Evidence for pseudo-gap behavior in defect-doped infinite layer (Ca, Sr)CuO₂ thin films

A distinct crossover between the high temperature and low temperature normal state dc transport properties of epitaxial (Ca, Sr)CuO₂ single crystal thin films has been observed. Defect-doped (Ca, Sr)CuO₂ films with dc conductivity behavior ranging from insulating to metallic-like were obtained by growing stoichiometric and cation-deficient material under various film growth conditions. The results indicate that 2D variable-range hopping is the dominant transport mechanism at low temperatures. The high temperature behavior depends on the magnitude of the conductivity, resulting in either activated conduction or a ln ρ ∼ 1/T^1/2 dependence that is reminiscent of thermally-activated tunneling in a granular metal/insulator matrix. The distinct crossover in behavior suggests the possibility of gap formation in the temperature range 150-200 K.