Impurity Effects on Superconductivity and Charge Density Waves in NbSe₃

Abstract

The superconducting transition temperature T_c and the sizes of the resistive anomalies due to both the q₁- and the q₂-CDW formations are examined on single crystals of Ta-doped NbSe₃. With addition of Ta, T_c increases steeply from below 70 mK to 1.7 K but turns to decrease at 5 at%Ta. The sizes of the resistive anomalies due to both CDW formations initially decrease but that due to the q₁-CDW begins to increase at 5 at% where T_c turns to decrease. These results are discussed in terms of the competition between superconductivity and the CDW formations. The upper critical field is also measured in the superconducting state induced by Ta doping. The critical field shows one-dimensional anisotropy, which originates from the anisotropy of the electronic band structure.