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 La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp 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 (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/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<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.