646 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
Spin Diffusion in Trapped Gases: Anisotropy in Dipole and Quadrupole Modes
Recent experiments in a mixture of two hyperfine states of trapped Bose gases
show behavior analogous to a spin-1/2 system, including transverse spin waves
and other familiar Leggett-Rice-type effects. We have derived the kinetic
equations applicable to these systems, including the spin dependence of
interparticle interactions in the collision integral, and have solved for
spin-wave frequencies and longitudinal and transverse diffusion constants in
the Boltzmann limit. We find that, while the transverse and longitudinal
collision times for trapped Fermi gases are identical, the Bose gas shows
unusual diffusion anisotropy in both dipole and quadrupole modes. Moreover, the
lack of spin isotropy in the interactions leads to the non-conservation of
transverse spin, which in turn has novel effects on the hydrodynamic modes.Comment: 18 pages, 9 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
Spin Liquid in the Multiple-Spin Exchange model on the Triangular lattice: 3He on graphite
Using exact diagonalizations, we investigate the T=0 phase diagram of the
Multi-Spin Exchange (MSE) model on the triangular lattice: we find a transition
separating a ferromagnetic phase from a non-magnetic gapped Spin Liquid phase.
Systems far enough from the ferromagnetic transition have a metamagnetic
behavior with magnetization plateaus at m/m_sat=0 and 1/2. The MSE has been
proposed to describe solid 3He films adsorbed onto graphite, thus we compute
the MSE heat capacity for parameters in the low density range of the 2nd layer
and find a double-peak structure.Comment: Revtex, 4 pages, 4 figures. Accepted to Phys. Rev. Let
An investigation of the quantum model on the honeycomb lattice
We have investigated the quantum model on the honeycomb lattice
with exact diagonalizations and linear spin-wave calculations for selected
values of , and antiferromagnetic () or
ferromagnetic () nearest neighbor interactions. We found a variety of
quantum effects: "order by disorder" selection of a N{\'e}el ordered
ground-state, good candidates for non-classical ground-states with dimer long
range order or spin-liquid like. The purely antiferromagnetic Heisenberg model
is confirmed to be N{\'e}el ordered. Comparing these results with those
observed on the square and triangular lattices, we enumerate some conjectures
on the nature of the quantum phases in the isotropic models.Comment: 14 pages, 22 Postscript figures, uses svjour.cls and svepj.clo,
submitted to European Physical Journal B: condensed matter physi
How to detect weak emergent broken-symmetries of the Kagome antiferromagnet from Raman spectroscopy
We show that the magnetic Raman response of a spin-liquid is independent of
the polarizations of the light for triangular symmetries. In contrast, a
ground-state that has a broken symmetry shows characteristic oscillations when
the polarizations are rotated. This would allow to detect weak broken
symmetries and emergent order-parameters. We focus on the Kagome
antiferromagnet where no conventional long-range order has been found so far,
and present the Raman cross-section of a spin-liquid and a valence bond crystal
(VBC) using a random phase approximation.Comment: 4 pages, 2 figures, v2. intro partially rewritte
Exact diagonalization Studies of Two-dimensional Frustrated Antiferromagnet Models
We describe the four kinds of behavior found in two-dimensional isotropic
quantum antiferromagnets. Two of them display long range order at T=0: the
N\'eel state and the Valence Bond Crystal. The last two are Spin-Liquids.
Properties of these different states are shortly described and open questions
are underlined.Comment: 7 pages; invited talk at "HFM 2000" (Waterloo, June 2000); submitted
to Can. J. Phy
Some remarks on the Lieb-Schultz-Mattis theorem and its extension to higher dimensions
The extension of the Lieb-Schultz-Mattis theorem to dimensions larger than
one is discussed. It is explained why the variational wave-function built by
the previous authors is of no help to prove the theorem in dimension larger
than one. The short range R.V.B. picture of Sutherland, Rokhsar and Kivelson,
Read and Chakraborty gives a strong support to the assertion that the theorem
is indeed valid in any dimension. Some illustrations of the general ideas are
displayed on exact spectra.Comment: 12 pages, LaTeX with 4 EPS figures embedded in the documen
Quantum phase transition induced by Dzyaloshinskii-Moriya in the kagome antiferromagnet
We argue that the S=1/2 kagome antiferromagnet undergoes a quantum phase
transition when the Dzyaloshinskii-Moriya coupling is increased. For
the system is in a moment-free phase and for the system develops
antiferromagnetic long-range order. The quantum critical point is found to be
using exact diagonalizations and finite-size scaling. This
suggests that the kagome compound ZnCu_6_3$ may be in a quantum
critical region controlled by this fixed point.Comment: 5 pages, 4 figures; v2: add. data included, show that D=0.1J is at a
quantum critical poin
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