3,521 research outputs found
Disorder- and correlation-driven metal-insulator transitions
Metal-insulator transitions driven by disorder (Delta) and/or by electron
correlations (U) are investigated within the Anderson-Hubbard model with local
binary-alloy disorder using a simple but consistent mean-field approach. The
Delta-U phase diagram is derived and discussed for T=0 and finite temperatures.Comment: 2 pages, 2 figures, submitted to the SCES'04, Ref.4 update
Dynamical variational principles for strongly correlated electron systems
The self-energy-functional approach (SFA) is discussed in the context of
different variational principles for strongly correlated electron systems.
Formal analogies between static and dynamical variational approaches, different
types of approximation strategies and the relations to density-functional and
dynamical mean-field theory are emphasized. The discussion elucidates the
strengths of the SFA in the construction of new non-perturbative approximations
but also the limitations of the approach and thereby opens up future
perspectives.Comment: 12 pages, 1 eps figures included, Adv. Solid State Phys. (in press
Systematics of approximations constructed from dynamical variational principles
The systematics of different approximations within the self-energy-functional
theory (SFT) is discussed for fermionic lattice models with local interactions.
In the context of the SFT, an approximation is essentially given by specifying
a reference system with the same interaction but a modified non-interacting
part of the Hamiltonian which leads to a partial decoupling of degrees of
freedom. The reference system defines a space of trial self-energies on which
an optimization of the grand potential as a functional of the self-energy
Omega[Sigma] is performed. As a stationary point is not a minimum in general
and does not provide a bound for the exact grand potential, however, it is {\em
a priori} unclear how to judge on the relative quality of two different
approximations. By analyzing the Euler equation of the SFT variational
principle, it is shown that a stationary point of the functional on a subspace
given by a reference system composed of decoupled subsystems is also a
stationary point in case of the coupled reference system. On this basis a
strategy is suggested which generates a sequence of systematically improving
approximations. The discussion is actually relevant for any variational
approach that is not based on wave functions and the Rayleigh-Ritz principle.Comment: 14 pages, 5 figures, for the proceedings of the conference on
''Effective Models for Low-Dimensional Strongly Correlated Systems'',
Peyresq, September 2005, to be published by AI
Self-energy-functional approach to systems of correlated electrons
The grand potential of a system of interacting electrons is considered as a
stationary point of a self-energy functional. It is shown that a rigorous
evaluation of the functional is possible for self-energies that are
representable within a certain reference system. The variational scheme allows
to construct new non-perturbative and thermodynamically consistent
approximations. Numerical results illustrate the practicability of the method.Comment: 8 pages, 5 figure
Fast simulation of Gaussian random fields
Fast Fourier transforms are used to develop algorithms for the fast
generation of correlated Gaussian random fields on d-dimensional rectangular
regions. The complexities of the algorithms are derived, simulation results and
error analysis are presented.Comment: 15 pages, 8 figures. Typos corrected in Algorithm 3, Remark (4),
Algorithm 4, Remark (5), and Algorithm 5, Remark (5
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
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