94 research outputs found
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
Dynamics of Impurity and Valence Bands in GaMnAs within the Dynamical Mean Field Approximation
We calculate the density-of-states and the spectral function of GaMnAs within
the dynamical mean-field approximation. Our model includes the competing
effects of the strong spin-orbit coupling on the J=3/2 GaAs hole bands and the
exchange interaction between the magnetic ions and the itinerant holes. We
study the quasi-particle and impurity bands in the paramagnetic and
ferromagnetic phases for different values of impurity-hole coupling at the Mn
doping of x=0.05. By analyzing the anisotropic angular distribution of the
impurity band carriers at T=0, we conclude that the carrier polarization is
optimal when the carriers move along the direction parallel to the average
magnetization.Comment: 6 pages, 4 figure
Dynamical mean field study of the Mott transition in the half-filled Hubbard model on a triangular lattice
We employ dynamical mean field theory (DMFT) with a Quantum Monte Carlo (QMC)
atomic solver to investigate the finite temperature Mott transition in the
Hubbard model with the nearest neighbor hopping on a triangular lattice at
half-filling. We estimate the value of the critical interaction to be in units of the hopping amplitude through the evolution of the
magnetic moment, spectral function, internal energy and specific heat as the
interaction and temperature are varied. This work also presents a
comparison between DMFT and finite size determinant Quantum Monte Carlo (DQMC)
and a discussion of the advantages and limitations of both methods.Comment: 7 pages, 5 figure
Effect of inhomogeneity on s-wave superconductivity in the attractive Hubbard model
Inhomogeneous s-wave superconductivity is studied in the two-dimensional,
square lattice attractive Hubbard Hamiltonian using the Bogoliubov-de Gennes
(BdG) mean field approximation. We find that at weak coupling, and for
densities mainly below half-filling, an inhomogeneous interaction in which the
on-site interaction takes on two values, results in a larger
zero temperature pairing amplitude, and that the superconducting can also
be significantly increased, relative to a uniform system with on all
sites. These effects are observed for stripe, checkerboard, and even random
patterns of the attractive centers, suggesting that the pattern of
inhomogeneity is unimportant. Monte Carlo calculations which reintroduce some
of the fluctuations neglected within the BdG approach see the same effect, both
for the attractive Hubbard model and a Hamiltonian with d-wave pairing
symmetry.Comment: 5 pages, 4 figure
Magnetism in semiconductors: A dynamical mean-field study of ferromagnetism in Ga1-xMnxAs
We employ the dynamical mean-field approximation to perform a systematic study of magnetism in Ga1-xMnxAs. Our model incorporates the effects of the strong spin-orbit coupling on the J=32 GaAs valence bands and of the exchange interaction between the randomly distributed magnetic ions and the itinerant holes. The ferromagnetic phase transition temperature Tc is obtained for different values of the impurity-hole coupling Jc and of the hole concentration nh at the Mn doping of x=0.05. We also investigate the temperature dependence of the local magnetization and spin polarization of the holes. By comparing our results with those for a single band Hamiltonian, we conclude that the spin-orbit coupling in Ga1-xMnxAs gives rise to frustration in the ferromagnetic order, strengthening recent findings by Zaránd and Jankó [Phys. Rev. Lett. 89, 047201 (2002)]. © 2005 The American Physical Society
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Dynamical Cluster Approximation Employing FLEX as a Cluster Solver
We employ the Dynamical Cluster Approximation (DCA) in conjunction with the
Fluctuation Exchange Approximation (FLEX) to study the Hubbard model. The DCA
is a technique to systematically restore the momentum conservation at the
internal vertices of Feynman diagrams relinquished in the Dynamical Mean Field
Approximation (DMFA). FLEX is a perturbative diagrammatic approach in which
classes of Feynman diagrams are summed over analytically using geometric
series. The FLEX is used as a tool to investigate the complementarity of the
DCA and the finite size lattice technique with periodic boundary conditions by
comparing their results for the Hubbard model. We also study the microscopic
theory underlying the DCA in terms of compact (skeletal) and non-compact
diagrammatic contributions to the thermodynamic potential independent of a
specific model. The significant advantages of the DCA implementation in
momentum space suggests the development of the same formalism for the frequency
space. However, we show that such a formalism for the Matsubara frequencies at
finite temperatures leads to acausal results and is not viable. However, a real
frequency approach is shown to be feasible.Comment: 15 pages, 24 figures. Submitted to Physical Review B as a Regular
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