  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 of
the resistivity maximum around a 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 $x\ge x_c$ and finally
disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon
is thought to arise from competition between substitution induced strengthening
of potential barriers (which hamper the charge hopping between neighboring $Mn$
sites) and weakening of carrier's kinetic energy. The data are well fitted
assuming a nonthermal tunneling conductivity theory with randomly distributed
hopping sites.
