Theory of Manganites Exhibiting Colossal Magnetoresistance

TL;DR

This paper develops a theoretical framework explaining colossal magnetoresistance in manganites through the coexistence of localized and broad band electronic states caused by Jahn Teller coupling, aligning with experimental observations.

Contribution

It introduces a new microscopic model of electron-lattice interactions in manganites, revealing the coexistence of localized and broad band states as key to their properties.

Findings

Localized and broad band states coexist due to Jahn Teller coupling.

Model explains orbital liquid regime in manganites.

Results align with experimental observations of colossal magnetoresistance.

Abstract

The electronic properties of many transition metal oxide systems require new ideas concerning the behaviour of electrons in solids for their explanation. A recent example, subsequent to that of cuprate superconductors, is of rare earth manganites doped with alkaline earths, namely $Re_{1-x}A_x MnO_3$, which exhibit colossal magnetoresistance, metal insulator transition and many other poorly understood phenomena. Here we show that the strong Jahn Teller coupling between the twofold degenerate ($d_{x^2 -y^2}$ and $d_{3z^2 -r^2}$) $e_g$ orbitals of $Mn$ and lattice modes of vibration (of the oxygen octahedra surrounding the $Mn$ ions) dynamically reorganizes the former into a set of states (which we label $\ell$) which are localized with large local lattice distortion and exponentially small intersite overlap, and another set (labelled $b$) which form a broad band. This hitherto unsuspected but microscopically inevitable $coexistence$ of radically different $\ell$ and $b$ states, and their relative energies and occupation as influenced by doping $x$, temperature $T$, local Coulomb repulsion $U$ etc., underlies the unique effects seen in manganites. We present results from strong correlation calculations using the dynamical mean-field theory which accord with a variety of observations in the orbital liquid regime (say, for $0.2\stackrel{<}\sim x \stackrel{<}\sim 0.5$).We outline extensions to include intersite $\ell$ coherence and spatial correlations/long range order.