2,511 research outputs found
Slow quench dynamics of Mott-insulating regions in a trapped Bose gas
We investigate the dynamics of Mott-insulating regions of a trapped bosonic
gas as the interaction strength is changed linearly with time. The bosonic gas
considered is loaded into an optical lattice and confined to a parabolic
trapping potential. Two situations are addressed: the formation of Mott domains
in a superfluid gas as the interaction is increased, and their melting as the
interaction strength is lowered. In the first case, depending on the local
filling, Mott-insulating barriers can develop and hinder the density and energy
transport throughout the system. In the second case, the density and local
energy adjust rapidly whereas long range correlations require longer time to
settle. For both cases, we consider the time evolution of various observables:
the local density and energy, and their respective currents, the local
compressibility, the local excess energy, the heat and single particle
correlators. The evolution of these observables is obtained using the
time-dependent density-matrix renormalization group technique and comparisons
with time-evolutions done within the Gutzwiller approximation are provided.Comment: 15 pages, 13 figure
Influence of a magnetic field on the viscosity of a dilute gas consisting of linear molecules.
The viscomagnetic effect for two linear molecules, N2 and CO2, has been calculated in the dilute-gas limit directly from the most accurate ab initio intermolecular potential energy surfaces presently available. The calculations were performed by means of the classical trajectory method in the temperature range from 70 K to 3000 K for N2 and 100 K to 2000 K for CO2, and agreement with the available experimental data is exceptionally good. Above room temperature, where no experimental data are available, the calculations provide the first quantitative information on the magnitude and the behavior of the viscomagnetic effect for these gases. In the presence of a magnetic field, the viscosities of nitrogen and carbon dioxide decrease by at most 0.3% and 0.7%, respectively. The results demonstrate that the viscomagnetic effect is dominated by the contribution of the jj¯ polarization at all temperatures, which shows that the alignment of the rotational axes of the molecules in the presence of a magnetic field is primarily responsible for the viscomagnetic effect
Blood pressure and indices of glomerular filtration area in hypertensive and normotensive Prague rats
The involvement of the kidney in the pathogenesis of hypertension has long been recognised, although the specific renal mechanisms underlying this phenomenon are still unknown. A current hypothesis attributes hyper tension to a reduction in glomerular filtration area by glomerular loss, The present study analyses the relationship between glomerular number and volume and conscious systolic blood pressure (SBP) in 4- to 53-week-old hypertensive (PHR) and normotensive (PNR) rats of the Prague strain. Adult PHRs had higher SEP, were larger and had larger kidneys than PNRs, but 20% fewer glomeruli, A significant negative correlation between SEP and glomerular number was found in PHR males, but not in PHR females or PNRs. There was no correlation at all between glomerular volume and SEP and, in young animals, both SEP and glomerular number were higher in PHRs than in PNRs. In addition, in adult PHRs, glomerular volume and SEP were higher in males than in females. In summary, a generally valid, causal relation-ship linking raised blood pressure to decreased glomerular number or volume could not be demonstrated in the Prague rat model of genetically determined hypertension. The nature of the renal mechanism(s) determining the hypertension in this model remains unknown. Copyright (C) 2000 S. Karger AG, Basel
Dynamical simulations of charged soliton transport in conjugated polymers with the inclusion of electron-electron interactions
We present numerical studies of the transport dynamics of a charged soliton
in conjugated polymers under the influence of an external time-dependent
electric field. All relevant electron-phonon and electron-electron interactions
are nearly fully taken into account by simulating the monomer displacements
with classical molecular dynamics (MD) and evolving the wavefunction for the
electrons by virtue of the adaptive time-dependent density matrix
renormalization group (TDDMRG) simultaneously and nonadiabatically. It is found
that after a smooth turn-on of the external electric field the charged soliton
is accelerated at first up to a stationary constant velocity as one entity
consisting of both the charge and the lattice deformation. An ohmic region (6
mV/ 12 mV/) where the stationary
velocity increases linearly with the electric field strength is observed. The
relationship between electron-electron interactions and charged soliton
transport is also investigated in detail. We find that the dependence of the
stationary velocity of a charged soliton on the on-site Coulomb interactions
and the nearest-neighbor interactions is due to the extent of
delocalization of the charged soliton defect.Comment: 25 pages, 15 figure
A Lorentz-Violating Alternative to Higgs Mechanism?
