122 research outputs found
Self-localization of composite spin-lattice polarons
Self-localization of holes in the Holstein t-J model is studied in the
adiabatic limit using exact diagonalization and the retraceable path
approximation. It is shown that the critical electron-phonon coupling \lambda_c
decreases with increasing J and that this behavior is determined mainly by the
incoherent rather than by the coherent motion of the hole. The obtained spin
correlation functions in the localized region can be understood within a
percolation picture where antiferromagnetic order can persist up to a
substantial hole doping. These results restrict the possibility of
self-localization of holes in lightly doped cuprates.Comment: 4 pages, 5 figure
Spin-orbital physics in the optical conductivity of quarter-filled manganites
Using finite-temperature diagonalization we investigate the optical
conductivity and spin-orbital dynamics in the CE phase of
half-doped manganites. We find characterized by a broad
spectrum with pronounced optical gap due to charge ordering induced by Coulomb
and further neighbor Jahn-Teller interactions. With increasing temperature the
conductivity shows a significant change over a wide energy range with a
characteristic shift towards lower frequencies. In the low temperature CE phase
we observe in-gap absorption due to combined orbiton-spin excitations.Comment: 4 pages, 5 figure
Doping dependence of density response and bond-stretching phonons in cuprates
We explain the anomalous doping dependence of zone boundary and
bond-stretching phonons in the high-temperature superconductor
LaSrCuO in the doping range . Our
calculations are based on a theory for the density response of doped
Mott-Hubbard insulators.Comment: 2 pages, 3 figures, SCES conference, Karlsruhe 200
Orbital fluctuations in the VO perovskites
The properties of Mott insulators with orbital degrees of freedom are
described by spin-orbital superexchange models, which provide a theoretical
framework for understanding their magnetic and optical properties. We introduce
such a model derived for configuration of V ions in
the VO perovskites, =Lu,Yb,,La, and demonstrate that
orbital fluctuations along the axis are responsible for the
huge magnetic and optical anisotropies observed in the almost perfectly cubic
compound LaVO. We argue that the GdFeO distortion and the large
difference in entropy of -AF and -AF phases is responsible for the second
magnetic transition observed at in YVO. Next we address the
variation of orbital and magnetic transition temperature, and
, in the VO perovskites, after extending the spin-orbital model
by the crystal-field and the orbital interactions which arise from the
GdFeO and Jahn-Teller distortions of the VO octahedra. We further find
that the orthorhombic distortion which increases from LaVO to LuVO
plays a crucial role by controlling the orbital fluctuations, and via the
modified orbital correlations influences the onset of both magnetic and orbital
order.Comment: 25 pages, 10 figure
Optical conductivity of colossal magnetorestistance compounds: Role of orbital degeneracy in the ferromagnetic phase
Recent optical conductivity experiments have revealed an
anomalous spectral distribution in the ferromagnetic phase of the perovskite
system . Using finite temperature diagonalization techniques
we investigate for a model that contains only the
-orbital degrees of freedom. Due to strong correlations the orbital model
appears as a generalized t-J model with anisotopic interactions and 3-site
hopping. In the orbital t-J model is characterized by a broad
incoherent spectrum with increasing intensity as temperature is lowered, and a
Drude peak with small weight, consistent with experiment. Our calculations for
two-dimensional systems, which may have some particular relevance for the
double-layer manganites, show that the scattering from orbital fluctuations can
explain the order of magnitude of the incoherent part of in
the low temperature ferromagnetic phase. Moreover orbital correlation functions
are studied and it is shown that - orbital order is prefered in the
doped planar model at low temperature.Comment: Revtex, 14 pages, 14 figure
Defects, disorder and strong electron correlations in orbital degenerate, doped Mott insulators
We elucidate the effects of defect disorder and - interaction on the
spectral density of the defect states emerging in the Mott-Hubbard gap of doped
transition-metal oxides, such as YCaVO. A soft gap of
kinetic origin develops in the defect band and survives defect disorder for
- interaction strengths comparable to the defect potential and hopping
integral values above a doping dependent threshold, otherwise only a pseudogap
persists. These two regimes naturally emerge in the statistical distribution of
gaps among different defect realizations, which turns out to be of Weibull
type. Its shape parameter determines the exponent of the power-law
dependence of the density of states at the chemical potential () and hence
distinguishes between the soft gap () and the pseudogap ()
regimes. Both and the effective gap scale with the hopping integral and the
- interaction in a wide doping range. The motion of doped holes is
confined by the closest defect potential and the overall spin-orbital
structure. Such a generic behavior leads to complex non-hydrogen-like defect
states that tend to preserve the underlying -type spin and -type orbital
order and can be detected and analyzed via scanning tunneling microscopy.Comment: 5 pages, 4 figure
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