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An unusual temperature and concentration behavior of resistivity in$La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting ofthe resistivity maximum around a metal-insulator transition temperature$T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-freemaximum which steadily increases with doping, the second (satellite) peakremains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finallydisappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenonis thought to arise from competition between substitution induced strengtheningof potential barriers (which hamper the charge hopping between neighboring $Mn$sites) and weakening of carrier's kinetic energy. The data are well fittedassuming a nonthermal tunneling conductivity theory with randomly distributedhopping sites.Comment: 10 REVTEX pages, 2 PostScript figures (epsf.sty); to be published in  JETP Letter

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