840 research outputs found
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
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
Solving the Richardson equations for Fermions
Forty years ago Richardson showed that the eigenstates of the pairing
Hamiltonian with constant interaction strength can be calculated by solving a
set of non-linear coupled equations. However, in the case of Fermions these
equations lead to singularities which made them very hard to solve. This letter
explains how these singularities can be avoided through a change of variables
making the Fermionic pairing problem numerically solvable for arbitrary single
particle energies and degeneracies.Comment: 5 pages, 4 figures, submitted to Phys.Rev.
A solvable model of a one-dimensional quantum gas with pair interaction
We propose a solvable model of a one-dimensional harmonic oscillator quantum
gas of two sorts of particles, fermions or bosons, which allows to describe the
formation of pairs due to a suitable pair interaction. These pairs we call
"pseudo-bosons" since the system can be approximated by an ideal bose gas for
low temperatures. We illustrate this fact by considering the specific heat and
the entropy function for N=8 pairs. The model can also be evaluated in the
thermodynamic limit if the harmonic oscillator potential is suitable scaled
The effect of a multispecies probiotic on the composition of the faecal microbiota and bowel habits in chronic obstructive pulmonary disease patients treated with antibiotics
Short-term antibiotic treatment profoundly affects the intestinal microbiota, which may lead to sustained changes in microbiota composition. Probiotics may restore such a disturbance. The objective of the present study was to investigate the effect of a multispecies probiotic on the faecal microbiota during and after antibiotic intake in patients with a history of frequent antibiotic use. In this randomised, placebo-controlled, double-blind study, thirty chronic obstructive pulmonary disease (COPD) patients treated with antibiotics for a respiratory tract infection received 5 g of a multispecies probiotic or placebo twice daily for 2 weeks. Faecal samples were collected at 0, 7, 14 and 63 d. Changes in the composition of the dominant faecal microbiota were determined by PCR-denaturing gradient gel electrophoresis (DGGE). Changes in bacterial subgroups were determined by quantitative PCR and culture. Bowel movements were scored daily according to the Bristol stool form scale. During and after antibiotic treatment, DGGE-based similarity indices (SI) were high ( >/= 84 %) and band richness was relatively low, both remaining stable over time. No difference in SI was observed between patients with and without diarrhoea-like bowel movements. The multispecies probiotic had a modest effect on the bacterial subgroups. Nevertheless, it affected neither the composition of the dominant faecal microbiota nor the occurrence of diarrhoea-like bowel movements. The dominant faecal microbiota was not affected by antibiotics in this COPD population, suggesting an existing imbalance of the microbiota, which may also have contributed to the lack of effect by probiotic intak
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
Optimal Monte Carlo Updating
Based on Peskun's theorem it is shown that optimal transition matrices in
Markov chain Monte Carlo should have zero diagonal elements except for the
diagonal element corresponding to the largest weight. We will compare the
statistical efficiency of this sampler to existing algorithms, such as
heat-bath updating and the Metropolis algorithm. We provide numerical results
for the Potts model as an application in classical physics. As an application
in quantum physics we consider the spin 3/2 XY model and the Bose-Hubbard model
which have been simulated by the directed loop algorithm in the stochastic
series expansion framework.Comment: 6 pages, 5 figures, replaced with published versio
General Aviation Trends in the European Market Memorandum Report
In 1969, as western Europe was becoming a common market, significant economic growth was projected. His Highness, The Aga Kahn requested that Economics Research Associates (ERA) perform a memorandum report and forecast the trends for private aviation potentials in Europe over the decade of the 1970s. The resulting report by ERA reviews the history of European aviation, current and future economic trends in the common European market area as these pertain to the growth of private aviation, advantages and disadvantages involved in noncommercial flying, and future trends in private aviation. Data was gathered from such sources as aviation publications, interviews with persons connected with private aviation, and market studies and projections of commercial organizations. The report includes the following sections: 1) introduction, 2) summary and conclusions, 3) the western European economy, 4) general aviation, 5) general aviation and future trends in aviation design, 6) impact of general aviation on tourism. The report includes four tables
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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