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The multitude of orbital (OO) and magnetic ordered (MO) ground states
in perovskite manganites, R1-xAxMnO3 with R=La,Pr,.. and A=Ca,Sr,
as a function of hole doping, x, are an unambiguous
fingerprint of the crucial role of multi-orbital electronic correlations in
these Mott-Hubbard systems. Understanding these ordered
states lies at the heart of a consistent resolution of the colossal magneto-resi
stance (CMR) observed in manganites, where melting of the OO induces a low-$T$
insulator-metal transition for 0.25 x 0.45. Recently, a d-wave pseudogap in the "bad metallic" phase of bilayer manganites has been extracted from
angle-resolved photoemission spectroscopy by Dessau and co-workers. This has
prompted inevitable comparison with similar features observed in underdoped
cuprates. However, given the very different physics of these two systems,
its origin in manganites is a enigmatic, and promises to shed invaluable light
on our microscopic understanding of these materials.
We have used a doubly-degenerate Hubbard model with: (i) realistic hopping structure characteristic of the eg-based perovskites, (ii) strong, multi-orbital coulomb interactions and Jahn-Teller distortions, to study these aspects of manganite physics. Using (a) multi-orbital DMFT, and (b) cluster diagonalisation studies, we focus on the orbital ordered states as a function of hole doping, x, as well as on the AFOO-FI to PI-FM transition. For x=0 (pure LaMnO3), we find the correct AFOO (of 3x2-r2/3y2-r2 orbitals) Mott insulating state. Moreover, the zig-zag (diagonal) as well as the stripe OO phases observed for x=1/2 and for x=2/3,3/4 are exactly obtained. These various OO states are EXACTLY representable as an ``alloy'' ordering of the 3x2-r2/3y2-r2 orbitals on each Mn3+ site. Remarkably, the "bad metallic" state in the DMFT turns out to have a d-wave pseudogap, in full agreement with ARPES. Implications for understanding CMR will be touched upon. |
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