786 research outputs found
Mechanism of structural phase transitions in KCrF3
We study the origin of the cubic to tetragonal and tetragonal to monoclinic
structural transitions in KCrF3, and the associated change in orbital order,
paying particular attention to the relevance of super-exchange in both phases.
We show that super-exchange is not the main mechanism driving these
transitions. Specifically, it is not strong enough to be responsible for the
high-temperature cubic to tetragonal transition and does not yield the type of
orbital order observed in the monoclinic phase. The energy difference between
the tetragonal and the monoclinic structure is tiny, and most likely results
from the interplay between volume, covalency, and localization effects. The
transition is rather driven by Slater exchange than super-exchange.
Nevertheless, once the monoclinic distortions are present, super-exchange helps
in stabilizing the low symmetry structure. The orbital order we obtain for this
monoclinic phase is consistent with the magnetic transition at 80 K.Comment: 8 pages, 6 figure
Thermally assisted ordering in Mott insulators
Ginzburg-Landau theory describes phase transitions as the competition between
energy and entropy: The ordered phase has lower energy, while the disordered
phase has larger entropy. When heating the system, ordering is reduced
entropically until it vanishes at the critical temperature. This established
picture implicitly assumes that the energy difference between ordered and
disordered phase does not change with temperature. We show that for the Mott
insulator KCuF3 this assumption is strongly violated: thermal expansion
energetically stabilizes the orbitally-ordered phase to such and extent that no
phase transition is observed. This new mechanism explains not only the absence
of a phase transition in KCuF3 but even suggests the possibility of an inverted
transition in closed-shell systems, where the ordered phase emerges only at
high temperatures.Comment: 5 pages, 5 figure
Dielectric screening in doped Fullerides
For conventional superconductors the electron-electron interaction is
strongly reduced by retardation effects, making the formation of Cooper pairs
possible. In the alkali-doped Fullerides, however, there are no strong
retardation effects. But dielectric screening can reduce the electron-electron
interaction sufficiently, if we assume that the random-phase approximation
(RPA) is valid. It is not clear, however, if this assumption holds, since the
alkali-doped Fullerides are strongly correlated systems close to a Mott
transition. To test the validity of the RPA for these systems we have
calculated the screening of a test charge using quantum Monte Carlo.Comment: 4 pages, 1 eps figure included; to be published in the proceedings of
the International Winterschool on Electronic Properties of Novel Materials,
Kirchberg/Tirol, 1998; additional information is available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Metal-Insulator transitions in generalized Hubbard models
We study the Mott transition in Hubbard models with a degenerate band on
different 3-dimensional lattices. While for a non-degenerate band only the
half-filled system may exhibit a Mott transition, with degeneracy there can be
a transition for any integer filling. We analyze the filling dependence of the
Mott transition and find that (the Hubbard interaction at which the
transition takes place) decreases away from half-filling. In addition we can
change the lattice structure of the model. This allows us to study the
influence of frustration on the Mott transition. We find that frustration
increases , compared to bipartite systems. The results were obtained from
fixed-node diffusion Monte Carlo calculations using trial functions which allow
us to systematically vary the magnetic character of the system. To gain a
qualitative understanding of the results, we have developed simple hopping
arguments that help to rationalize the doping dependence and the influence of
frustration on the Mott transition. Choosing the model parameters to describe
the doped Fullerides, we can make contact with experiment and understand why
some of the Fullerides are metals, while others, which according to density
functional theory should also be metallic, actually are insulators.Comment: 4 pages LaTeX with 4 eps figures; submitted to Computer Physics
Communications, Proceedings of the CPP'99/Centennial Meeting, Atlanta, GA;
additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Filling dependence of the Mott transition in the degenerate Hubbard model
Describing the doped Fullerenes using a generalized Hubbard model, we study
the Mott transition for different integer fillings of the t_1u band. We use the
opening of the energy-gap E_g as a criterion for the transition. E_g is
calculated as a function of the on-site Coulomb interaction U using fixed-node
diffusion Monte Carlo. We find that for systems with doping away from
half-filling the Mott transitions occurs at smaller U than for the half-filled
system. We give a simple model for the doping dependence of the Mott
transition.Comment: 7 pages RevTeX with 10 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Multiplet effects in orbital and spin ordering phenomena: A hybridization-expansion quantum impurity solver study
Orbital and spin ordering phenomena in strongly correlated systems are
commonly studied using the local-density approximation + dynamical mean-field
theory approach. Typically, however, such simulations are restricted to
simplified models (density-density Coulomb interactions, high symmetry
couplings and few-band models). In this work we implement an efficient general
hybridization-expansion continuous-time quantum Monte Carlo impurity solver
(Krylov approach) which allows us to investigate orbital and spin ordering in a
more realistic setting, including interactions that are often neglected (e.g.,
spin-flip and pair-hopping terms), enlarged basis sets (full d versus eg),
low-symmetry distortions, and reaching the very low-temperature (experimental)
regime. We use this solver to study ordering phenomena in a selection of
exemplary low-symmetry transition-metal oxides: LaMnO3 and rare-earth
manganites as well as the perovskites CaVO3 and YTiO3. We show that spin-flip
and pair hopping terms do not affect the Kugel-Khomskii orbital-order melting
transition in rare-earth manganites, or the suppression of orbital fluctuations
driven by crystal field and Coulomb repulsion. For the Mott insulator YTiO3 we
find a ferromagnetic transition temperature 50 K, in remarkably good agreement
with experiments. For LaMnO3 we show that the classical t2g-spin approximation,
commonly adopted for studying manganites, yields indeed an occupied eg orbital
in very good agreement with that obtained for the full d 5-orbital Hubbard
model, while the spin-spin e_g-t_{2g} correlation function calculated from the
full d model is 0.74, very close to the value expected for aligned eg and t2g
spins; the eg spectral function matrix is also well reproduced. Finally, we
show that the t2g screening reduces the eg-eg Coulomb repulsion by about 10%Comment: 9 pages, 5 figure
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