Random mixtures with orientational order, and the anisotropic resistivity tensor of high-T c superconductors

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Abstract
By generalizing effective‐medium theory to the case of orientationally ordered but positionally disordered two‐component mixtures, it is shown that the anisotropic dielectric tensor of oxide superconductors can be extracted from microwave measurements on oriented crystallites of YBa2Cu3O7−x embedded in epoxy. Surprisingly, this technique appears to be the only one which can access the resistivity perpendicular to the copper‐oxide planes in crystallites that are too small for depositing electrodes. This possibility arises in part because the real part of the dielectric constant of oxide superconductors has a large magnitude. The validity of the effective‐medium approach for orientationally ordered mixtures is corroborated by simulations on two‐dimensional anisotropic random resistor networks. Analysis of the experimental data suggests that the zero‐temperature limit of the finite‐frequency resistivity does not vanish along the c axis, a result which would imply the existence of states at the Fermi surface, even in the superconducting state.