9,951 research outputs found
Decay of super-currents in condensates in optical lattices
In this paper we discuss decay of superfluid currents in boson lattice
systems due to quantum tunneling and thermal activation mechanisms. We derive
asymptotic expressions for the decay rate near the critical current in two
regimes, deep in the superfluid phase and close to the superfluid-Mott
insulator transition. The broadening of the transition at the critical current
due to these decay mechanisms is more pronounced at lower dimensions. We also
find that the crossover temperature below which quantum decay dominates is
experimentally accessible in most cases. Finally, we discuss the dynamics of
the current decay and point out the difference between low and high currents.Comment: Contribution to the special issue of Journal of Superconductivity in
honor of Michael Tinkham's 75th birthda
Dynamical instability of a spin spiral in an interacting Fermi gas as a probe of the Stoner transition
We propose an experiment to probe ferromagnetic phenomena in an ultracold
Fermi gas, while alleviating the sensitivity to three-body loss and competing
many-body instabilities. The system is initialized in a small pitch spin
spiral, which becomes unstable in the presence of repulsive interactions. To
linear order the exponentially growing collective modes exhibit critical
slowing down close to the Stoner transition point. Also, to this order, the
dynamics are identical on the paramagnetic and ferromagnetic sides of the
transition. However, we show that scattering off the exponentially growing
modes qualitatively alters the collective mode structure. The critical slowing
down is eliminated and in its place a new unstable branch develops at large
wave vectors. Furthermore, long-wavelength instabilities are quenched on the
paramagnetic side of the transition. We study the experimental observation of
the instabilities, specifically addressing the trapping geometry and how
phase-contrast imaging will reveal the emerging domain structure. These probes
of the dynamical phenomena could allow experiments to detect the transition
point and distinguish between the paramagnetic and ferromagnetic regimes
Phase Diagram of the Bose-Hubbard Model with T_3 symmetry
In this paper we study the quantum phase transition between the insulating
and the globally coherent superfluid phases in the Bose-Hubbard model with T_3
structure, the "dice lattice". Even in the absence of any frustration the
superfluid phase is characterized by modulation of the order parameter on the
different sublattices of the T_3 structure. The zero-temperature critical point
as a function of a magnetic field shows the characteristic "butterfly" form. At
fully frustration the superfluid region is strongly suppressed. In addition,
due to the existence of the Aharonov-Bohm cages at f=1/2, we find evidence for
the existence of an intermediate insulating phase characterized by a zero
superfluid stiffness but finite compressibility. In this intermediate phase
bosons are localized due to the external frustration and the topology of the
T_3 lattice. We name this new phase the Aharonov-Bohm (AB) insulator. In the
presence of charge frustration the phase diagram acquires the typical
lobe-structure. The form and hierarchy of the Mott insulating states with
fractional fillings, is dictated by the particular topology of the T_3 lattice.
The results presented in this paper were obtained by a variety of analytical
methods: mean-field and variational techniques to approach the phase boundary
from the superconducting side, and a strongly coupled expansion appropriate for
the Mott insulating region. In addition we performed Quantum Monte Carlo
simulations of the corresponding (2+1)D XY model to corroborate the analytical
calculations with a more accurate quantitative analysis. We finally discuss
experimental realization of the T_3 lattice both with optical lattices and with
Josephson junction arrays.Comment: 16 pages, 17 figure
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