Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase

Pulsed magnetic field (0-4.4 T) was used to study the magnetic field dependent resistivity (12-350 K) and thermoelectric power (0-1.5 T) of Na and K-doped La(0.7)Ca(0.7-y)A(y)MnO(3) (0.0less than or equal toyless than or equal to0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature T-p. Na/K-doping increases both conductivity and T-p. In La1-xCaxMnO3, an increase of T-p and conductivity with an increase of Ca (for xless than or equal to0.33) are small and the small polaron coupling constant (gamma) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, gamma is small and for ygreater than or equal to0.05, Motts' condition of strong el-ph interaction breaks down in the high temperature (T>T-p) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of gamma, binding energy (W-p), hopping energy (W-H), and effective mass (m(p)) of the polarons leading to the melting (we call it) of polarons in the T>T-p phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80-300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (T-p<T<theta(D)/2, theta(D) being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>theta(D)/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<T-p) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein's condition of the adiabatic "small polaron" hopping conduction mechanism for the region T>T-p. (C) 2003 American Institute of Physics.