A variational theory of magnetic field dependent thermoelectric power of Anderson lattice model: An application to colossal magnetoresistive manganites (Re1-xAxMnO3)

Abstract

Using a simple variational method, we have studied the Magnetic field dependent thermoelectric power Q (T) as a function of temperature of rare earth manganites doped with alkaline earths namely Re1-xAxMnO3 (where Re=La, Pr, Nd etc., and A= Ca, Sr, Ba etc.) which exhibit Colossal Magnetoresistance (CMR), metal- insulator transition & many other poorly understood phenomena. We have recently developed a two band (ℓ-b) Anderson lattice model Hamiltonian alongwith(ℓ-b) hybridization recently studied by us for manganites in the strong electron- lattice Jahn-Teller (JT) coupling regime an approach similar to the two-fluid models. We have already used this variational method to study the electrical resistivity & magnetic susceptibility of doped CMR manganites. In the present study, we find that the thermoelectric power Q (T) decreases with increasing magnetic field and magnetic transition temperature Tc shifts towards lower temperature region. An interesting feature of Q (T) is that it rises gradually with decreasing temperatures and a broad peak appears at low temperatures and drops sharply (i.e. Q →0 as T →0) on further lowering of temperature which is suggested to originate from the weak carrier localization effect. In the external magnetic field, the phonon- drag effect is expected to become weaker so that the value of Q (T) reduces. We also report the effects of the model parameters e.g. local Coulomb repulsion U, strong ferromagnetic Hund’s coupling JH between eg and t2g spins, hybridization VK between ℓ– polarons & d- electrons of same spins on Q(T) at fixed magnetic parameters h, m. It has been found from our results that Q (T) is positive at low temperatures & shows a pronounced peak at the metal – insulator transition which gets sharpened with alkaline earth doping x& shifts towards the high temperature region. Similar effects are observed on increasing JH also but on the contrary the magnitude of Q(T) decreases on increasing Vk and peak at low T becomes broader & shifts towards the high temperature region. The observed broad peak may be explained on the basis of the spin – wave theory and may be attributed to the magnon drag effect which increases with x or JH value. Q (T) as a function of temperature computed from this method is, generally, in good agreement with the available experimental data.