Low-temperature resistance minimum in non-superconducting 3R-Nb1+xS2 and 3R-GaxNbS2

We report the structural and electron transport properties of 3R-Nb1+xS2 (x greater than or equal to 0.07) and 3R-GaxNbS2 (0.1 less than or equal to x less than or equal to 0.33) prepared as polycrystalline pellets as well as single crystals grown by vapour transport. We observe a resistance minimum in these compounds between 20 and 60 K, with T-min proportional to x. The resistance scales as rho/rho (min) (T/T-min) in the range 0.2 < T/T-min < 2 for different phases with x less than or equal to 0.25 whose resistivity differs by an order of magnitude. Powder x-ray diffraction also shows progressively increasing intensity of superlattice lines with cation concentration. The thermopower changes sign around the resistance minimum. The explanation of the resistance minimum and the simultaneous rapid suppression of superconductivity is sought in electron-electron scattering effects in the presence of cation disorder in these narrow-band anisotropic materials.