2,464 research outputs found

### Finite temperature analysis of a quasi2D dipolar gas

We present finite temperature analysis of a quasi2D dipolar gas. To do this,
we use the Hartree Fock Bogoliubov method within the Popov approximation. This
formalism is a set of non-local equations containing the dipole-dipole
interaction and the condensate and thermal correlation functions, which are
solved self-consistently. We detail the numerical method used to implement the
scheme. We present density profiles for a finite temperature dipolar gas in
quasi2D, and compare these results to a gas with zero-range interactions.
Additionally, we analyze the excitation spectrum and study the impact of the
thermal exchange

### Excited states of a static dilute spherical Bose condensate in a trap

The Bogoliubov approximation is used to study the excited states of a dilute
gas of $N$ atomic bosons trapped in an isotropic harmonic potential
characterized by a frequency $\omega_0$ and an oscillator length $d_0 =
\sqrt{\hbar/m\omega_0}$. The self-consistent static Bose condensate has
macroscopic occupation number $N_0 \gg 1$, with nonuniform spherical condensate
density $n_0(r)$; by assumption, the depletion of the condensate is small ($N'
\equiv N - N_0\ll N_0$). The linearized density fluctuation operator $\hat
\rho'$ and velocity potential operator $\hat\Phi'$ satisfy coupled equations
that embody particle conservation and Bernoulli's theorem. For each angular
momentum $l$, introduction of quasiparticle operators yields coupled eigenvalue
equations for the excited states; they can be expressed either in terms of
Bogoliubov coherence amplitudes $u_l(r)$ and $v_l(r)$ that determine the
appropriate linear combinations of particle operators, or in terms of
hydrodynamic amplitudes $\rho_l'(r)$ and $\Phi_l'(r)$. The hydrodynamic picture
suggests a simple variational approximation for $l >0$ that provides an upper
bound for the lowest eigenvalue $\omega_l$ and an estimate for the
corresponding zero-temperature occupation number $N_l'$; both expressions
closely resemble those for a uniform bulk Bose condensate.Comment: 5 pages, RevTeX, contributed paper accepted for Low Temperature
Conference, LT21, August, 199

### Energy and Vorticity in Fast Rotating Bose-Einstein Condensates

We study a rapidly rotating Bose-Einstein condensate confined to a finite
trap in the framework of two-dimensional Gross-Pitaevskii theory in the strong
coupling (Thomas-Fermi) limit. Denoting the coupling parameter by 1/\eps^2
and the rotational velocity by $\Omega$, we evaluate exactly the next to
leading order contribution to the ground state energy in the parameter regime
|\log\eps|\ll \Omega\ll 1/(\eps^2|\log\eps|) with \eps\to 0. While the TF
energy includes only the contribution of the centrifugal forces the next order
corresponds to a lattice of vortices whose density is proportional to the
rotational velocity.Comment: 19 pages, LaTeX; typos corrected, clarifying remarks added, some
rearrangements in the tex

### Thermodynamics of Solitonic Matter Waves in a Toroidal Trap

We investigate the thermodynamic properties of a Bose-Einstein condensate
with negative scattering length confined in a toroidal trapping potential. By
numerically solving the coupled Gross-Pitaevskii and Bogoliubov-de Gennes
equations, we study the phase transition from the uniform state to the
symmetry-breaking state characterized by a bright-soliton condensate and a
localized thermal cloud. In the localized regime three states with a finite
condensate fraction are present: the thermodynamically stable localized state,
a metastable localized state and also a metastable uniform state. Remarkably,
the presence of the stable localized state strongly increases the critical
temperature of Bose-Einstein condensation.Comment: 4 pages, 4 figures, to be published in Physical Review A as a Rapid
Communication. Related papers can be found at
http://www.padova.infm.it/salasnich/tdqg.htm

### Spin-dependent Hedin's equations

Hedin's equations for the electron self-energy and the vertex were originally
derived for a many-electron system with Coulomb interaction. In recent years it
has been increasingly recognized that spin interactions can play a major role
in determining physical properties of systems such as nanoscale magnets or of
interfaces and surfaces. We derive a generalized set of Hedin's equations for
quantum many-body systems containing spin interactions, e.g. spin-orbit and
spin-spin interactions. The corresponding spin-dependent GW approximation is
constructed.Comment: 5 pages, 1 figur

### A simple mean field equation for condensates in the BEC-BCS crossover regime

We present a mean field approach based on pairs of fermionic atoms to
describe condensates in the BEC-BCS crossover regime. By introducing an
effective potential, the mean field equation allows us to calculate the
chemical potential, the equation of states and the atomic correlation function.
The results agree surprisingly well with recent quantum Monte Carlo
calculations. We show that the smooth crossover from the bosonic mean field
repulsion between molecules to the Fermi pressure among atoms is associated
with the evolution of the atomic correlation function

### Quantum Monte Carlo study of dilute neutron matter at finite temperatures

We report results of fully non-perturbative, Path Integral Monte Carlo (PIMC)
calculations for dilute neutron matter. The neutron-neutron interaction in the
s channel is parameterized by the scattering length and the effective range. We
calculate the energy and the chemical potential as a function of temperature at
the density \dens=0.003\fm^{-3}. The critical temperature \Tc for the
superfluid-normal phase transition is estimated from the finite size scaling of
the condensate fraction. At low temperatures we extract the spectral weight
function $A(p,\omega)$ from the imaginary time propagator using the methods of
maximum entropy and singular value decomposition. We determine the
quasiparticle spectrum, which can be accurately parameterized by three
parameters: an effective mass $m^*$, a mean-field potential $U$, and a gap
$\Delta$. Large value of \Delta/\Tc indicates that the system is not a
BCS-type superfluid at low temperatures.Comment: 4 pages, 3 figure

### Thermodynamic properties of a dipolar Fermi gas

Based on the semi-classical theory, we investigate the thermodynamic
properties of a dipolar Fermi gas. Through a self-consistent procedure, we
numerically obtain the phase space distribution function at finite temperature.
We show that the deformations in both momentum and real space becomes smaller
and smaller as one increases the temperature. For homogeneous case, we also
calculate pressure, entropy, and heat capacity. In particular, at low
temperature limit and in weak interaction regime, we obtain an analytic
expression for the entropy, which agrees qualitatively with our numerical
result. The stability of a trapped gas at finite temperature is also explored

### Hartree shift in unitary Fermi gases

The Hartree energy shift is calculated for a unitary Fermi gas. By including
the momentum dependence of the scattering amplitude explicitly, the Hartree
energy shift remains finite even at unitarity. Extending the theory also for
spin-imbalanced systems allows calculation of polaron properties. The results
are in good agreement with more involved theories and experiments.Comment: 31 pages, many figure

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