475 research outputs found
Localized collapse and revival of coherence in an ultracold Bose gas
We study the collapse and revival of coherence induced by dipolar spin waves
in a trapped gas of Rb-87 atoms. In particular we observe spatially localized
collapse and revival of Ramsey fringe contrast and show how the pattern of
coherence depends on strength of the spin wave excitation. We find that the
spatial character of the coherence dynamics is incompatible with a simple model
based only on position-space overlap of wave functions. This phenomenon
requires a full phase-space description of the atomic spin using a quantum
Boltzmann transport equation, which highlights spin wave-induced coherent spin
currents and the ensuing dynamics they drive.Comment: 5 pages, 4 figure
The Antiferromagnetic Heisenberg Model on Fullerene-Type Symmetry Clusters
The nearest neighbor antiferromagnetic Heisenberg model is
considered for spins sitting on the vertices of clusters with the connectivity
of fullerene molecules and a number of sites ranging from 24 to 32. Using
the permutational and spin inversion symmetries of the Hamiltonian the low
energy spectrum is calculated for all the irreducible representations of the
symmetry group of each cluster. Frustration and connectivity result in
non-trivial low energy properties, with the lowest excited states being
singlets except for . Same hexagon and same pentagon correlations are the
most effective in the minimization of the energy, with the
symmetry cluster having an unusually strong singlet intra-pentagon correlation.
The magnetization in a field shows no discontinuities unlike the icosahedral
fullerene clusters, but only plateaux with the most pronounced for
. The spatial symmetry as well as the connectivity of the clusters appear
to be important for the determination of their magnetic properties.Comment: Extended to include low energy spectra, correlation functions and
magnetization data of clusters up to 32 site
A Schwinger-boson approach to the kagome with Dzyaloshinskii-Moriya interactions: phase diagram and dynamical structure factors
We have obtained the zero-temperature phase diagram of the kagome
antiferromagnet with Dzyaloshinskii-Moriya interactions in Schwinger-boson
mean-field theory. We find quantum phase transitions (first or second order)
between different topological spin liquids and Neel ordered phases (either the
state or the so-called Q=0 state). In the regime of
small Schwinger-boson density, the results bear some resemblances with exact
diagonalization results and we briefly discuss some issues of the mean-field
treatment. We calculate the equal-time structure factor (and its angular
average to allow for a direct comparison with experiments on powder samples),
which extends earlier work on the classical kagome to the quantum regime. We
also discuss the dynamical structure factors of the topological spin liquid and
the Neel ordered phase.Comment: 8 pages, 9 figure
Internal state conversion in ultracold gases
We consider an ultracold gas of (non-condensed) bosons or fermions with two
internal states, and study the effect of a gradient of the transition frequency
between these states. When a RF pulse is applied to the sample,
exchange effects during collisions transfer the atoms into internal states
which depend on the direction of their velocity. This results, after a short
time, in a spatial separation between the two states. A kinetic equation is
solved analytically and numerically; the results agree well with the recent
observations of Lewandowski et al.Comment: Accepted version, to appear in PR
Chirality and vortices in an Heisenberg spin model on the kagom\'e lattice
The phase diagram of the classical \jj model on the \kag lattice is
investigated using extensive \mc simulations. In a realistic range of
parameters, this model has a low-temperature chiral-ordered phase without
long-range spin order. We show that the critical transition marking the
destruction of chiral order is preempted by the first order proliferation of
\Zdeux point defects. The core energy of these vortices appears to vanish when
approaching the T=0 phase boundary, where both \Zdeux defects and gapless
magnons contribute to disordering the system at very low temperature. This
situation might be typical of a large class of frustrated magnets. Possible
relevance for real materials is also discussed.Comment: 4 pages, 4 figure
Spontaneous order in the highly frustrated spin-1/2 Ising-Heisenberg model on the triangulated Kagome lattice due to the Dzyaloshinskii-Moriya anisotropy
The spin-1/2 Ising-Heisenberg model on the triangulated Kagome
(triangles-in-triangles) lattice is exactly solved by establishing a precise
