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

Abstract

Nearly a 50% decrease of the resistivity ρ(T, x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1−x CuxO3. When the observed phenomenon is attributed to a decrease of the spin-polaron energy E σ(x) below T C (x), all of the data are found to be well fitted by the nonthermal coherent tunneling expression $$\rho (T,x) = \rho _0 e^{ - \gamma M^2 (T,x)} $$ , assuming that the magnetization in the ferromagnetic state is given by the expression $$M(T,x) = M_R (x) + M_0 (x)\tanh \left\{ {\sqrt {\left[ {T_C (x)/T} \right]^2 - 1} } \right\}$$ . The best fits through all the data points suggest $$M_0 (x) \simeq \sqrt {1 - x} M_0 (0), M_R (x) \simeq \sqrt x M_0 (0)$$ and E σ(x)≃E σ(0)(1−x)4 for the explicit x dependence of the Cu-induced modifications of the Mn-spin-dominated zero-temperature spontaneous magnetization, residual paramagnetic contribution, and spin-polaron tunneling energy, respectively, with E σ(0)=0.12 eV.