673 research outputs found
Consequences of the Pauli exclusion principle for the Bose-Einstein condensation of atoms and excitons
The bosonic atoms used in present day experiments on Bose-Einstein
condensation are made up of fermionic electrons and nucleons. In this Letter we
demonstrate how the Pauli exclusion principle for these constituents puts an
upper limit on the Bose-Einstein-condensed fraction. Detailed numerical results
are presented for hydrogen atoms in a cubic volume and for excitons in
semiconductors and semiconductor bilayer systems. The resulting condensate
depletion scales differently from what one expects for bosons with a repulsive
hard-core interaction. At high densities, Pauli exclusion results in
significantly more condensate depletion. These results also shed a new light on
the low condensed fraction in liquid helium II.Comment: 4 pages, 2 figures, revised version, now includes a direct comparison
with hard-sphere QMC results, submitted to Phys. Rev. Let
Integrable two-channel p_x+ip_y-wave superfluid model
We present a new two-channel integrable model describing a system of spinless
fermions interacting through a p-wave Feshbach resonance. Unlike the BCS-BEC
crossover of the s-wave case, the p-wave model has a third order quantum phase
transition. The critical point coincides with the deconfinement of a single
molecule within a BEC of bound dipolar molecules. The exact many-body
wavefunction provides a unique perspective of the quantum critical region
suggesting that the size of the condensate wavefunction, that diverges
logarithmically with the chemical potential, could be used as an experimental
indicator of the phase transition.Comment: 4 pages, 4 figure
Existence of Density Functionals for Excited States and Resonances
We show how every bound state of a finite system of identical fermions,
whether a ground state or an excited one, defines a density functional.
Degeneracies created by a symmetry group can be trivially lifted by a
pseudo-Zeeman effect. When complex scaling can be used to regularize a
resonance into a square integrable state, a DF also exists.Comment: 4 pages, no figure
Ultracold atoms in one-dimensional optical lattices approaching the Tonks-Girardeau regime
Recent experiments on ultracold atomic alkali gases in a one-dimensional
optical lattice have demonstrated the transition from a gas of soft-core bosons
to a Tonks-Girardeau gas in the hard-core limit, where one-dimensional bosons
behave like fermions in many respects. We have studied the underlying many-body
physics through numerical simulations which accommodate both the soft-core and
hard-core limits in one single framework. We find that the Tonks-Girardeau gas
is reached only at the strongest optical lattice potentials. Results for
slightly higher densities, where the gas develops a Mott-like phase already at
weaker optical lattice potentials, show that these Mott-like short range
correlations do not enhance the convergence to the hard-core limit.Comment: 4 pages, 3 figures, replaced with published versio
Integrable models for asymmetric Fermi superfluids: Emergence of a new exotic pairing phase
We introduce an exactly-solvable model to study the competition between the
Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) and breached-pair superfluid in
strongly interacting ultracold asymmetric Fermi gases. One can thus investigate
homogeneous and inhomogeneous states on an equal footing and establish the
quantum phase diagram. For certain values of the filling and the interaction
strength, the model exhibits a new stable exotic pairing phase which combines
an inhomogeneous state with an interior gap to pair-excitations. It is proven
that this phase is the exact ground state in the strong coupling limit, while
numerical examples demonstrate that also at finite interaction strength it can
have lower energy than the breached-pair or LOFF states.Comment: Revised version accepted for publicatio
A quantum Monte-Carlo method for fermions, free of discretization errors
In this work we present a novel quantum Monte-Carlo method for fermions,
based on an exact decomposition of the Boltzmann operator . It
can be seen as a synthesis of several related methods. It has the advantage
that it is free of discretization errors, and applicable to general
interactions, both for ground-state and finite-temperature calculations. The
decomposition is based on low-rank matrices, which allows faster calculations.
As an illustration, the method is applied to an analytically solvable model
(pairing in a degenerate shell) and to the Hubbard model.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
Quantum Monte Carlo simulation in the canonical ensemble at finite temperature
A quantum Monte Carlo method with non-local update scheme is presented. The
method is based on a path-integral decomposition and a worm operator which is
local in imaginary time. It generates states with a fixed number of particles
and respects other exact symmetries. Observables like the equal-time Green's
function can be evaluated in an efficient way. To demonstrate the versatility
of the method, results for the one-dimensional Bose-Hubbard model and a nuclear
pairing model are presented. Within the context of the Bose-Hubbard model the
efficiency of the algorithm is discussed.Comment: 11 pages, 8 figure
Heat capacity and pairing transition in nuclei
A simple model based on the canonical-ensemble theory is outlined for hot
nuclei. The properties of the model are discussed with respect to the Fermi gas
model and the breaking of Cooper pairs. The model describes well the
experimental level density of deformed nuclei in various mass regions. The
origin of the so-called S-shape of the heat capacity curve Cv(T) is discussed.Comment: 6 pages + 8 figure
Exactly solvable pairing Hamiltonian for heavy nuclei
We present a new exactly solvable Hamiltonian with a separable pairing
interaction and non-degenerate single-particle energies. It is derived from the
hyperbolic family of Richardson-Gaudin models and possesses two free
parameters, one related to an interaction cutoff and the other to the pairing
strength. These two parameters can be adjusted to give an excellent
reproduction of Gogny self-consistent mean-field calculations in the canonical
basis.Comment: 4 pages, 3 figure
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