184 research outputs found

### A Non-Crossing Approximation for the Study of Intersite Correlations

We develop a Non-Crossing Approximation (NCA) for the effective cluster
problem of the recently developed Dynamical Cluster Approximation (DCA). The
DCA technique includes short-ranged correlations by mapping the lattice problem
onto a self-consistently embedded periodic cluster of size $N_c$. It is a fully
causal and systematic approximation to the full lattice problem, with
corrections ${\cal{O}}(1/N_c)$ in two dimensions. The NCA we develop is a
systematic approximation with corrections ${\cal{O}}(1/N_c^3)$. The method will
be discussed in detail and results for the one-particle properties of the
Hubbard model are shown. Near half filling, the spectra display pronounced
features including a pseudogap and non-Fermi-liquid behavior due to
short-ranged antiferromagnetic correlations.Comment: 12 pages, 11 figures, EPJB styl

### Spectral Properties and Bandstructure of Correlated Electron Systems

We present $\vec{k}$-dependent one-particle spectra and corresponding
effective bandstructures for the $2d$ Hubbard model calculated within the
dynamical molecular field theory (DMFT). This method has proven to yield highly
nontrivial results for a variety of quantities but the question remains open to
what extent it is applicable to relevant physical situations.
To address this problem we compare our results for spectral functions to
those obtained by QMC simulations. The good agreement supports our notion that
the DMFT is indeed a sensible ansatz for correlated models even in to $d=2$.Comment: Paper presented at SCES '95, Sept. 27 - 30 1995, Goa. To be published
in Physica B. 10 pages, figures include

### Magnetism and Phase Separation in the Ground State of the Hubbard Model

We discuss the ground state magnetic phase diagram of the Hubbard model off
half filling within the dynamical mean-field theory. The effective
single-impurity Anderson model is solved by Wilson's numerical renormalization
group calculations, adapted to symmetry broken phases. We find a phase
separated, antiferromagnetic state up to a critical doping for small and
intermediate values of U, but could not stabilise a Neel state for large U and
finite doping. At very large U, the phase diagram exhibits an island with a
ferromagnetic ground state. Spectral properties in the ordered phases are
discussed.Comment: 9 pages, 11 figure

### On the Analyticity of Solutions in the Dynamical Mean-Field Theory

The unphysical solutions of the periodic Anderson model obtained by H. Keiter
and T. Leuders [Europhys. Lett. 49, 801(2000)] in dynamical mean-field theory
(DMFT) are shown to result from the author's restricted choice of the
functional form of the solution, leading to a violation of the analytic
properties of the exact solution. By contrast, iterative solutions of the
self-consistency condition within the DMFT obtained by techniques which
preserve the correct analytic properties of the exact solution (e.g., quantum
Monte-Carlo simulations or the numerical renormalization group) always lead to
physical solutions.Comment: 4 pages, 1 figur

### Dynamical Magnetic Susceptibility for the $t$-$J$ Model

We present results for the {\em dynamical}\/ magnetic susceptibility of the
$t$-$J$ model, calculated with the dynamical mean field theory. For $J=0$ we
find enhanced ferromagnetic correlations but an otherwise relatively
$\vec{q}$-independent dynamical magnetic susceptibility. For $J>0$ the explicit
antiferromagnetic exchange leads to a dynamic spin structure factor with the
expected peak at the antiferromagnetic Bragg point.Comment: 3 pages LaTeX, postscript figures included, submitted as contribution
to SCES' 96, to appear in Physica

### Transport Properties of the Infinite Dimensional Hubbard Model

Results for the optical conductivity and resistivity of the Hubbard model in
infinite spatial dimensions are presented. At half filling we observe a gradual
crossover from a normal Fermi-liquid with a Drude peak at $\omega=0$ in the
optical conductivity to an insulator as a function of $U$ for temperatures
above the antiferromagnetic phase transition. When doped, the ``insulator''
becomes a Fermi-liquid with a corresponding temperature dependence of the
optical conductivity and resistivity. We find a $T^2$-coefficient in the low
temperature resistivity which suggests that the carriers in the system acquire
a considerable mass-enhancement due to the strong local correlations. At high
temperatures, a crossover into a semi-metallic regime takes place.Comment: 14 page

### Phase diagram of the frustrated Hubbard model

The Mott-Hubbard metal-insulator transition in the paramagnetic phase of the
one-band Hubbard model has long been used to describe similar features in real
materials like V$_2$O$_3$. Here we show that this transition is hidden inside a
rather robust antiferromagnetic insulator even in the presence of comparatively
strong magnetic frustration. This result raises the question of the relevance
of the Mott-Hubbard metal-insulator transition for the generic phase diagram of
the one-band Hubbard model.Comment: 4 pages, 6 figure

### The Dynamical Cluster Approximation (DCA) versus the Cellular Dynamical Mean Field Theory (CDMFT) in strongly correlated electrons systems

We are commenting on the article Phys. Rev. {\bf B 65}, 155112 (2002) by G.
Biroli and G. Kotliar in which they make a comparison between two cluster
techniques, the {\it Cellular Dynamical Mean Field Theory} (CDMFT) and the {\it
Dynamical Cluster Approximation} (DCA). Based upon an incorrect implementation
of the DCA technique in their work, they conclude that the CDMFT is a faster
converging technique than the DCA. We present the correct DCA prescription for
the particular model Hamiltonian studied in their article and conclude that the
DCA, once implemented correctly, is a faster converging technique for the
quantities averaged over the cluster. We also refer to their latest response to
our comment where they argue that instead of averaging over the cluster, local
observables should be calculated in the bulk of the cluster which indeed makes
them converge much faster in the CDMFT than in the DCA. We however show that in
their original work, the authors themselves use the cluster averaged quantities
to draw their conclusions in favor of using the CDMFT over the DCA.Comment: Comment on Phys. Rev. B 65, 155112 (2002). 3 pages, 2 figure

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