Two-dimensional variable range hopping and superconducting transition temperatures in Y 1Ba 2Cu 3O 7−y films on MgO and impurity doped ceramics

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The electrical transport studies in semiconducting and superconducting Y 1Ba 2Cu 3O 7−y thin films on MgO and ceramics are made. Effects of the impurity doping on the electrical resistivity of the semiconducting phase (nominally 1% Ni, Zn, Mg, Al etc. for Cu) and the superconducting phase are examined in the ceramics. The resistivities in Zn, Mg, or Al doped semiconducting samples quenched from 930°C are found to obey the typical two dimensional variable range hopping (2D-VRH) law over four orders of magnitude below 100 K. The resistivity of a semiconducting film reveals 2D-VRH. The localization lengthes are reasonably estimated by assuming the density of states at the Fermi level as 0.5 states/eV · cell. The typical superconducting film shows the superconducting onset at 80 K and the end at 77 K. The decrease of T c from the bulk T c (92 K) in the superconducting film is apparently due to disorder induced by the substitution of Mg for Cu in the CuO planes and chains caused by less than 1% Mg atom diffusion from the MgO substrate into the film during annealing processes.