  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 new 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. The
present 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 t_2g orbital flavors in the perovskite vanadates
doped by divalent Sr or Ca ions, such as in La_(1-x)Sr_xVO_3 and
Y_(1-x)Ca_xVO_3 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.

Strongly Correlated Electrons