K and pressure independent; measurements at the 'magic angle' indicate an orbital contribution (T₁T)^-1 approximately=0.9+or-0.9 (sK)^-1. The results are consistent with the onset of incommensurate charge density waves (ICDW) at T₀=33K at P=1 bar and T₀=26K at 21 kbar. Below T₀, the lineshapes of the m to m-1 transitions agree with a local distribution of KS and EFG respecting the symmetry of a triple ICDW with a maximum value of 9% for the redistribution of conduction electrons within an atomic cell. Above T₀, nuclear spin-spin relaxation measurements demonstrate that the observed pretransitional broadening of the lines is due to static CDW fluctuations, induced by impurities.