37,568 research outputs found
Large-N expansion based on the Hubbard operator path integral representation and its application to the t-J model II. The case for finite
We have introduced a new perturbative approach for model where
Hubbard operators are treated as fundamental objects. Using our vertices and
propagators we have developed a controllable large-N expansion to calculate
different correlation functions. We have investigated charge density-density
response and the phase diagram of the model. The charge correlations functions
are not very sensitive to the value of and they show collective peaks (or
zero sound) which are more pronounced when they are well separated (in energy)
from the particle-hole continuum. For a given a Fermi liquid state is found
to be stable for doping larger than a critical doping .
decreases with decreasing . For the physical region of the
parameters and, for , the system enters in an incommensurate
flux or DDW phase. The inclusion of the nearest-neighbors Coulomb repulsion
leads to a CDW phase when is larger than a critical value . The
dependence of with and is shown. We have compared the
results with other ones in the literature.Comment: 10 pages, 8 figures, to appear in Phys. Rev.
Increasing the coherence time of Bose-Einstein-condensate interferometers with optical control of dynamics
Atom interferometers using Bose-Einstein condensate that is confined in a
waveguide and manipulated by optical pulses have been limited by their short
coherence times. We present a theoretical model that offers a physically simple
explanation for the loss of contrast and propose the method for increasing the
fringe contrast by recombining the atoms at a different time. A simple,
quantitatively accurate, analytical expression for the optimized recombination
time is presented and used to place limits on the physical parameters for which
the contrast may be recovered.Comment: 34 Pages, 8 Figure
Undamped nonequilibrium dynamics of a nondegenerate Bose gas in a 3D isotropic trap
We investigate anomalous damping of the monopole mode of a non-degenerate 3D
Bose gas under isotropic harmonic confinement as recently reported by the JILA
TOP trap experiment [D. S. Lob- ser, A. E. S. Barentine, E. A. Cornell, and H.
J. Lewandowski (in preparation)]. Given a realistic confining potential, we
develop a model for studying collective modes that includes the effects of
anharmonic corrections to a harmonic potential. By studying the influence of
these trap anharmonicities throughout a range of temperatures and collisional
regimes, we find that the damping is caused by the joint mechanisms of
dephasing and collisional relaxation. Furthermore, the model is complimented by
Monte Carlo simulations which are in fair agreement with data from the JILA
experiment.Comment: 11 pages, 6 figure
Phase separation in a boson-fermion mixture of Lithium atoms
We use a semiclassical three-fluid model to analyze the conditions for
spatial phase separation in a mixture of fermionic Li-6 and a (stable)
Bose-Einstein condensate of Li-7 atoms under cylindrical harmonic confinement,
both at zero and finite temperature. We show that with the parameters of the
Paris experiment [F. Schrek et al., Phys. Rev. Lett. 87 080403 (2001)] an
increase of the boson-fermion scattering length by a factor five would be
sufficient to enter the phase-separated regime. We give examples of
configurations for the density profiles in phase separation and estimate that
the transition should persist at temperatures typical of current experiments.
For higher values of the boson-fermion coupling we also find a new phase
separation between the fermions and the bosonic thermal cloud at finite
temperature.Comment: 8 pages, 4 figures, new version of Fig. 4 and typos correcte
A Compact Microchip-Based Atomic Clock Based on Ultracold Trapped Rb Atoms
We propose a compact atomic clock based on ultracold Rb atoms that are
magnetically trapped near the surface of an atom microchip. An interrogation
scheme that combines electromagnetically-induced transparency (EIT) with
Ramsey's method of separated oscillatory fields can achieve atomic shot-noise
level performance of 10^{-13}/sqrt(tau) for 10^6 atoms. The EIT signal can be
detected with a heterodyne technique that provides noiseless gain; with this
technique the optical phase shift of a 100 pW probe beam can be detected at the
photon shot-noise level. Numerical calculations of the density matrix equations
are used to identify realistic operating parameters at which AC Stark shifts
are eliminated. By considering fluctuations in these parameters, we estimate
that AC Stark shifts can be canceled to a level better than 2*10^{-14}. An
overview of the apparatus is presented with estimates of duty cycle and power
consumption.Comment: 15 pages, 11 figures, 5 table
- …