Normal state tunneling conductance of perovskite oxides: implications for high-T_c superconductors

Abstract

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+⋎V/V^∗⋎ⁿ] with 1≥n≥0.5 and where V^∗ is an intrinsic energy scale. The exponent n graduall y increases from 0.5 to 1 as the metal-insulator (M-I) transition is approached. In the high-T_c 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 ∂G/∂V⋍G^α₀ with α⋍1 and the intrinsic energy scale V⋍25-75 meV in the different oxides investigated.