628 research outputs found
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
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
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
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
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
Energy-level ordering and ground-state quantum numbers for frustrated two-leg spin-1/2 ladder model
The Lieb-Mattis theorem about antiferromagnetic ordering of energy levels on
bipartite lattices is generalized to finite-size two-leg spin-1/2 ladder model
frustrated by diagonal interactions. For reflection-symmetric model with
site-dependent interactions we prove exactly that the lowest energies in
sectors with fixed total spin and reflection quantum numbers are monotone
increasing functions of total spin. The nondegeneracy of most levels is proved
also. We also establish the uniqueness and obtain the spin value of the
lowest-level multiplet in the whole sector formed by reflection-symmetric
(antisymmetric) states. For a wide range of coupling constants, we prove that
the ground state is a unique spin singlet. For other values of couplings, it
may be also a unique spin triplet or may consist of both multiplets. Similar
results have been obtained for the ladder with arbitrary boundary impurity
spin. Some partial results have also been obtained in the case of periodical
boundary conditions.Comment: 17 page
Observation of a Transient Magnetization Plateau in a Quantum Antiferromagnet on the Kagome Lattice
The magnetization process of an S=1/2 antiferromagnet on the kagome lattice,
[Cu_3(titmb)_2(OCOCH_3)_6]H_2O {titmb= 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6
trimethylbenzene} has been measured at very low temperatures in both pulsed and
steady fields. We have found a new dynamical behavior in the magnetization
process: a plateau at one third of the saturation magnetization appears in the
pulsed field experiments for intermediate sweep rates of the magnetic field and
disappears in the steady field experiments. A theoretical analysis using exact
diagonalization yields J_1=-19K and J_2=6K, for the nearest neighbor and second
nearest neighbor interactions, respectively. This set of exchange parameters
explains the very low saturation field and the absence of the plateau in the
thermodynamic equilibrium as well as the two-peak feature in the magnetic heat
capacity. Supported by numerical results we argue that a dynamical order by
disorder phenomenon could explain the transient appearance of the 1/3 plateau
in pulsed field experiments.Comment: 7 pages, 5 figure
Classical Limit of Demagnetization in a Field Gradient
We calculate the rate of decrease of the expectation value of the transverse
component of spin for spin-1/2 particles in a magnetic field with a spatial
gradient, to determine the conditions under which a previous classical
description is valid. A density matrix treatment is required for two reasons.
The first arises because the particles initially are not in a pure state due to
thermal motion. The second reason is that each particle interacts with the
magnetic field and the other particles, with the latter taken to be via a
2-body central force. The equations for the 1-body Wigner distribution
functions are written in a general manner, and the places where quantum
mechanical effects can play a role are identified. One that may not have been
considered previously concerns the momentum associated with the magnetic field
gradient, which is proportional to the time integral of the gradient. Its
relative magnitude compared with the important momenta in the problem is a
significant parameter, and if their ratio is not small some non-classical
effects contribute to the solution.
Assuming the field gradient is sufficiently small, and a number of other
inequalities are satisfied involving the mean wavelength, range of the force,
and the mean separation between particles, we solve the integro- partial
differential equations for the Wigner functions to second order in the strength
of the gradient. When the same reasoning is applied to a different problem with
no field gradient, but having instead a gradient to the z-component of
polarization, the connection with the diffusion coefficient is established, and
we find agreement with the classical result for the rate of decrease of the
transverse component of magnetization.Comment: 22 pages, no figure
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