1,285 research outputs found
Comparison of Variational Approaches for the Exactly Solvable 1/r-Hubbard Chain
We study Hartree-Fock, Gutzwiller, Baeriswyl, and combined
Gutzwiller-Baeriswyl wave functions for the exactly solvable one-dimensional
-Hubbard model. We find that none of these variational wave functions is
able to correctly reproduce the physics of the metal-to-insulator transition
which occurs in the model for half-filled bands when the interaction strength
equals the bandwidth. The many-particle problem to calculate the variational
ground state energy for the Baeriswyl and combined Gutzwiller-Baeriswyl wave
function is exactly solved for the~-Hubbard model. The latter wave
function becomes exact both for small and large interaction strength, but it
incorrectly predicts the metal-to-insulator transition to happen at infinitely
strong interactions. We conclude that neither Hartree-Fock nor Jastrow-type
wave functions yield reliable predictions on zero temperature phase transitions
in low-dimensional, i.e., charge-spin separated systems.Comment: 23 pages + 3 figures available on request; LaTeX under REVTeX 3.
Optical excitations of Peierls-Mott insulators with bond disorder
The density-matrix renormalization group (DMRG) is employed to calculate
optical properties of the half-filled Hubbard model with nearest-neighbor
interactions. In order to model the optical excitations of oligoenes, a Peierls
dimerization is included whose strength for the single bonds may fluctuate.
Systems with up to 100 electrons are investigated, their wave functions are
analyzed, and relevant length-scales for the low-lying optical excitations are
identified. The presented approach provides a concise picture for the size
dependence of the optical absorption in oligoenes.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Application of the Density Matrix Renormalization Group in momentum space
We investigate the application of the Density Matrix Renormalization Group
(DMRG) to the Hubbard model in momentum-space. We treat the one-dimensional
models with dispersion relations corresponding to nearest-neighbor hopping and
hopping and the two-dimensional model with isotropic nearest-neighbor
hopping. By comparing with the exact solutions for both one-dimensional models
and with exact diagonalization in two dimensions, we first investigate the
convergence of the ground-state energy. We find variational convergence of the
energy with the number of states kept for all models and parameter sets. In
contrast to the real-space algorithm, the accuracy becomes rapidly worse with
increasing interaction and is not significantly better at half filling. We
compare the results for different dispersion relations at fixed interaction
strength over bandwidth and find that extending the range of the hopping in one
dimension has little effect, but that changing the dimensionality from one to
two leads to lower accuracy at weak to moderate interaction strength. In the
one-dimensional models at half-filling, we also investigate the behavior of the
single-particle gap, the dispersion of spinon excitations, and the momentum
distribution function. For the single-particle gap, we find that proper
extrapolation in the number of states kept is important. For the spinon
dispersion, we find that good agreement with the exact forms can be achieved at
weak coupling if the large momentum-dependent finite-size effects are taken
into account for nearest-neighbor hopping. For the momentum distribution, we
compare with various weak-coupling and strong-coupling approximations and
discuss the importance of finite-size effects as well as the accuracy of the
DMRG.Comment: 15 pages, 11 eps figures, revtex
Local Electronic Correlation at the Two-Particle Level
Electronic correlated systems are often well described by dynamical mean
field theory (DMFT). While DMFT studies have mainly focused hitherto on
one-particle properties, valuable information is also enclosed into local
two-particle Green's functions and vertices. They represent the main ingredient
to compute momentum-dependent response functions at the DMFT level and to treat
non-local spatial correlations at all length scales by means of diagrammatic
extensions of DMFT. The aim of this paper is to present a DMFT analysis of the
local reducible and irreducible two-particle vertex functions for the Hubbard
model in the context of an unified diagrammatic formalism. An interpretation of
the observed frequency structures is also given in terms of perturbation
theory, of the comparison with the atomic limit, and of the mapping onto the
attractive Hubbard model.Comment: 29 pages, 26 Figures. Accepted for publication in Phys. Rev.
