713 research outputs found
The Hubbard model with smooth boundary conditions
We apply recently developed smooth boundary conditions to the quantum Monte
Carlo simulation of the two-dimensional Hubbard model. At half-filling, where
there is no sign problem, we show that the thermodynamic limit is reached more
rapidly with smooth rather than with periodic or open boundary conditions. Away
from half-filling, where ordinarily the simulation cannot be carried out at low
temperatures due to the existence of the sign problem, we show that smooth
boundary conditions allow us to reach significantly lower temperatures. We
examine pairing correlation functions away from half-filling in order to
determine the possible existence of a superconducting state. On a
lattice for , at a filling of and an inverse
temperature of , we did find enhancement of the -wave correlations
with respect to the non-interacting case, a possible sign of -wave
superconductivity.Comment: 16 pages RevTeX, 9 postscript figures included (Figure 1 will be
faxed on request
Charge-density waves in the Hubbard chain: evidence for 4k_F instability
Charge density waves in the Hubbard chain are studied by means of
finite-temperature Quantum Monte Carlo simulations and Lanczos diagonalizations
for the ground state. We present results both for the charge susceptibilities
and for the charge structure factor at densities \rho=1/6 and 1/3; for \rho=1/2
(quarter filled) we only present results for the charge structure factor. The
data are consistent with a 4k_F instability dominating over the 2k_F one, at
least for sufficiently large values of the Coulomb repulsion, U. This can only
be reconciled with the Luttinger liquid analyses if the amplitude of the 2k_F
contribution vanishes above some U^*(\rho).Comment: RevTeX, 4 two-column pages with 7 colour figures embedded in tex
Charge-density waves in one-dimensional Hubbard superlattices
We study the formation of charge density waves (CDW's) in one-dimensional
Hubbard superlattices, modeled by a repeated pattern of repulsive (U>0) and
free (U=0) sites. By means of Lanczos diagonalizations for the ground state, we
calculate the charge structure factor. Our results show that while the
superlattice structure affects the modulation of the charge density waves, the
periodicity can still be predicted through an effective density. We also show
that, for a fixed repulsive layer thickness, the periodicity of the CDW is an
oscillatory function of the free layer thickness.Comment: 4 pages, 4 figure
Path Integral Description of a Semiclassical Su-Schrieffer-Heeger Model
The electron motion along a chain is described by a continuum version of the
Su-Schrieffer-Heeger Hamiltonian in which phonon fields and electronic
coordinates are mapped onto the time scale. The path integral formalism allows
us to derive the non local source action for the particle interacting with the
oscillators bath. The method can be applied for any value of the {\it e-ph}
coupling. The path integral dependence on the model parameters has been
analysed by computing the partition function and some thermodynamical
properties from up to room temperature. A peculiar upturn in the low
temperature {\it heat capacity over temperature} ratio (pointing to a glassy
like behavior) has been ascribed to the time dependent electronic hopping along
the chain
A schematic model for QCD I: Low energy meson states
A simple model for QCD is presented, which is able to reproduce the meson
spectrum at low energy. The model is a Lipkin type model for quarks coupled to
gluons. The basic building blocks are pairs of quark-antiquarks coupled to a
definite flavor and spin. These pairs are coupled to pairs of gluons with spin
zero. The multiplicity problem, which dictates that a given experimental state
can be described in various manners, is removed when a particle-mixing
interaction is turned on. In this first paper of a series we concentrates on
the discussion of meson states at low energy, the so-called zero temperature
limit of the theory. The treatment of baryonic states is indicated, also.Comment: 29 pages, 6 figures. submitted to Phys. Rev.
