451 research outputs found
Exotic magnetic orders for high spin ultracold fermions
We study Hubbard models for ultracold bosonic or fermionic atoms loaded into
an optical lattice. The atoms carry a high spin , and interact on site
via strong repulsive Van der Waals forces. Making convenient rearrangements of
the interaction terms, and exploiting their symmetry properties, we derive low
energy effective models with nearest-neighbor interactions, and their
properties. We apply our method to , and 5/2 fermions on two-dimensional
square lattice at quarter, and 1/6 fillings, respectively, and investigate
mean-field equations for repulsive couplings. We find for fermions that
the plaquette state appearing in the highly symmetric SU(4) case does not
require fine tuning, and is stable in an extended region of the phase diagram.
This phase competes with an SU(2) flux state, that is always suppressed for
repulsive interactions in absence of external magnetic field. The SU(2) flux
state has, however, lower energy than the plaquette phase, and stabilizes in
the presence of weak applied magnetic field. For fermions a similar
SU(2) plaquette phase is found to be the ground state without external magnetic
field.Comment: final version, 6 pages, 4 figures, epl forma
Spin squeezing of high-spin, spatially extended quantum fields
Investigations of spin squeezing in ensembles of quantum particles have been
limited primarily to a subspace of spin fluctuations and a single spatial mode
in high-spin and spatially extended ensembles. Here, we show that a wider range
of spin-squeezing is attainable in ensembles of high-spin atoms, characterized
by sub-quantum-limited fluctuations in several independent planes of
spin-fluctuation observables. Further, considering the quantum dynamics of an
ferromagnetic spinor Bose-Einstein condensate, we demonstrate
theoretically that a high degree of spin squeezing is attained in multiple
spatial modes of a spatially extended quantum field, and that such squeezing
can be extracted from spatially resolved measurements of magnetization and
nematicity, i.e.\ the vector and quadrupole magnetic moments, of the quantum
gas. Taking into account several experimental limitations, we predict that the
variance of the atomic magnetization and nematicity may be reduced as far as 20
dB below the standard quantum limits.Comment: 18 pages, 5 figure
How to fix a broken symmetry: Quantum dynamics of symmetry restoration in a ferromagnetic Bose-Einstein condensate
We discuss the dynamics of a quantum phase transition in a spin-1
Bose-Einstein condensate when it is driven from the magnetized
broken-symmetry phase to the unmagnetized ``symmetric'' polar phase. We
determine where the condensate goes out of equilibrium as it approaches the
critical point, and compute the condensate magnetization at the critical point.
This is done within a quantum Kibble-Zurek scheme traditionally employed in the
context of symmetry-breaking quantum phase transitions. Then we study the
influence of the nonequilibrium dynamics near a critical point on the
condensate magnetization. In particular, when the quench stops at the critical
point, nonlinear oscillations of magnetization occur. They are characterized by
a period and an amplitude that are inversely proportional. If we keep driving
the condensate far away from the critical point through the unmagnetized
``symmetric'' polar phase, the amplitude of magnetization oscillations slowly
decreases reaching a non-zero asymptotic value. That process is described by
the equation that can be mapped onto the classical mechanical problem of a
particle moving under the influence of harmonic and ``anti-friction'' forces
whose interplay leads to surprisingly simple fixed-amplitude oscillations. We
obtain several scaling results relating the condensate magnetization to the
quench rate, and verify numerically all analytical predictions.Comment: 15 pages, 11 figures, final version accepted in NJP (slight changes
with respect to the former submission
Mapping giant magnetic fields around dense solid plasmas by high resolution magneto-optical microscopy
We investigate distribution of magnetic fields around dense solid plasmas
generated by intense p-polarized laser (~10^{16} W.cm^{-2}, 100 fs) irradiation
of magnetic tapes, using high sensitivity magneto optical microscopy. We
present evidence for giant axial magnetic fields and map out for the first time
the spatial distribution of these fields. By using the axial magnetic field
distribution as a diagnostic tool we uncover evidence for angular momentum
associated with the plasma. We believe this study holds significance for
investigating the process under which a magnetic material magnetizes or
demagnetizes under the influence of ultrashort intense laser pulses.Comment: 17 pages of text with 4 figure
Strongly enhanced inelastic collisions in a Bose-Einstein condensate near Feshbach resonances
The properties of Bose-Einstein condensed gases can be strongly altered by
tuning the external magnetic field near a Feshbach resonance. Feshbach
resonances affect elastic collisions and lead to the observed modification of
the scattering length. However, as we report here, this is accompanied by a
strong increase in the rate of inelastic collisions. The observed three-body
loss rate in a sodium Bose-Einstein condensation increased when the scattering
length was tuned to both larger or smaller values than the off-resonant value.
This observation and the maximum measured increase of the loss rate by several
orders of magnitude are not accounted for by theoretical treatments. The strong
losses impose severe limitations for using Feshbach resonances to tune the
properties of Bose-Einstein condensates. A new Feshbach resonance in sodium at
1195 G was observed.Comment: 4 pages, 3 figure
Ground state energy of the spinor Bose-Einstein condensates
We calculate, in the standard Bogoliubov approximation, the ground state
energy of the spinor BEC with hyperfine spin where the two-body repulsive
hard-core and spin exchange interactions are both included. The coupling
constants characterized these two competing interactions are expressed in terms
of the corresponding s-wave scattering lengths using second-order perturbation
methods. We show that the ultraviolet divergence arising in the ground state
energy corrections can be exactly eliminated.Comment: 14 pages, no figures, submitted to PR
Periodically-dressed Bose-Einstein condensates: a superfluid with an anisotropic and variable critical velocity
Two intersecting laser beams can produce a spatially-periodic coupling
between two components of an atomic gas and thereby modify the dispersion
relation of the gas according to a dressed-state formalism. Properties of a
Bose-Einstein condensate of such a gas are strongly affected by this
modification. A Bogoliubov transformation is presented which accounts for
interparticle interactions to obtain the quasiparticle excitation spectrum in
such a condensate. The Landau critical velocity is found to be anisotropic and
can be widely tuned by varying properties of the dressing laser beams.Comment: 5 pages, 4 figure
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