10,913 research outputs found
Magnetic quasicrystals: What can we expect to see in their neutron diffraction data?
The theory of magnetic symmetry in quasicrystals is used to characterize the
nature of magnetic peaks, expected in elastic neutron diffraction experiments.
It is established that there is no symmetry-based argument which forbids the
existence of quasiperiodic long-range magnetic order. Suggestions are offered
as to where one should look for the simplest kinds of antiferromagnetic
quasicrystals.Comment: 4 pages. Submitted to Materials Science and Engineering
Magnetic point groups and space groups
We review the notion of magnetic symmetry of finite structures as well as
infinite periodic and quasiperiodic crystals. We describe one of the most
direct consequences of having magnetic symmetry in crystals which is the
extinction of magnetic Bragg peaks in neutron diffraction patterns. We finish
by mentioning the generalization of magnetic groups to spin groups and color
groups.Comment: Written for the Encyclopedia of Condensed Matter Physics. Contains 2
color figures - gray scale version available from the author's website:
http://www.tau.ac.il/~ronlif
Kinetics of the Phase Separation Transition in Cold-Atom Boson-Fermion Mixtures
We study the kinetics of the first order phase separation transition in
boson-fermion cold-atom mixtures. At sufficiently low temperatures such a
transition is driven by quantum fluctuations responsible for the formation of
critical nuclei of a stable phase. Based on a microscopic description of
interacting boson-fermion mixtures we derive an effective action for the
critical droplet and obtain an asymptotic expression for the nucleation rate in
the vicinity of the phase transition and near the spinodal instability of the
mixed phase. We also discuss effects of dissipation which play a dominant role
close to the transition point, and identify the regimes where quantum
nucleation can be experimentally observed in cold-atom systems.Comment: 4 pages 1 figure, typos correcte
Comment on "Quantum Quasicrystals of Spin-Orbit-Coupled Dipolar Bosons"
In a recent Letter, Gopalakrishnan, Martin, and Demler [Phys. Rev. Lett. 111
(2013) 185304] show that quasi-two-dimensional dipolar Bose gases, subject to a
Rashba spin-orbit coupling, exhibit a variety of spatially ordered, or
crystalline, ground states, including a pentagonal quasicrystal. Indeed, as the
authors say, realizing quasicrystalline condensates would provide new ways to
explore the physics of quasicrystals, and in particular to study the quantum
dynamics of their unique collective phason modes. Yet, the authors conclude
that "there are typically additional phasons in quantum-mechanical
quasicrystals, when compared with their classical equivalents." In this Comment
I review the notion of phason modes in quasicrystals, and explain why their
number does not depend on whether they are classical or quantum
Lattice Color Groups of Quasicrystals
Lattice color groups are introduced and used to study the partitioning of a
periodically- or quasiperiodically-ordered set of points into N
symmetry-related subsets. Applications range from magnetic structure to
superlattice ordering in periodic and quasiperiodic crystals.Comment: 5 pages, 1 color figure. Uses World Scientific style file:
sprocl.sty. Final version to appear in the Proceedings of the 6th
International Conference on Quasicrystals, Tokyo, Japan, May 26-30, 199
Comment on "Phonon Spectrum and Dynamical Stability of a Dilute Quantum Degenerate Bose-Fermi Mixture
We show that the conclusions of a recent PRL by Pu et al is incorrect.Comment: late
Casimir Force between a Small Dielectric Sphere and a Dielectric Wall
The possibility of repulsive Casimir forces between small metal spheres and a
dielectric half-space is discussed. We treat a model in which the spheres have
a dielectric function given by the Drude model, and the radius of the sphere is
small compared to the corresponding plasma wavelength. The half-space is also
described by the same model, but with a different plasma frequency. We find
that in the retarded limit, the force is quasi-oscillatory. This leads to the
prediction of stable equilibrium points at which the sphere could levitate in
the Earth's gravitational field. This seems to lead to the possibility of an
experimental test of the model. The effects of finite temperature on the force
are also studied, and found to be rather small at room temperature. However,
thermally activated transitions between equilibrium points could be significant
at room temperature.Comment: 16 pages, 5 figure
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