NMR Study of the Peierls Transition and the Superconductivity in Quasi One-Dimensional Metallic Compounds: Nb_<1-x>Ta_xSe_3(LOCALIZATION AND SUPERCONDUCTIVITY IN ID CONDUCTORS, International Symposium on NONLINEAR TRANSPORT AND RELATED PHENOMENA IN INORGANIC QUASI ONE DIMENSIONAL CONDUCTORS)

Abstract

We describe a NMR study attempting to understand the microscopic mechanism of the antagonistic behavior between the CDW formations and the superconducting transition in Ta-doped NbSe_3, both competing for the electronic states near the Fermi surface. The static and dynamic properties of the conduction electrons are probed through their influences on the nuclear spin relaxation time T_1 of ^<93>Nb nuclei. From the characteristic temperature dependence of T_1 in the range 1.4-200K, it is shown that the two CDWs take place within the most conductive column at T_<C1> and within intermediately conductive at T_<C2> of the three inequivalent columns in the unit cell of NbSe_3. The Ta-doping in NbSe3 is found to reduce the density of states o_F for the two columns, resulting in the suppression of the CDW transition temperatures T_C'S. The low temperature conductivity and further the superconductivity occurrence when doped with Ta are analyzed to mainly originate from the remaining less conductive column. A ω^<1/2> frequency dependence of the experimental T_1 indicates the cohesive motion of conduction electrons to be one dimensional diffusive above the electronic Larmor frequency range for both of the pure and Ta-doped NbSe_3. The density of states at low temperatures is also diminished by ～45 % for an introduction of 5 at.% Ta, which conflicts with the increase of the superconducing transition temperature. It is suggested that the fluctuations of the superconductivity may play an important role for the appearance of the superconducting transition.