mapping correspondence to the simple spin-1/2 Ising model on Kagome lattice. It
is shown that the disordered spin liquid state, which otherwise occurs in the
ground state of this frustrated spin system on assumption that there is a
sufficiently strong antiferromagnetic intra-trimer interaction, is eliminated
from the ground state by arbitrary but non-zero Dzyaloshinskii-Moriya
anisotropy.Comment: 4 pages, 3 figures, to be presented at conference Highly Frustrated
Magnetism, 7-12 September 2008, Braunschweig, German
Longitudinal spin waves in a dilute Bose gas
We present a kinetic theory for a dilute noncondensed Bose gas of two-level
atoms that predicts the transient spin segregation observed in a recent
experiment. The underlying mechanism driving spin currents in the gas is due to
a mean field effect arising from the quantum interference between the direct
and exchange scattering of atoms in different spin states. We numerically solve
the spin Boltzmann equation, using a one dimensional model, and find excellent
agreement with experimental data.Comment: 4.5 pages, 3 embedded color figure
Observation of anomalous spin-state segregation in a trapped ultra-cold vapor
We observe counter-intuitive spin segregation in an inhomogeneous sample of
ultra-cold, non-condensed Rubidium atoms in a magnetic trap. We use spatially
selective microwave spectroscopy to verify a model that accounts for the
differential forces on two internal spin states. In any simple understanding of
the cloud dynamics, the forces are far too small to account for the dramatic
transient spin polarizations observed. The underlying mechanism remains to be
elucidated.Comment: 5 pages, 3 figure
High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt
The Cretaceous Normal Superchron (CNS, 84–121 Ma) is a singular period of the geodynamo's history, identified by a prolonged absence of polarity reversals. To better characterize the paleosecular variation (PSV) of the geomagnetic field at the end of this interval, we sampled seven continuous sequences of lava flows from the Okhotsk-Chukotka Volcanic Belt, emplaced 84–89 Ma in the vicinity of the Kupol ore deposit (NE Russia). From a collection of 1,024 paleomagnetic cores out of 82 investigated lava flows, we successfully determined the paleodirections of 78 lava flows, which led to 57 directional groups after removing the serial correlations. The resulting paleomagnetic pole is located at 170.0°E, 76.8°N (A95 = 5.2°, N = 57), in good agreement with previous estimates for north-eastern Eurasia. Aiming at quantifying PSV at a reconstructed paleolatitude (λ) of ∼80°N, we obtained a virtual geomagnetic pole (VGP) scatter
, the value of which was corrected for within-site dispersion and is little dependent on the choice of the selection criteria. Compared to previous paleodirectional data sets characterizing PSV at various paleolatitudes during the CNS, our Sb estimate confirms a relative latitudinal increase Sb(λ = 90°)/Sb(λ = 0°) on the order of 2–2.5. Focusing on PSV at high paleolatitude within the 70°–90° range, we show that Sb was ∼15% lower at the end of the CNS than during the past 10 Myr, confirming that the singular polarity regime of the geodynamo observed during the CNS is likely accompanied with reduced PSV
The Antiferromagnetic Heisenberg Model on Clusters with Icosahedral Symmetry
The antiferromagnetic Heisenberg model is considered for spins
located on the vertices of the dodecahedron and the icosahedron, which belong
to the point symmetry group . Taking into account the permutational and
spin inversion symmetries of the Hamiltonian results in a drastic reduction of
the dimensionality of the problem, leading to full diagonalization for both
clusters. There is a strong signature of the frustration present in the systems
in the low energy spectrum, where the first excited states are singlets.
Frustration also results in a doubly-peaked specific heat as a function of
temperature for the dodecahedron. Furthermore, there is a discontinuity in the
magnetization as a function of magnetic field for the dodecahedron, where a
specific total spin sector never becomes the ground state in a field. This
discontinuity is accompanied by a magnetization plateau. The calculation is
also extended for where both systems again have singlet excitations.
The magnetization of the dodecahedron has now two discontinuities in an
external field and also magnetization plateaux, and the specific heat of the
icosahedron a two-peak structure as a function of temperature. The similarities
between the two systems suggest that the antiferromagnetic Heisenberg model on
a larger cluster with the same symmetry, the 60-site cluster, will have similar
properties
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