Dielectric response of charge induced correlated state in the quasi-one-dimensional conductor (TMTTF)2PF6
Conductivity and permittivity of the quasi-one-dimensionsional organic
transfer salt (TMTTF)2PF6 have been measured at low frequencies (10^3-10^7 Hz)
between room temperature down to below the temperature of transition into the
spin-Peierls state. We interpret the huge real part of the dielectric
permittivity (up to 10^6) in the localized state as the realization in this
compound of a charge ordered state of Wigner crystal type due to long range
Coulomb interaction.Comment: 11 pages, 3 .eps figure
Magnetic and Dynamic Properties of the Hubbard Model in Infinite Dimensions
An essentially exact solution of the infinite dimensional Hubbard model is
made possible by using a self-consistent mapping of the Hubbard model in this
limit to an effective single impurity Anderson model. Solving the latter with
quantum Monte Carlo procedures enables us to obtain exact results for the one
and two-particle properties of the infinite dimensional Hubbard model. In
particular we find antiferromagnetism and a pseudogap in the single-particle
density of states for sufficiently large values of the intrasite Coulomb
interaction at half filling. Both the antiferromagnetic phase and the
insulating phase above the N\'eel temperature are found to be quickly
suppressed on doping. The latter is replaced by a heavy electron metal with a
quasiparticle mass strongly dependent on doping as soon as . At half
filling the antiferromagnetic phase boundary agrees surprisingly well in shape
and order of magnitude with results for the three dimensional Hubbard model.Comment: 32 page
Self-Consistent Quasi-Particle RPA for the Description of Superfluid Fermi Systems
Self-Consistent Quasi-Particle RPA (SCQRPA) is for the first time applied to
a more level pairing case. Various filling situations and values for the
coupling constant are considered. Very encouraging results in comparison with
the exact solution of the model are obtained. The nature of the low lying mode
in SCQRPA is identified. The strong reduction of the number fluctuation in
SCQRPA vs BCS is pointed out. The transition from superfluidity to the normal
fluid case is carefully investigated.Comment: 23 pages, 18 figures and 1 table, submitted to Phys. Rev.
Evolution of deformation and recrystallization textures in high-purity Ni and the Ni-5 at. pct W alloy
An attempt has been made to study the evolution of texture in high-purity Ni and Ni-5 at. pct W alloy prepared by the powder metallurgy route followed by heavy cold rolling (∼95 pct deformation) and recrystallization. The deformation textures of the two materials are of typical pure metal or Cu-type texture. Cube-oriented ({001} {100}) regions are present in the deformed state as long thin bands, elongated in the rolling direction (RD). These bands are characterized by a high orientation gradient inside, which is a result of the rotation of the cube-oriented cells around the RD toward the RD-rotated cube ({013} {100}). Low-temperature annealing produces a weak cube texture along with the {013} {100} component, with the latter being much stronger in high-purity Ni than in the Ni-W alloy. At higher temperatures, the cube texture is strengthened considerably in the Ni-W alloy; however, the cube volume fraction in high-purity Ni is significantly lower because of the retention of the {013} {100} component. The difference in the relative strengths of the cube, and the {013} {100} components in the two materials is evident from the beginning of recrystallization in which more {013} {100} -oriented grains than near cube grains form in high-purity Ni. The preferential nucleation of the near cube and the {013} {100} grains in these materials seems to be a result of the high orientation gradients associated with the cube bands that offer a favorable environment for early nucleation
Left ventricular strain-volume loops in bicuspid aortic valve disease:new insights in cardiomechanics
AIMS: By combining temporal changes in left ventricular (LV) global longitudinal strain (GLS) with LV volume, LV strain-volume loops can assess cardiac function across the cardiac cycle. This study compared LV strain-volume loops between bicuspid aortic valve (BAV) patients and controls, and investigated the loop’s prognostic value for clinical events.METHODS AND RESULTS: From a prospective cohort of congenital heart disease patients, BAV patients were selected and compared with healthy volunteers, who were matched for age and sex at group level. GLS7 analysis from apical views were used to construct strain-volume loops. Associations with clinical events, i.e. a composite of all-cause mortality, heart failure, arrhythmias and aortic valve replacement, were assessed by Cox regression. 113 BAV patients were included (median age 32 years, 40% female). BAV patients demonstrated lower Sslope (0.21%/mL, [Q1-Q3: 0.17-0.28] vs. 0.27%/mL [0.24-0.34], p<0.001) and ESslope (0.19%/mL [0.12-0.25] vs. 0.29%/mL [0.21-0.43], p<0.001) compared to controls, but also greater uncoupling during early (0.48±1.29 vs. 0.06±1.2, p=0.018) and late diastole (0.66±1.01 vs -0.06±1.09, p<0.001). Median follow-up duration was 9.9 [9.3-10.4] years. Peak aortic jet velocity (HR 1.22, p=0.03), enlarged left atrium (HR 3.16, p=0.003), E/e’ ratio (HR 1.17, p=0.002), GLS (HR 1.16, p=0.008) and ESslope (HR 0.66, p=0.04) were associated with the occurrence of clinical events.CONCLUSION: Greater uncoupling and lower systolic and diastolic slopes were observed in BAV patients compared to healthy controls, suggesting presence of altered LV cardiomechanics. Moreover, lower ESslope was associated with clinical events, highlighting the strain-volume loop’s potential as prognostic marker
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