A variational theory of zero field electrical resistivity of colossal magnetoresistive manganites (Re1-xAxMnO3)

Abstract

Using a simple variational method, we have studied the zero field electrical resistivity ρ(T) of rare earth manganites doped with alkaline earths namely Re1-xAxMnO3which exhibit colossal magnetoresistance (CMR), metal–insulator transition and many other poorly understood phenomena. We take the two band model Hamiltonian for manganites in the strong electron-lattice Jahn–Teller (JT) coupling regime. This model is constructed for the doped manganites which exhibit colossal magnetoresistance (CMR) involving a broad spin-majority (eg-spins) conduction band (b-band) as well as nearly localized spin-minority (t2g-spins) electron states (ℓ-band). Two band models involving itinerant and localized states were also suggested earlier by both experimentalists and theorists. We have also studied the temperature dependence of electrical resistivity ρ(T) at H = 0 of these materials and observed the role of the model parameters e.g. local Coulomb repulsion U, strong ferromagnetic Hund's rule coupling JHbetween egand t2gspins and hybridization V between ℓ-polarons and b-electrons of the same spins on ρ(T). We find from the resistivity results that as the temperature is lowered below a critical temperature Tc(~ 200 K), there is a sudden drop in electrical resistivity ρ(T) at H = 0 resembling with the key feature of many CMR compounds like La2/3( Pb , Ca )1/3MnO3and ( Sm1-yGdy)0.55Sr0.45MnO3at y = 0.5. This anomaly in ρ(T) arises from the onset of magnetic ordering at 200 K and vanishes on increasing V or JHvalue. T-dependence of ρ(T) is metallic-like below Tc(~ 200 K), above which it shows insulator/semiconducting-like behavior.