19,137 research outputs found
Bose-Einstein condensation of trapped atoms with dipole interactions
The path integral Monte Carlo method is used to simulate dilute trapped Bose
gases and to investigate the equilibrium properties at finite temperatures. The
quantum particles have a long-range dipole-dipole interaction and a short-range
s-wave interaction. Using an anisotropic pseudopotential for the long-range
dipolar interaction and a hard-sphere potential for the short-range s-wave
interaction, we calculate the energetics and structural properties as a
function of temperature and the number of particles. Also, in order to
determine the effects of dipole-dipole forces and the influence of the trapping
field on the dipolar condensate, we use two cylindrically symmetric harmonic
confinements (a cigar-shaped trap and a disk-shaped trap). We find that the net
effect of dipole-dipole interactions is governed by the trapping geometry. For
a cigar-shaped trap, the net contribution of dipolar interactions is attractive
and the shrinking of the density profiles is observed. For a disk-shaped trap,
the net effect of long-range dipolar forces is repulsive and the density
profiles expand
Comment on "Off-diagonal Long-range Order in Bose Liquids: Irrotational Flow and Quantization of Circulation"
In the context of an application to superfluidity, it is elaborated how to do
quantum mechanics of a system with a rotational velocity. Especially, in both
the laboratory frame and the non-inertial co-rotating frame, the canonical
momentum, which corresponds to the quantum mechanical momentum operator,
contains a part due to the rotational velocity.Comment: 2 page, comment on cond-mat/010435
Scaling of in heavy ion collisions
We interpret the scaling of the corrected elliptic flow parameter w.r.t. the
corrected multiplicity, observed to hold in heavy ion collisions for a wide
variety of energies and system sizes. We use dimensional analysis and
power-counting arguments to place constraints on the changes of initial
conditions in systems with different center of mass energy .
Specifically, we show that a large class of changes in the (initial) equation
of state, mean free path, and longitudinal geometry over the observed
are likely to spoil the scaling in observed experimentally. We
therefore argue that the system produced at most Super Proton Synchrotron (SPS)
and Relativistic Heavy Ion Collider (RHIC) energies is fundamentally the same
as far as the soft and approximately thermalized degrees of freedom are
considered. The ``sQGP'' (Strongly interacting Quark-Gluon Plasma) phase, if it
is there, is therefore not exclusive to RHIC. We suggest, as a goal for further
low-energy heavy ion experiments, to search for a ``transition''
where the observed scaling breaks.Comment: Accepted for publication by Phys. Rev. C Based on presentation in
mini-symposium on QGP collective properties, Frankfurt. Discussion expanded,
results adde
Bose-Einstein Condensation Temperature of a Homogeneous Weakly Interacting Bose Gas : PIMC study
Using a finite-temperature Path Integral Monte Carlo simulation (PIMC) method
and finite-size scaling, we have investigated the interaction-induced shift of
the phase transition temperature for Bose-Einstein condensation of homogeneous
weakly interacting Bose gases in three dimensions, which is given by a proposed
analytical expression , where
is the critical temperature for an ideal gas, is the s-wave
scattering length, and is the number density. We have used smaller number
densities and more time slices than in the previous PIMC simulations [Gruter
{\it et al.}, Phys. Rev. Lett. {\bf 79}, 3549 (1997)] in order to understand
the difference in the value of the coefficient between their results
and the (apparently) other reliable results in the literature. Our results show
that depends strongly on the
interaction strength while the previous PIMC results are
considerably flatter and smaller than our results. We obtain = 1.32
0.14, in agreement with results from recent Monte Carlo methods of
three-dimensional O(2) scalar field theory and variational
perturbation theory
3+1D hydrodynamic simulation of relativistic heavy-ion collisions
We present MUSIC, an implementation of the Kurganov-Tadmor algorithm for
relativistic 3+1 dimensional fluid dynamics in heavy-ion collision scenarios.
This Riemann-solver-free, second-order, high-resolution scheme is characterized
by a very small numerical viscosity and its ability to treat shocks and
discontinuities very well. We also incorporate a sophisticated algorithm for
the determination of the freeze-out surface using a three dimensional
triangulation of the hyper-surface. Implementing a recent lattice based
equation of state, we compute p_T-spectra and pseudorapidity distributions for
Au+Au collisions at root s = 200 GeV and present results for the anisotropic
flow coefficients v_2 and v_4 as a function of both p_T and pseudorapidity. We
were able to determine v_4 with high numerical precision, finding that it does
not strongly depend on the choice of initial condition or equation of state.Comment: 16 pages, 11 figures, version accepted for publication in PRC,
references added, minor typos corrected, more detailed discussion of
freeze-out routine adde
A heuristic approach to the weakly interacting Bose gas
Some thermodynamic properties of weakly interacting Bose systems are derived
from dimensional and heuristic arguments and thermodynamic relations, without
resorting to statistical mechanics
Graphene nanoribbons subject to gentle bends
Since graphene nanoribbons are thin and flimsy, they need support. Support
gives firm ground for applications, and adhesion holds ribbons flat, although
not necessarily straight: ribbons with high aspect ratio are prone to bend. The
effects of bending on ribbons' electronic properties, however, are unknown.
Therefore, this article examines the electromechanics of planar and gently bent
graphene nanoribbons. Simulations with density-functional tight-binding and
revised periodic boundary conditions show that gentle bends in armchair ribbons
can cause significant widening or narrowing of energy gaps. Moreover, in zigzag
ribbons sizeable energy gaps can be opened due to axial symmetry breaking, even
without magnetism. These results infer that, in the electronic measurements of
supported ribbons, such bends must be heeded.Comment: 5 pages, 4 figure
Weakly nonlinear stability analysis of MHD channel flow using an efficient numerical approach
We analyze weakly nonlinear stability of a flow of viscous conducting liquid
driven by pressure gradient in the channel between two parallel walls subject
to a transverse magnetic field. Using a non-standard numerical approach, we
compute the linear growth rate correction and the first Landau coefficient,
which in a sufficiently strong magnetic field vary with the Hartmann number as
and
. These
coefficients describe a subcritical transverse velocity perturbation with the
equilibrium amplitude
which exists at Reynolds numbers below the linear stability threshold
We find that the flow
remains subcritically unstable regardless of the magnetic field strength. Our
method for computing Landau coefficients differs from the standard one by the
application of the solvability condition to the discretized rather than
continuous problem. This allows us to bypass both the solution of the adjoint
problem and the subsequent evaluation of the integrals defining the inner
products, which results in a significant simplification of the method.Comment: 16 pages, 10 figures, revised version (to appear in Phys Fluids
- …