Defect states and excitations in a Mott insulator with orbital degrees of freedom: Mott-Hubbard gap versus optical and transport gaps in doped systems

Adolfo Avella, Peter Horsch, and Andrzej M. Oleś
Phys. Rev. B 87, 045132 – Published 31 January 2013

Abstract

We address the role played by charged defects in doped Mott insulators with active orbital degrees of freedom. It is observed that defects feature a rather complex and rich physics, which is well captured by a degenerate Hubbard model extended by terms that describe crystal-field splittings and orbital-lattice coupling, as well as by terms generated by defects such as the Coulomb potential terms that act both on doped holes and on electrons within occupied orbitals at undoped sites. We show that the multiplet structure of the excited states generated in such systems by strong electron interactions is well described within the unrestricted Hartree-Fock approximation, once the symmetry breaking caused by the onset of magnetic and orbital order is taken into account. Furthermore, we uncover spectral features that arise within the Mott-Hubbard gap and in the multiplet spectrum at high energies due to the presence of defect states and strong correlations. These features reflect the action on electrons/holes of the generalized defect potential that affects charge and orbital degrees of freedom, and indirectly also spin ones. This study elucidates the mechanism behind the Coulomb gap appearing in the band of defect states and investigates the dependence on the electron-electron interactions and the screening by the orbital-polarization field. As an illustrative example of our general approach, we present explicit calculations for the model describing three t2g orbital flavors in the perovskite vanadates doped by divalent Sr or Ca ions, such as in La1xSrxVO3 and Y1xCaxVO3 systems. We analyze the orbital densities at vanadium ions in the vicinity of defects and the excited defect states which determine the optical and transport gaps in doped systems.

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  • Received 26 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.045132

©2013 American Physical Society

Authors & Affiliations

Adolfo Avella1,2,3,4, Peter Horsch1, and Andrzej M. Oleś1,5

  • 1Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
  • 2Dipartimento di Fisica “E.R. Caianiello,” Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy
  • 3CNR-SPIN, UoS di Salerno, I-84084 Fisciano (SA), Italy
  • 4Unità CNISM di Salerno, Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy
  • 5Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4, PL-30059 Kraków, Poland

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Vol. 87, Iss. 4 — 15 January 2013

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