Theory of Manganites Exhibiting Colossal Magnetoresistance

Ramakrishnan, T. V.; Krishnamurthy, H. R.; Hassan, S. R.; Pai, G. Venketeswara

Theory of Manganites Exhibiting Colossal Magnetoresistance

Authors

Description

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.
26pages, 4 figures, to appear as a chapter in "Colossal Magnetoresistive Manganites", ed. T. Chatterji, Kluwer Academic Publishers, Dordrecht, Netherlands (2003)