We consider a four-dimensional field-theory model with two massless fermions,
coupled to an Abelian vector field without flavour mixing, and to another
Abelian vector field with flavour mixing. Both Abelian vectors have a
Lorentz-violating kinetic term, introducing a Lorentz-violation mass scale ,
from which fermions and the flavour-mixing vector get their dynamical masses,
whereas the vector coupled without flavour mixing remains massless. When the
two coupling constants have similar values in order of magnitude, a mass
hierarchy pattern emerges, in which one fermion is very light compared to the
other, whilst the vector mass is larger than the mass of the heavy fermion. The
work presented here may be considered as a Lorentz-symmetry-Violating
alternative to the Higgs mechanism, in the sense that no scalar particle
(fundamental or composite) is necessary for the generation of the vector-meson
mass. However, the model is not realistic given that, as a result of Lorentz
Violation, the maximal (light-cone) speed seen by the fermions is smaller than
that of the massless gauge boson (which equals the speed of light in vacuo) by
an amount which is unacceptably large to be compatible with the current tests
of Lorentz Invariance, unless the gauge couplings assume unnaturally small
values. Possible ways out of this phenomenological drawback are briefly
discussed, postponing a detailed construction of more realistic models for
future work.Comment: 16 pages revtex, three eps figures incorporate
Magnetism in 2D BNO and BSiN: polarized itinerant and local electrons
We use density functional theory based first-principles methods to study the
magnetism in a 2D hexagonal BN sheet induced by the different concentrations of
oxygen and silicon atoms substituting for nitrogen (O) and boron
(Si) respectively. We demonstrate the possible formation of three
distinct phases based on the magnetization energy calculated self-consistently
for the ferromagnetic (ME) and antiferromagnetic
(ME) states, i.e. the paramagnetic phase with
ME=ME, the ferromagnetic phase with
MEME and finally the polarized itinerant
electrons with finite ME but zero ME. While
the O system was found to exist in all three phases, no tendency
towards the formation of the polarized itinerant electrons was observed for the
Si system though the existence of the other two phases was
ascertained. The different behavior of these two systems is associated with the
diverse features in the magnetization energy as a function of the oxygen and
silicon concentrations. Finally, the robustness of the polarized itinerant
electron phase is also discussed with respect to the O substitute atom
distributions and the applied strains to the system.Comment: accepted by RP
Entanglement and Quantum Phase Transition Revisited
We show that, for an exactly solvable quantum spin model, a discontinuity in
the first derivative of the ground state concurrence appears in the absence of
quantum phase transition. It is opposed to the popular belief that the
non-analyticity property of entanglement (ground state concurrence) can be used
to determine quantum phase transitions. We further point out that the
analyticity property of the ground state concurrence in general can be more
intricate than that of the ground state energy. Thus there is no one-to-one
correspondence between quantum phase transitions and the non-analyticity
property of the concurrence. Moreover, we show that the von Neumann entropy, as
another measure of entanglement, can not reveal quantum phase transition in the
present model. Therefore, in order to link with quantum phase transitions, some
other measures of entanglement are needed.Comment: RevTeX 4, 4 pages, 1 EPS figures. some modifications in the text.
Submitted to Phys. Rev.
Analysis of band-gap formation in squashed arm-chair CNT
The electronic properties of squashed arm-chair carbon nanotubes are modeled
using constraint free density functional tight binding molecular dynamics
simulations. Independent from CNT diameter, squashing path can be divided into
{\it three} regimes. In the first regime, the nanotube deforms with negligible
force. In the second one, there is significantly more resistance to squashing
with the force being nN/per CNT unit cell. In the last regime,
the CNT looses its hexagonal structure resulting in force drop-off followed by
substantial force enhancement upon squashing. We compute the change in band-gap
as a function of squashing and our main results are: (i) A band-gap initially
opens due to interaction between atoms at the top and bottom sides of CNT. The
orbital approximation is successful in modeling the band-gap opening at
this stage. (ii) In the second regime of squashing, large
interaction at the edges becomes important, which can lead to band-gap
oscillation. (iii) Contrary to a common perception, nanotubes with broken
mirror symmetry can have {\it zero} band-gap. (iv) All armchair nanotubes
become metallic in the third regime of squashing. Finally, we discuss both
differences and similarities obtained from the tight binding and density
functional approaches.Comment: 16 pages and 6 figures, To appear in PR
Stability in the instantaneous Bethe-Salpeter formalism: harmonic-oscillator reduced Salpeter equation
A popular three-dimensional reduction of the Bethe-Salpeter formalism for the
description of bound states in quantum field theory is the Salpeter equation,
derived by assuming both instantaneous interactions and free propagation of all
bound-state constituents. Numerical (variational) studies of the Salpeter
equation with confining interaction, however, observed specific instabilities
of the solutions, likely related to the Klein paradox and rendering (part of
the) bound states unstable. An analytic investigation of this problem by a
comprehensive spectral analysis is feasible for the reduced Salpeter equation
with only harmonic-oscillator confining interactions. There we are able to
prove rigorously that the bound-state solutions correspond to real discrete
energy spectra bounded from below and are thus free of any instabilities.Comment: 23 pages, 3 figures, extended conclusions, version to appear in Phys.
Rev.
Energy average formula of photon gas rederived by using the generalized Hermann-Feynman theorem
By virtue of the generalized Hermann-Feynmam theorem and the method of
characteristics we rederive energy average formula of photon gas, this is
another useful application of the theorem.Comment: 2 page
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