1,289 research outputs found
Spatial separation in a thermal mixture of ultracold Yb and Rb atoms
We report on the observation of unusually strong interactions in a thermal
mixture of ultracold atoms which cause a significant modification of the
spatial distribution. A mixture of Rb and Yb with a temperature
of a few K is prepared in a hybrid trap consisting of a bichromatic
optical potential superimposed on a magnetic trap. For suitable trap parameters
and temperatures, a spatial separation of the two species is observed. We infer
that the separation is driven by a large interaction strength between
Yb and Rb accompanied by a large three-body recombination rate.
Based on this assumption we have developed a diffusion model which reproduces
our observations
A Q-Ising model application for linear-time image segmentation
A computational method is presented which efficiently segments digital
grayscale images by directly applying the Q-state Ising (or Potts) model. Since
the Potts model was first proposed in 1952, physicists have studied lattice
models to gain deep insights into magnetism and other disordered systems. For
some time, researchers have realized that digital images may be modeled in much
the same way as these physical systems (i.e., as a square lattice of numerical
values). A major drawback in using Potts model methods for image segmentation
is that, with conventional methods, it processes in exponential time. Advances
have been made via certain approximations to reduce the segmentation process to
power-law time. However, in many applications (such as for sonar imagery),
real-time processing requires much greater efficiency. This article contains a
description of an energy minimization technique that applies four Potts
(Q-Ising) models directly to the image and processes in linear time. The result
is analogous to partitioning the system into regions of four classes of
magnetism. This direct Potts segmentation technique is demonstrated on
photographic, medical, and acoustic images.Comment: 7 pages, 8 figures, revtex, uses subfigure.sty. Central European
Journal of Physics, in press (2010
Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals
A theoretical study of the magnon-drag Peltier and Seebeck effects in
ferromagnetic metals is presented. A magnon heat current is described
perturbatively from the microscopic viewpoint with respect to electron--magnon
interactions and the electric field. Then, the magnon-drag Peltier coefficient
\Pi_\MAG is obtained as the ratio between the magnon heat current and the
electric charge current. We show that \Pi_\MAG=C_\MAG T^{5/2} at a low
temperature ; that the coefficient C_\MAG is proportional to the spin
polarization of the electric conductivity; and that for C_\MAG<0,
but . From experimental results for magnon-drag Peltier
effects, we estimate that the strength of the electron--magnon interaction is
about 0.3 eV for permalloy.Comment: 3 pages, 2 figures, accepted for publication in Journal of the
Physical Society of Japa
Observation of a Phase Transition at 55 K in Single-Crystal CaCu1.7As2
We present the structural, magnetic, thermal and ab-plane electronic
transport properties of single crystals of CaCu1.7As2 grown by the self-flux
technique that were investigated by powder x-ray diffraction, magnetic
susceptibility chi, isothermal magnetization M, specific heat Cp, and
electrical resistivity rho measurements as a function of temperature T and
magnetic field H. X-ray diffraction analysis of crushed crystals at room
temperature confirm the collapsed tetragonal ThCr2Si2-type structure with \sim
15% vacancies on the Cu sites as previously reported, corresponding to the
composition CaCu1.7As2. The chi(T) data are diamagnetic, anisotropic and nearly
independent of T. The chi is larger in the ab-plane than along the c-axis, as
also observed previously for SrCu2As2 and for pure and doped BaFe2As2. The
Cp(T) and rho(T) data indicate metallic sp-band character. In contrast to the
chi(T) and Cp(T) data that do not show any evidence for phase transitions below
300 K, the rho(T) data exhibit a sharp increase in slope on cooling below a
temperature Tt = 54-56 K, depending on the crystal. The \rho(T) data show no
hysteresis on warming and cooling through Tt and the transition appears to be
second order. The phase transition may arise from spatial ordering of the
vacancies on the Cu sublattice. The Tt is found to be independent of H for H
\leq 8 T. A positive magnetoresistance is observed below Tt that increases with
decreasing T and attains a value in H = 8.0 T of 8.7% at T = 1.8 K.Comment: 9 pages, 7 figures, 4 tables, 38 references; v2: Added discussion of
Fe vacancies in AFe2Se2 compounds. Added new resistivity data for CaCu1.7As2
and new discussion of them; v3: minor changes, published versio
Cooling the Collective Motion of Trapped Ions to Initialize a Quantum Register
We report preparation in the ground state of collective modes of motion of
two trapped 9Be+ ions. This is a crucial step towards realizing quantum logic
