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

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

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 "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

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