A sharp decrease of resistivity in La0.7Ca0.3Mn0.96Cu0.04O3: Evidence for Cu-assisted coherent tunneling of spin polarons

Nearly a 50% decrease of the resistivity rho(T,x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1-xCuxO3. When the observed phenome non is attributed to a decrease of the spin-polaron energy E-sigma(x) below T-C(x), all of the data are found to be well fitted by the nonthermal cohe rent tunneling expression rho(T,x)=rho(0)e(-gamma M2(T,x)), assuming that t he magnetization in the ferromagnetic state is given by the expression M(T, x)=M-R(x)+M-0(x)tanh{T-[root(C)(x)/T](2)-1}. The best fits through all the data points suggest M-0(x)similar or equal to root 1-xM(0)(0), M-R(x)simila r or equal to x root M-0(0), and E-sigma(x)similar or equal to E-sigma(0)(1 -x)(4) for the explicit x dependence of the Cu-induced modifications of the Mn-spin-dominated zero-temperature spontaneous magnetization, residual par amagnetic contribution, and spin-polaron tunneling energy, respectively, wi th E-sigma(0)=0.12 eV. (C) 1999 American Institute of Physics. [S0021-3640( 99)00811-7].