3,811 research outputs found
The confining trailing string
We extend the holographic trailing string picture of a heavy quark to the
case of a bulk geometry dual to a confining gauge theory. We compute the
classical trailing confining string solution for a static as well as a
uniformly moving quark. The trailing string is infinitely extended and
approaches a confining horizon, situated at a critical value of the radial
coordinate, along one of the space-time directions, breaking boundary
rotational invariance. We compute the equations for the fluctuations around the
classical solutions, which are used to obtain boundary force correlators
controlling the Langevin dynamics of the quark. The imaginary part of the
correlators has a non-trivial low-frequency limit, which gives rise to a
viscous friction coefficient induced by the confining vacuum. The vacuum
correlators are used to define finite-temperature dressed Langevin correlators
with an appropriate high-frequency behavior.Comment: 63 pages plus appendices, 19 figures; version accepted for
publication in JHE
Zero-temperature phase diagram of Yukawa bosons
We study the zero-temperature phase diagram of bosons interacting via
screened Coulomb (Yukawa) potential by means of the diffusion Monte Carlo
method. The Yukawa potential is used as a model interaction in the neutron
matter, dusty plasmas and charged colloids. As shown by D. S. Petrov et al.
[Phys. Rev. Lett. 99, 130407 (2007)], interactions between weakly bound
molecules of heavy and light fermionic atoms are described by an effective
Yukawa potential with a strength related to the heavy-light mass ratio M/m
which might lead to crystallization in a two-dimensional geometry if the mass
ratio of heavy-light fermions exceeds a certain critical value. In the present
work we do a thorough study of the quantum three-dimensional Yukawa system. For
strong interactions (equivalently, large mass ratios) the system experiences
several phase transitions as the density is increased, passing from gas to
solid and to gas phase again. Weakly interacting Yukawa particles do not
crystallize at any density. We find the minimal interaction strength at which
the crystallization happens. In terms of the two-component fermionic system,
this strength corresponds to a heavy-light mass ratio of M/m ~ 180, so that it
is impossible to realize the gas-crystal transition in a conventional bulk
system. For the Yukawa model of fermionic mixtures we also analyze the
possibility of building molecular systems with very large effective mass ratios
by confining the heavy component to a sufficiently deep optical lattice. We
show how the effective mass of the heavy component can be made arbitrarily
large by increasing the lattice depth, thus leading to a tunable effective mass
ratio that can be used to realize a molecular superlattice.Comment: added figure with finite-size dependence of the energy; comments and
references added; title change
Low-dimensional weakly interacting Bose gases: non-universal equations of state
The zero-temperature equation of state is analyzed in low-dimensional bosonic
systems. In the dilute regime the equation of state is universal in terms of
the gas parameter, i.e. it is the same for different potentials with the same
value of the s-wave scattering length. Series expansions of the universal
equation of state are reported for one- and two- dimensional systems. We
propose to use the concept of energy-dependent s-wave scattering length for
obtaining estimations of non-universal terms in the energy expansion. We test
this approach by making a comparison to exactly solvable one-dimensional
problems and find that the generated terms have the correct structure. The
applicability to two-dimensional systems is analyzed by comparing with results
of Monte Carlo simulations. The prediction for the non-universal behavior is
qualitatively correct and the densities, at which the deviations from the
universal equation of state become visible, are estimated properly. Finally,
the possibility of observing the non-universal terms in experiments with
trapped gases is also discussed.Comment: 11 pages, 4 figure
Energy and Structure of Hard-Sphere Bose Gases in three and two dimensions
The energy and structure of dilute gases of hard spheres in three dimensions
is discussed, together with some aspects of the corresponding 2D systems. A
variational approach in the framework of the Hypernetted Chain Equations (HNC)
is used starting from a Jastrow wavefunction that is optimized to produce the
best two--body correlation factor with the appropriate long range. Relevant
quantities describing static properties of the system are studied as a function
of the gas parameter where , and are the density,
--wave scattering length of the potential and dimensionality of the space,
respectively. The occurrence of a maximum in the radial distribution function
and in the momentum distribution is a natural effect of the correlations when
increases. Some aspects of the asymptotic behavior of the functions
characterizing the structure of the systems are also investigated.Comment: Proceedings of the QFS2004 conference in Trento. To appear in JLT
Ground-State Properties of a One-Dimensional System of Hard Rods
A quantum Monte Carlo simulation of a system of hard rods in one dimension is
presented and discussed. The calculation is exact since the analytical form of
the wavefunction is known, and is in excellent agreement with predictions
obtained from asymptotic expansions valid at large distances. The analysis of
the static structure factor and the pair distribution function indicates that a
solid-like and a gas-like phases exist at high and low densities, respectively.
The one-body density matrix decays following a power-law at large distances and
produces a divergence in the low density momentum distribution at k=0 which can
be identified as a quasi-condensate.Comment: 4 pages, 4 figure
Single-particle vs. pair superfluidity in a bilayer system of dipolar bosons
We consider the ground state of a bilayer system of dipolar bosons, where
dipoles are oriented by an external field in the direction perpendicular to the
parallel planes. Quantum Monte Carlo methods are used to calculate the
ground-state energy, the one-body and two-body density matrix, and the
superfluid response as a function of the separation between layers. We find
that by decreasing the interlayer distance for fixed value of the strength of
the dipolar interaction, the system undergoes a quantum phase transition from a
single-particle to a pair superfluid. The single-particle superfluid is
characterized by a finite value of both the atomic condensate and the
super-counterfluid density. The pair superfluid phase is found to be stable
against formation of many-body cluster states and features a gap in the
spectrum of elementary excitations.Comment: 4 figure
Gapped spectrum in pair-superfluid bosons
We study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum on both sides of this transition by using two microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both the density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value.Postprint (author's final draft
Optical lattices as a tool to study defect-induced superfluidity
We study the superfluid response, the energetic and structural properties of
a one-dimensional ultracold Bose gas in an optical lattice of arbitrary
strength. We use the Bose-Fermi mapping in the limit of infinitely large
repulsive interaction and the diffusion Monte Carlo method in the case of
finite interaction. For slightly incommensurate fillings we find a superfluid
behavior which is discussed in terms of vacancies and interstitials. It is
shown that both the excitation spectrum and static structure factor are
different for the cases of microscopic and macroscopic fractions of defects.
This system provides a extremely well-controlled model for studying
defect-induced superfluidity.Comment: 14 pages, 13 figures, published versio
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