999 research outputs found
Layer-dependent magnetization at the surface of a band-ferromagnet
The temperature-dependence of the magnetization near the surface of a
band-ferromagnet is measured with monolayer resolution. The simultaneous
application of novel highly surface-sensitive techniques enables one to deduce
the layer-dependent magnetization curves at a Fe(100) surface. Analysis of data
is based on a simple mean-field approach. Implications for modern theories of
itinerant-electron ferromagnetism are discussed.Comment: 4 pages, 1 figure, Phys. Rev. B, rapid (in press
``Linearized'' Dynamical Mean-Field Theory for the Mott-Hubbard transition
The Mott-Hubbard metal-insulator transition is studied within a simplified
version of the Dynamical Mean-Field Theory (DMFT) in which the coupling between
the impurity level and the conduction band is approximated by a single pole at
the Fermi energy. In this approach, the DMFT equations are linearized, and the
value for the critical Coulomb repulsion U_{\rm c} can be calculated
analytically. For the symmetric single-band Hubbard model at zero temperature,
the critical value is found to be given by 6 times the square root of the
second moment of the free (U=0) density of states. This result is in good
agreement with the numerical value obtained from the Projective Selfconsistent
Method and recent Numerical Renormalization Group calculations for the Bethe
and the hypercubic lattice in infinite dimensions. The generalization to more
complicated lattices is discussed. The ``linearized DMFT'' yields plausible
results for the complete geometry dependence of the critical interaction.Comment: 8 page
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Spin state transition in LaCoO3 by variational cluster approximation
The variational cluster approximation is applied to the calculation of
thermodynamical quantities and single-particle spectra of LaCoO3. Trial
self-energies and the numerical value of the Luttinger-Ward functional are
obtained by exact diagonalization of a CoO6 cluster. The VCA correctly predicts
LaCoO3 as a paramagnetic insulator and a gradual and relatively smooth increase
of the occupation of high-spin Co3+ ions causes the temperature dependence of
entropy and magnetic susceptibility. The single particle spectral function
agrees well with experiment, the experimentally observed temperature dependence
of photoelectron spectra is reproduced satisfactorily. Remaining discrepancies
with experiment highlight the importance of spin orbit coupling and local
lattice relaxation.Comment: Revtex file with 10 eps figure
Correlated band structure of NiO, CoO and MnO by variational cluster approximation
The variational cluster approximation proposed by Potthoff is applied to the
calculation of the single-particle spectral function of the transition metal
oxides MnO, CoO and NiO. Trial self-energies and the numerical value of the
Luttinger-Ward functional are obtained by exact diagonalization of a
TMO6-cluster. The single-particle parameters of this cluster serve as
variational parameters to construct a stationary point of the grand potential
of the lattice system. The stationary point is found by a crossover procedure
which allows to go continuously from an array of disconnected clusters to the
lattice system. The self-energy is found to contain irrelevant degrees of
freedom which have marginal impact on the grand potential and which need to be
excluded to obtain meaningful results. The obtained spectral functions are in
good agreement with experimental data.Comment: 14 pages, 17 figure
Mott transition in one dimension: Benchmarking dynamical cluster approaches
The variational cluster approach (VCA) is applied to the one-dimensional
Hubbard model at zero temperature using clusters (chains) of up to ten sites
with full diagonalization and the Lanczos method as cluster solver. Within the
framework of the self-energy-functional theory (SFT), different cluster
reference systems with and without bath degrees of freedom, in different
topologies and with different sets of variational parameters are considered.
Static and one-particle dynamical quantities are calculated for half-filling as
a function of U as well as for fixed U as a function of the chemical potential
to study the interaction- and filling-dependent metal-insulator (Mott)
transition. The recently developed Q-matrix technique is used to compute the
SFT grand potential. For benchmarking purposes we compare the VCA results with
exact results available from the Bethe ansatz, with essentially exact dynamical
DMRG data, with (cellular) dynamical mean-field theory and full diagonalization
of isolated Hubbard chains. Several issues are discussed including convergence
of the results with cluster size, the ability of cluster approaches to access
the critical regime of the Mott transition, efficiency in the optimization of
correlated-site vs. bath-site parameters and of multi-dimensional parameter
optimization. We also study the role of bath sites for the description of
excitation properties and as charge reservoirs for the description of filling
dependencies. The VCA turns out to be a computationally cheap method which is
competitive with established cluster approaches.Comment: 19 pages, 19 figures, v3 with minor corrections, extended discussio
Mott transitions in correlated electron systems with orbital degrees of freedom
Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard
model are studied by means of a generalization of the linearized dynamical
mean-field theory. The method allows for an efficient and reliable
determination of the critical interaction U_c for any integer filling n and
different M at zero temperature. For half-filling a linear dependence of U_c on
M is found. Inclusion of the (full) Hund's rule exchange J results in a strong
reduction of U_c. The transition turns out to change qualitatively from
continuous for J=0 to discontinuous for any finite J
Two-site dynamical mean-field theory
It is shown that a minimum realization of the dynamical mean-field theory
(DMFT) can be achieved by mapping a correlated lattice model onto an impurity
model in which the impurity is coupled to an uncorrelated bath that consists of
a single site only. The two-site impurity model can be solved exactly. The
mapping is approximate. The self-consistency conditions are constructed in a
way that the resulting ``two-site DMFT'' reduces to the previously discussed
linearized DMFT for the Mott transition. It is demonstrated that a reasonable
description of the mean-field physics is possible with a minimum computational
effort. This qualifies the simple two-site DMFT for a systematic study of more
complex lattice models which cannot be treated by the full DMFT in a feasible
way. To show the strengths and limitations of the new approach, the single-band
Hubbard model is investigated in detail. The predictions of the two-site DMFT
are compared with results of the full DMFT. Internal consistency checks are
performed which concern the Luttinger sum rule, other Fermi-liquid relations
and thermodynamic consistency.Comment: LaTeX, 14 pages, 8 eps figures included, Phys. Rev. B (in press
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