7,472 research outputs found
Finite temperature phase diagram of a polarized Fermi gas in an optical lattice
We present phase diagrams for a polarized Fermi gas in an optical lattice as
a function of temperature, polarization, and lattice filling factor. We
consider the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma or breached pair
(BP), and BCS phases, and the normal state and phase separation. We show that
the FFLO phase appears in a considerable portion of the phase diagram. The
diagrams have two critical points of different nature. We show how various
phases leave clear signatures to momentum distributions of the atoms which can
be observed after time of flight expansion.Comment: Journal versio
Exotic superfluid states of lattice fermions in elongated traps
We present real-space dynamical mean-field theory calculations for
attractively interacting fermions in three-dimensional lattices with elongated
traps. The critical polarization is found to be 0.8, regardless of the trap
elongation. Below the critical polarization, we find unconventional superfluid
structures where the polarized superfluid and
Fulde-Ferrell-Larkin-Ovchinnikov-type states emerge across the entire core
region
From Trapped Atoms to Liberated Quarks
We discuss some aspects of cold atomic gases in the unitarity limit that are
of interest in connection with the physics of dense hadronic matter. We
consider, in particular, the equation of state at zero temperature, the
magnitude of the pairing gap, and the phase diagram at non-zero polarization.Comment: 13 pages, 5 figures; to appear in the proceedings of the
International Symposium on Heavy Ion Physics 2006, Frankfurt, Germany;
International Journal of Modern Physics E, in pres
Local electronic structure near oxygen dopants in BSCCO-2212: a window on the high-Tc pair mechanism?
The cuprate material BSCCO-2212 is believed to be doped by a combination of
cation switching and excess oxygen. The interstitial oxygen dopants are of
particular interest because scanning tunnelling microscopy (STM) experiments
have shown that they are positively correlated with the local value of the
superconducting gap, and calculations suggest that the fundamental attraction
between electrons is modulated locally. In this work, we use density functional
theory to try to ascertain which locations in the crystal are energetically
most favorable for the O dopant atoms, and how the surrounding cage of atoms
deforms. Our results provide support for the identification of STM resonances
at -1eV with dopant interstitial O atoms, and show how the local electronic
structure is modified nearby.Comment: 5 pages, 3 figure
Shot-noise-driven escape in hysteretic Josephson junctions
We have measured the influence of shot noise on hysteretic Josephson
junctions initially in macroscopic quantum tunnelling (MQT) regime. Escape
threshold current into the resistive state decreases monotonically with
increasing average current through the scattering conductor, which is another
tunnel junction. Escape is predominantly determined by excitation due to the
wide-band shot noise. This process is equivalent to thermal activation (TA)
over the barrier at temperatures up to about four times above the critical
temperature of the superconductor. The presented TA model is in excellent
agreement with the experimental results
Stability of the Breached Pair State for a Two-species Fermionic System in the Presence of Feshbach Resonance
We investigate the phenomenon of fermionic pairing with mismatched Fermi
surfaces in a two-species system in the presence of Feshbach resonance, where
the resonantly-paired fermions combine to form bosonic molecules. We observe
that the Feshbach parameters control the critical temperature of the gapped BCS
superfluid state, and also determine the range over which a gapless breached
pair state may exist. Demanding the positivity of the superfluid density, it is
shown that although a breached pair state with two Fermi surfaces is always
unstable, its single Fermi-surface counterpart can be stable if the chemical
potentials of the two pairing species have opposite signs. This condition is
satisfied only over a narrow region in the BEC side, characterized by an upper
and a lower limit for the magnetic field. We estimate these limits for a
mixture of two hyperfine states of Li using recent experimental data.Comment: 14 pages,5 figure
Effect of dopant atoms on local superexchange in cuprate superconductors: a perturbative treatment
Recent scanning tunneling spectroscopy experiments have provided evidence
that dopant impurities in high- Tc superconductors can strongly modify the
electronic structure of the CuO2 planes nearby, and possibly influence the
pairing. To investigate this connection, we calculate the local magnetic
superexchange J between Cu ions in the presence of dopants within the framework
of the three-band Hubbard model, up to fifth-order in perturbation theory. We
demonstrate that the sign of the change in J depends on the relative
dopant-induced spatial variation of the atomic levels in the CuO2 plane,
contrary to results obtained within the one-band Hubbard model. We discuss some
realistic cases and their relevance for theories of the pairing mechanism in
the cupratesComment: 5 pages, 4 figures, revised versio
Itinerant ferromagnetism in an interacting Fermi gas with mass imbalance
We study the emergence of itinerant ferromagnetism in an ultra-cold atomic
gas with a variable mass ratio between the up and down spin species. Mass
imbalance breaks the SU(2) spin symmetry leading to a modified Stoner
criterion. We first elucidate the phase behavior in both the grand canonical
and canonical ensembles. Secondly, we apply the formalism to a harmonic trap to
demonstrate how a mass imbalance delivers unique experimental signatures of
ferromagnetism. These could help future experiments to better identify the
putative ferromagnetic state. Furthermore, we highlight how a mass imbalance
suppresses the three-body loss processes that handicap the formation of a
ferromagnetic state. Finally, we study the time dependent formation of the
ferromagnetic phase following a quench in the interaction strength
Decoherence processes in a current biased dc SQUID
A current bias dc SQUID behaves as an anharmonic quantum oscillator
controlled by a bias current and an applied magnetic flux. We consider here its
two level limit consisting of the two lower energy states | 0 \right> and |
1 \right>. We have measured energy relaxation times and microwave absorption
for different bias currents and fluxes in the low microwave power limit.
Decoherence times are extracted. The low frequency flux and current noise have
been measured independently by analyzing the probability of current switching
from the superconducting to the finite voltage state, as a function of applied
flux. The high frequency part of the current noise is derived from the
electromagnetic environment of the circuit. The decoherence of this quantum
circuit can be fully accounted by these current and flux noise sources.Comment: 4 pages, 4 figure
Near-Zero Modes in Superconducting Graphene
Vortices in the simplest superconducting state of graphene contain very low
energy excitations, whose existence is connected to an index theorem that
applies strictly to an approximate form of the relevant Bogoliubov-deGennes
equations. When Zeeman interactions are taken into account, the zero modes
required by the index theorem are (slightly) displaced. Thus the vortices
acquire internal structure, that plausibly supports interesting dynamical
phenomena.Comment: 9 pages, to appear in Proceedings of the Nobel Symposium on Graphene
and Quantum Matte
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