Metal-insulator transition in La1.85Sr0.15CuO4 with various substitutions for Cu.

Abstract

We have measured the resistance and magnetoresistance of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ with five different impurities (Zn, Ni, Ga, Co and Fe) substituted for Cu, down to 50 mK, and in magnetic fields up to 8 T. The concentration ${\mathit{x}}_{\mathit{c}}$ at which superconductivity disappears is smaller than the concentration ${\mathit{x}}_{\mathrm{MI}}$ at which the metal-insulator (MI) transition occurs, leaving in each case a metallic, nonsuperconducting region of concentration. We show that ${\mathit{x}}_{\mathrm{MI}}$ is determined by a superposition of the impurity-induced disorder and the carrier concentration. ${\mathit{x}}_{\mathit{c}}$, on the other hand, is a function of the effective local magnetic moment induced by the impurity. In the metallic specimens \ensuremath{\sigma} varies with \ensuremath{\surd}T up to a temperature ${\mathit{T}}^{\mathrm{*}}$ which increases with the effective local magnetic moment induced by the impurity, and for Fe reaches 70 K. The magnetoresistance is negative except in the presence of superconducting fluctuations. We conclude that these features are the result of the influence of spin scattering on the electron-electron interactions. In strongly insulating specimens the resistivity varies as \ensuremath{\rho}=${\mathrm{\ensuremath{\rho}}}_{0}$exp(${\mathit{T}}_{0}$/T${)}^{1/2}$. We demonstrate that the behavior is consistent with variable-range hopping in the presence of a Coulomb gap, and describe the conditions under which the exponent may change to 1/4 in the vicinity of the MI transition.