Normal state tunneling conductance of perovskite oxides. Implications for high-Tc superconductors

We have investigated tunneling conductances in disordered, normally conducting perovskite oxides close to the metal-insulator transition. We show that the normal state tunneling conductance of perovskite oxides can be cast in a general form G(V)=G0[1+vertical strokeV/V*vertical stroken] with 1≥n≥0.5 and where V* is an intrinsic energy scale. The exponent n gradually increases from 0.5 to 1 as the metal-insulator (M-I) transition is approached. In the high-Tc Bi(2212) cuprates, the normally observed, linear G(V)(n=1) can be made sub-linear (n<1) by substitution of Ca with Y. From the similarity of the linear conductances, we suggest proximity to the M-I transition as a likely cause for this G(V)∝V dependence. In systems showing linear conductances (n≅1), we find that dG/dV≅G0α with α≅1 and the intrinsic energy scale V*≅25-75 meV in the different oxides investigated. (orig.)