Carriers binding to excitons: Crystal-field excitations in doped Mott-Hubbard insulators

Journal Article · Wed Sep 01 04:00:00 EDT 1993 · Physical Review, B: Condensed Matter; (United States)

DOI:https://doi.org/10.1103/PhysRevB.48.7197· OSTI ID:6242280

Zaanen, J ^[1]; Oles, A M ^[2]

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1. Lorentz Institute for Theoretical Physics, Leiden University, P.O. Box 9506, NL-2300 RA Leiden (Netherlands)
2. Institute of Physics, Jagellonian University, Reymonta 4, PL-30-059, Krakow (Poland)

We address the role played by orbital degeneracy in doped Mott-Hubbard insulators. We observe that in all but the simplest systems the carriers bind to [ital d]-[ital d] excitons because of Hund's-rule interactions. These three-particle bound states have distinct spectroscopic signatures and at least in one case these seem already confirmed experimentally. If the crystal-field gaps become of the order of the kinetic energy of the carriers, doping might tend to stabilize phases characterized by a finite occupation of [ital d]-[ital d] excitons in the ground state. If the total spin of both the carrier state and the spin background are at maximum, the relevant excitons do not involve a change in spin. As a consequence, the orbital channel can be in the first instance considered independently from the spin channel and we find an exciton-carrier coupling that in essence interpolates between the carrier-spin-wave couplings of the [ital t]-[ital J] model and the conventional couplings to optical phonons. We work out in detail a case involving high-spin holes in a cupratelike system and we show that the exciton-carrier coupling tends to stabilize an orthorhombic type of orbital order. On the other hand, if either the carriers or the background are in a low-spin state, the relevant excitons also change total spin locally and more-exotic order parameters are possible. We analyze in detail the case of a nickelate close to the high-spin--low-spin transition where we show that doping will tend to stabilize an ordering related to superpositions of low-spin and high-spin states, characterized by an overall spin-rotational invariance. We argue that such a state might be realized in [ital n]-type La[sub 2]NiO[sub 4].

• OSTI ID: 6242280
• Journal Information: Physical Review, B: Condensed Matter; (United States), Journal Name: Physical Review, B: Condensed Matter; (United States) Vol. 48:10; ISSN PRBMDO; ISSN 0163-1829
• Country of Publication: United States
• Language: English

Related Subjects

665000* -- Physics of Condensed Matter-- (1992-)
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BOUND STATE
CHARGE CARRIERS
COLLECTIVE EXCITATIONS
COPPER COMPOUNDS
CRYSTAL FIELD
CRYSTAL MODELS
CUPRATES
DIELECTRIC MATERIALS
ENERGY-LEVEL TRANSITIONS
EXCITATION
EXCITONS
HIGH-TC SUPERCONDUCTORS
HUBBARD MODEL
LANTHANUM COMPOUNDS
MATERIALS
MATHEMATICAL MODELS
NICKEL COMPOUNDS
NICKELATES
ORDER PARAMETERS
OXYGEN COMPOUNDS
QUASI PARTICLES
RARE EARTH COMPOUNDS
SUPERCONDUCTORS
TRANSITION ELEMENT COMPOUNDS