8,128 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

### 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 $h({\xi,\tau})$, where $\xi=-(k_Fa_s)^{-1}$ and
$\tau=T/T_F$. The function $h(\xi,\tau)$ 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 $f_c(\xi,\tau)$ in the closed channel can be expressed in terms of
$h(\xi,\tau)$ and thus have identical temperature dependence. The conclusions
should have testable consequences in future experiments

### 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

### 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 $E_F/\eta$, where $E_F$ is the Fermi energy of the gas in the absence of
interactions and $\eta$ 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 $U(1)\otimes U(1)$ quantum rotor Hamiltonian and a coupled XXZ
$\otimes$ 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

### 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 $2_1^+$ state is enhanced in the Sn isotopes with $A \geq 132$. The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to $r\sim 12-13$ 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

### "Gray" BCS condensate of excitons and internal Josephson effect

It has been recently suggested that the Bose-Einstein condensate formed by
excitons in the dilute limit must be dark, i.e., not coupled to photons. Here,
we show that, under a density increase, the dark exciton condensate must
acquire a bright component due to carrier exchange in which dark excitons turn
bright. This however requires a density larger than a threshold which seems to
fall in the forbidden region of the phase separation between a dilute exciton
gas and a dense electron-hole plasma. The BCS-like condensation which is likely
to take place on the dense side, must then have a dark and a bright component -
which makes it "gray". It should be possible to induce an internal Josephson
effect between these two coherent components, with oscillations of the
photoluminescence as a strong proof of the existence for this "gray" BCS-like
exciton condensate.Comment: 4 pages, typo correcte

### BCS-BEC crossover in a gas of Fermi atoms with a p-wave Feshbach resonance

We investigate unconventional superfluidity in a gas of Fermi atoms with an
anisotropic p-wave Feshbach resonance. Including the p-wave Feshbach resonance
as well as the associated three kinds of quasi-molecules with finite orbital
angular momenta $L_z=\pm1,0$, we calculate the transition temperature of the
superfluid phase. As one passes through the p-wave Feshbach resonance, we find
the usual BCS-BEC crossover phenomenon. The p-wave BCS state continuously
changes into the BEC of bound molecules with L=1. Our calculation includes the
effect of fluctuations associated with Cooper-pairs and molecules which are not
Bose-condensed.Comment: 9 pages, 3 figures, 1 tabl

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