gates which can entangle the ions' internal electronic states. We find that
heating of the modes of relative ion motion is substantially suppressed
relative to that of the center-of-mass modes, suggesting the importance of
these modes in future experiments.Comment: 5 pages, including 3 figures. RevTeX. PDF and PostScript available at
http://www.bldrdoc.gov/timefreq/ion/qucomp/papers.htm . final (published)
version. Eq. 1 and Table 1 slightly different from original submissio
Thermoelectric Effects in Magnetic Nanostructures
We model and evaluate the Peltier and Seebeck effects in magnetic multilayer
nanostructures by a finite-element theory of thermoelectric properties. We
present analytical expressions for the thermopower and the current-induced
temperature changes due to Peltier cooling/heating. The thermopower of a
magnetic element is in general spin-polarized, leading to spin-heat coupling
effects. Thermoelectric effects in spin valves depend on the relative alignment
of the magnetization directions and are sensitive to spin-flip scattering as
well as inelastic collisions in the normal metal spacer.Comment: 14 pages, 7 figures, 1 table. Publishe
Quantum state engineering on an optical transition and decoherence in a Paul trap
A single Ca+ ion in a Paul trap has been cooled to the ground state of
vibration with up to 99.9% probability. Starting from this Fock state |n=0> we
have demonstrated coherent quantum state manipulation on an optical transition.
Up to 30 Rabi oscillations within 1.4 ms have been observed. We find a similar
number of Rabi oscillations after preparation of the ion in the |n=1> Fock
state. The coherence of optical state manipulation is only limited by laser and
ambient magnetic field fluctuations. Motional heating has been measured to be
as low as one vibrational quantum in 190 ms.Comment: 4 pages, 5 figure
The NN scattering 3S1-3D1 mixing angle at NNLO
The 3S1-3D1 mixing angle for nucleon-nucleon scattering, epsilon_1, is
calculated to next-to-next-to-leading order in an effective field theory with
perturbative pions. Without pions, the low energy theory fits the observed
epsilon_1 well for momenta less than MeV. Including pions
perturbatively significantly improves the agreement with data for momenta up to
MeV with one less parameter. Furthermore, for these momenta the
accuracy of our calculation is similar to an effective field theory calculation
in which the pion is treated non-perturbatively. This gives phenomenological
support for a perturbative treatment of pions in low energy two-nucleon
processes. We explain why it is necessary to perform spin and isospin traces in
d dimensions when regulating divergences with dimensional regularization in
higher partial wave amplitudes.Comment: 17 pages, journal versio
Superfluidity and binary-correlations within clusters of fermions
We propose a method for simulating the behaviour of small clusters of
particles that explicitly accounts for all mean-field and binary-correlation
effects. Our approach leads to a set of variational equations that can be used
to study both the dynamics and thermodynamics of these clusters. As an
illustration of this method, we explore the BCS-BEC crossover in the simple
model of four fermions, interacting with finite-range potentials, in a harmonic
potential. We find, in the crossover regime, that the particles prefer to
occupy two distinct pair states as opposed to the one assumed by BCS theory
Phase transitions in the boson-fermion resonance model in one dimension
We study 1D fermions with photoassociation or with a narrow Fano-Feshbach
resonance described by the Boson-Fermion resonance model. Using thebosonization
technique, we derive a low-energy Hamiltonian of the system. We show that at
low energy, the order parameters for the Bose Condensation and fermion
superfluidity become identical, while a spin gap and a gap against the
formation of phase slips are formed. As a result of these gaps, charge density
wave correlations decay exponentially in contrast with the phases where only
bosons or only fermions are present. We find a Luther-Emery point where the
phase slips and the spin excitations can be described in terms of
pseudofermions. This allows us to provide closed form expressions of the
density-density correlations and the spectral functions. The spectral functions
of the fermions are gapped, whereas the spectral functions of the bosons remain
gapless. The application of a magnetic field results in a loss of coherence
between the bosons and the fermion and the disappearance of the gap. Changing
the detuning has no effect on the gap until either the fermion or the boson
density is reduced to zero. Finally, we discuss the formation of a Mott
insulating state in a periodic potential. The relevance of our results for
experiments with ultracold atomic gases subject to one-dimensional confinement
is also discussed.Comment: 31 pages, 8 EPS figures, RevTeX 4, long version of cond-mat/050570
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