Mott-Hubbard transition in infinite dimensions
We calculate the zero-temperature gap and quasiparticle weight of the
half-filled Hubbard model with a random dispersion relation. After
extrapolation to the thermodynamic limit, we obtain reliable bounds on these
quantities for the Hubbard model in infinite dimensions. Our data indicate that
the Mott-Hubbard transition is continuous, i.e., that the quasiparticle weight
becomes zero at the same critical interaction strength at which the gap opens.Comment: 4 pages, RevTeX, 5 figures included with epsfig Final version for
PRL, includes L=14 dat
Amphetamines induce tissue factor and impair tissue factor pathway inhibitor: role of dopamine receptor type 4
Aims Amphetamine intake is associated with acute vascular syndromes. Since these events are caused by arterial thrombosis and this in turn is triggered by tissue factor (TF), this study examines whether amphetamines regulate TF in human endothelial cells. Methods and results Amphetamine (10−7-10−4 mol/L) enhanced thrombin- and tumour necrosis factor (TNF)-α-induced as well as basal TF expression (P = 0.029, 0.0003, and 0.003 at maximal concentration), and TNF-α-induced plasminogen activator inhibitor (PAI)-1 expression (P = 0.003), whereas tissue factor pathway inhibitor expression was impaired (P = 0.008). Similarly, 3,4-methylenedioxymethamphetamine (10−7-10−4 mol/L) enhanced TF expression (P = 0.046). These effects were paralleled by an increased TF activity (P = 0.002); moreover, clotting time of human plasma was accelerated by supernatant from amphetamine-treated cells (P = 0.03). Amphetamine enhanced TF mRNA expression via phosphorylation of the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase (ERK) and p38 (P = 0.03 and 0.033), but not c-Jun NH2-terminal kinase (JNK; P = 0.81). The effect of amphetamine on TF expression was abrogated by the dopamine D4 receptor antagonists L-745,870 and L-750,667, but not D2 or D3 receptor antagonists; furthermore, L-745,870 blunted the amphetamine-induced activation of ERK and p38, but not JNK. Conclusion Amphetamines induce endothelial TF expression via stimulation of dopamine D4 receptor and activation of the MAPKs p38 and ERK. These effects occur at clinically relevant amphetamine concentrations and may account for the increased incidence of acute vascular syndromes after amphetamine consumptio
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
The anisotropic degenerate two-orbital Hubbard model is studied within
dynamical mean-field theory at low temperatures. High-precision calculations on
the basis of a refined quantum Monte Carlo (QMC) method reveal that two
distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2
even in the absence of spin-flip contributions to the Hund exchange. The second
transition -- not seen in earlier studies using QMC, iterative perturbation
theory, and exact diagonalization -- is clearly exposed in a low-frequency
analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure
Correlation-induced metal insulator transition in a two-channel fermion-boson model
We investigate charge transport within some background medium by means of an
effective lattice model with a novel form of fermion-boson coupling. The bosons
describe fluctuations of a correlated background. By analyzing groundstate and
spectral properties of this transport model, we show how a metal-insulator
quantum phase transition can occur for the half-filled band case. We discuss
the evolution of a mass-asymmetric band structure in the insulating phase and
establish connections to the Mott and Peierls transition scenarios.Comment: 4 pages, 4 figures, 1 table, revised version accepted for publication
in Phys. Rev. Let
International Students’ Adjustment Problems and Behaviors
This article focuses on the kinds of adjustment problems that international students had while they studied at a university in the United States, as well as the adjustment behaviors they used when faced with these problems. Qualitative data was collected and analyzed for over a decade through on-going interviews with 85 international students, as well as through observation field notes and student-written narratives about their adaptation experiences. Findings show that students were challenged by academics, social interaction, and emotional reactions to their new life. To manage their problems, students made use of behaviors that can facilitate adapting to the new culture, as well as behaviors that can obstruct them from adapting. Facilitative behaviors include coping strategies, use of supportive people, observation and imitation, and reflection. Behaviors interpreted as impeding adaptation include expecting others to adapt, complaining, and withdrawing
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