Anomalous temperature behavior of the resistivity in lightly doped manganites around a metal-insulator phase transition

An unusual temperature and concentration behavior of the resistivity ρ(T, x) in La_0.7Ca_0.3Mn_1−2x Cu_xO₃ is observed at slight Cu doping (0≤x≤0.05). Namely, the introduction of copper results in a splitting of the resistivity maximum around the metal-insulator transition temperature T ₀(x) into two differently evolving peaks. Unlike the original Cu-free maximum, which steadily increases with doping, the second (satellite) peak remains virtually unchanged for x<x _c , increases for x≥x _c , and finally disappears at x _m ≃2x _c , with x _c ≃ 0.03. The observed phenomenon is thought to arise from a competition between substitution-induced strengthening of the potential barriers (which hinder the charge hopping between neighboring Mn sites) and weakening of the kinetic energy of the carriers. The data are well fitted by assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 473–478 (10 October 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.