7,298 research outputs found
Temperature of a Decoherent Oscillator with Strong Coupling
The temperature of an oscillator coupled to the vacuum state of a heat bath
via ohmic coupling is non-zero, as measured by the reduced density matrix of
the oscillator. This paper shows that the actual temperature, as measured by a
thermometer is still zero (or in the thermal state of the bath, the temperature
of the bath). The decoherence temperature is due to "false-decoherence", with
the heat bath state being dragged along with the oscillator.Comment: 6 page
Transition from Band insulator to Bose-Einstein Condensate superfluid and Mott State of Cold Fermi Gases with Multiband Effects in Optical Lattices
We study two models realized by two-component Fermi gases loaded in optical
lattices. We clarify that multi-band effects inevitably caused by the optical
lattices generate a rich structure, when the systems crossover from the region
of weakly bound molecular bosons to the region of strongly bound atomic bosons.
Here the crossover can be controlled by attractive fermion interaction. One of
the present models is a case with attractive fermion interaction, where an
insulator-superfluid transition takes place. The transition is characterized as
the transition between a band insulator and a Bose-Einstein condensate (BEC)
superfluid state. Differing from the conventional BCS superfluid transition,
this transition shows unconventional properties. In contrast to the one
particle excitation gap scaled by the superfluid order parameter in the
conventional BCS transition, because of the multi-band effects, a large gap of
one-particle density of states is retained all through the transition although
the superfluid order grows continuously from zero. A reentrant transition with
lowering temperature is another unconventionality. The other model is the case
with coexisting attractive and repulsive interactions. Within a mean field
treatment, we find a new insulating state, an orbital ordered insulator. This
insulator is one candidate for the Mott insulator of molecular bosons and is
the first example that the orbital internal degrees of freedom of molecular
bosons appears explicitly. Besides the emergence of a new phase, a coexisting
phase also appears where superfluidity and an orbital order coexist just by
doping holes or particles. The insulating and superfluid particles show
differentiation in momentum space as in the high-Tc cuprate superconductors.Comment: 13 pages, 10 figure
Universal Properties of the Ultra-Cold Fermi Gas
We present some general considerations on the properties of a two-component
ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the
interaction energy and the ground state energy can be written in terms of a
single dimensionless function , where and
. The function incorporates all the many-body physics
and naturally occurs in other physical quantities as well. In particular, we
show that the RF-spectroscopy shift \bar{\d\o}(\xi,\tau) and the molecular
fraction in the closed channel can be expressed in terms of
and thus have identical temperature dependence. The conclusions
should have testable consequences in future experiments
BEC-BCS Crossover with Feshbach Resonance for a Three-Hyperfine-Species Model
We consider the behavior of an ultracold Fermi gas across a narrow Feshbach
resonance, where the occupation of the closed channel may not be negligible.
While the corrections to the single-channel formulae associated with the
nonzero chemical potential and with particle conservation have been considered
in the existing literature, there is a further effect, namely the
"inter-channel Pauli exclusion principle" associated with the fact that a
single hyperfine species may be common to the two channels. We focus on this
effect and show that, as intuitively expected, the resulting corrections are of
order , where is the Fermi energy of the gas in the absence of
interactions and is the Zeeman energy difference between the two
channels. We also consider the related corrections to the fermionic excitation
spectrum, and briefly discuss the collective modes of the system
Laser cooling all the way down to molecular condensate
Numerical simulations show that laser cooling of fermions on the repulsive
side of the Feshbach resonance can sympathetically cool molecules well below
their condensation temperature.Comment: 7 pages, 2 .eps figure
Tunneling out of metastable vacuum in a system consisting of two capacitively coupled phase qubits
Using a powerful combination of Coleman's instanton technique and the method
of Banks and Bender, the exponential factor for the zero temperature rate of
tunneling out of metastable vacuum in a system of two identical capacitively
coupled phase qubits is calculated in closed form to second order in asymmetry
parameter for a special case of intermediate coupling C=C_J/2.Comment: 10 pages, 5 figures (select PostScript to download Fig. 1). Corrected
version, to appear in PR
Mott states under the influence of fermion-boson conversion: invasion of superfluidity
I study the influence of fermion-boson conversion near Feshbach resonances on
Mott states of Cooper pairs and demonstrate possible invasion of superfluidity.
The quantum dynamics of Fermi-Bose gases is studied using both an effective
coupled quantum rotor Hamiltonian and a coupled XXZ
XXZ spin Hamiltonian. I also point out two distinct branches of
collective modes in superfluid states, one of which involves anti-symmetric
phase oscillations in fermionic and bosonic channels and is {\em always} gapped
because of fermion-boson conversion.Comment: 5 pages; typos correcte
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
Surface-enhanced pair transfer in quadrupole states of neutron-rich Sn isotopes
We investigate the neutron pair transfer modes associated with the low-lying
quadrupole states in neutron-rich Sn isotopes by means of the quasiparticle
random phase approximation based on the Skyrme-Hartree-Fock-Bogoliubov mean
field model. The transition strength of the quadrupole pair-addition mode
feeding the state is enhanced in the Sn isotopes with . The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to fm. The quadrupole
pair-addition mode reflects sensitively a possible increase of the effective
pairing interaction strength in the surface and exterior regions of
neutron-rich nuclei.Comment: 14 page
Comment on ``Phase and Phase Diffusion of a Split Bose-Einstein Condensate''
Recently Javanainen and Wilkens [Phys. Rev. Lett. 78, 4675 (1997)] have
analysed an experiment in which an interacting Bose condensate, after being
allowed to form in a single potential well, is "cut" by splitting the well
adiabatically with a very high potential barrier, and estimate the rate at
which, following the cut, the two halves of the condensate lose the "memory" of
their relative phase. We argue that, by neglecting the effect of interactions
in the initial state before the separation, they have overestimated the rate of
phase randomization by a numerical factor which grows with the interaction
strength and with the slowness of the separation process.Comment: 2 pages, no figures, to appear in Phys. Rev. Let
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