Superconductivity enhancement and CDW gap formation-NMR study of the pressure and impurity effects in NbSe3

Abstract

The authors present an experimental and theoretical study of the interplay of the superconductivity (SC) and the charge density wave (CDW) formations in the quasi-one-dimensional conductor NbSe3. The application of high pressure or the doping of impurities results in the coexistence of the SC and CDW and then raises the SC transition temperature TS while lowering two independent CDW transition temperatures TC1 and TC2. Under these perturbations, they have measured 93Nb relaxation rates in the normal CDW state (TS<T<TC2). In spite of the considerable rise in TS, they have found significant decreases in the density of states (DOS) at the Fermi level. This appears to conflict with the BCS prediction. They discussed the microscopic band structure of NbSe3 in the normal CDW state and then the origin of the Ts enhancement, but none of the existing theory can necessarily explain fully all the experimental results from both the application of pressure and the Ta doping. Finally they propose a possible mechanism of the Ts enhancement originating in the peaks of DOS near to the CDW gap edge within the BCS cut-off interval of the Debye energy, which might be the case for NbSe3.