41 research outputs found
Quasiperiodic Heisenberg antiferromagnets in two dimensions
This is a review of the properties of 2d quantum quasiperiodic
antiferromagnets as reported in studies that have been carried out in the last
decade. Many results have been obtained for perfectly ordered as well as for
disordered two dimensional bipartite quasiperiodic tilings. The theoretical
methods used include spin wave theory, and renormalization group along with
Quantum Monte Carlo simulations. These methods all show that the ground state
of these unfrustrated antiferromagnets have N\'eel type order but with a highly
complex spatial distribution of local staggered magnetization. The ground state
properties, excitation energies and spatial dependence, structure factor, and
local susceptibilities are presented. The effects of introducing geometrical
disorder on the magnetic properties are discussed.Comment: 21 pages, 29 figure
Open theoretical problems in the physics of aperiodic systems
Quasicrystals have intrigued and stimulated research in a large number of
disciplines. Mathematicians, physicists, chemists, metallurgists and materials
scientists have found in them a fertile ground for new insights and
discoveries. In the quarter century that has ensued since the publication of
the experimental observation of a quasiperiodic Al-Mn alloy \cite{shecht}, many
different kinds of quasiperiodic alloys have been manufactured and studied. The
physical properties of quasicrystals are no less interesting than the unusual
structural properties that led to their discovery in 1984. In this review, I
present some of the properties that characterize quasicrystals, briefly discuss
several types of theories that have been put forward, and describe some new
behaviors that might be investigated by experiment.Comment: 6 pages, 5 figures, plenary lecture for CMAC (Complex metallic
alloys) workshop (Zagreb, 2009
The eight-fold way for optical quasicrystals
In a recent Letter we proposed a means to realize a quasicrystal with
eight-fold symmetry by trapping particles in an optical potential created by
four lasers. The quasicrystals obtained in this way, which are closely related
to the well-known octagonal tiling, offer unique possibilities to study the
effects of quasiperiodicity on physical properties. This method allows to
transform the structures, to inflate or deflate them, include interactions or
disorder and thus realize a large variety of theoretical models, both classical
and quantum. In this paper we derive a number of interesting geometrical
properties of the optical quasicrystals, and present some results obtained by
numerical calculations.Comment: Paper following short report published in Europhys.Lett. vol.104 p.
66003 (2013
Geometry fluctuations and Casimir effect in a quantum antiferromagnet
We show the presence of a Casimir type force between domain walls in a two
dimensional Heisenberg antiferromagnet subject to geometrical fluctuations. The
type of fluctuations that we consider, called phason flips, are well known in
quasicrystals, but less so in periodic structures. As the classical ground
state energy of the antiferromagnet is unaffected by this type of fluctuation,
energy changes are purely of quantum origin. We calculate the effective
interaction between two parallel domain walls, defining a slab of thickness d,
in such an antiferromagnet within linear spin wave theory. The interaction is
anisotropic, and for a particular orientation of the slab we find that it
decays as 1/d, thus, more slowly than the electromagnetic Casimir effect in the
same geometry.Comment: 5 pages, 5 figures, minor modifications, accepted for publication in
EPJ
Tight-binding models in a quasiperiodic optical lattice
This paper describes how one can use four standing wave laser fields to
realize a two dimensional optical quasicrystal with eight-fold symmetry,
closely related to the well-known octagonal or Ammann-Beenker tiling
quasicrystal. We describe the structure and its properties, and the effective
tight-binding model for atoms in this optical quasicrystal. Such a system, if
realized experimentally, should provide valuable insights into the quantum
properties of quasicrystals.Comment: revised version (to appear in the Proceedings of the International
Conference on Quasicrystals ICQ12 Kracow, Poland
Closing of gaps and gap labeling and passage from molecular states to critical states in a 2D quasicrystal
The single electron spectrum and wavefunctions in quasicrystals continue to
be a fascinating problem, with few known solutions, especially in two and
higher dimensions. This paper investigates the energy spectra and gap
structures in tight-binding models on a quasiperiodic tiling in two dimensions.
By varying a continuous parameter, we follow the evolution of the band
structure from the discrete molecular or atomic states, to the multifractal
states well-known from previous studies. We propose a scheme for labeling gaps
in finite approximants. It is equivalent in the limit of infinite systems to
the one presented by Kellendonk and Putnam based on the algebraic structure of
this quasiperiodic system