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

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 La1−xSrxVO3 and Y1−xCaxVO3 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

Spin and orbital disordering by hole doping inPr1−xCaxVO3<

“…In addition to the conventional charge-lattice coupling, a doped hole in an orbitally degenerate Mott insulator may generate strong local distortions through the modification of the spin-orbital structure in its neighborhood, forming a polaron with large binding energy [28][29][30]32,39,40]. Its enhanced effective mass reduces the mobility and leads to the localization of carriers in the random potential of dopant ions.…”

“…In addition, modifications of the one-electron bandwidth through changes in the crystal structure induced by chemical substitution are also expected (Refs. [8][9][10][11][12][13] and [26]), and the substitutional disorder can strongly affect the transport and optical properties [27][28][29][30]. Calcium doped PrVO 3 can be considered as a special case because the ionic radii of Pr 3+ (1.126Å) and Ca 2+ (1.120Å) are almost identical (see Ref.…”

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

“…Given the above prescriptions, the derivation of the uHF equations is standard and we do not present it here in extenso; more details can be found, for instance, in Refs. [86,[91][92][93]. The essence of the derivation is that the e-e interactions are replaced by the terms containing mean fields acting on the single-particle electron densities.…”

On the Neglect of Local Coulomb Interaction on Oxygens in Perovskites Described by the Multiband d-p Model

“…The implementation of the model Hamiltonian (1) is the same as described in refs. [7,11,25,26]. The possibilities of different types of order were taken into account: nonmagnetic phase, ferromagnetic and antiferromagnetic with spins aligned along: (1,1,0) or (1,0,0) or (0,0,1) direction.…”