3,837 research outputs found
Superfluid-Insulator Transitions on the Triangular Lattice
We report on a phenomenological study of superfluid to Mott insulator
transitions of bosons on the triangular lattice, focusing primarily on the
interplay between Mott localization and geometrical charge frustration at
1/2-filling. A general dual vortex field theory is developed for arbitrary
rational filling factors f, based on the appropriate projective symmetry group.
At the simple non-frustrated density f=1/3, we uncover an example of a
deconfined quantum critical point very similar to that found on the half-filled
square lattice. Turning to f=1/2, the behavior is quite different. Here, we
find that the low-energy action describing the Mott transition has an emergent
nonabelian SU(2)\times U(1) symmetry, not present at the microscopic level.
This large nonabelian symmetry is directly related to the frustration-induced
quasi-degeneracy between many charge-ordered states not related by microscopic
symmetries. Through this ``pseudospin'' SU(2)symmetry, the charged excitations
in the insulator close to the Mott transition develop a skyrmion-like
character. This leads to an understanding of the recently discovered supersolid
phase of the triangular lattice XXZ model (cond-mat/0505258, cond-mat/0505257,
cond-mat/0505298) as a ``partially melted'' Mott insulator. The latter picture
naturally explains a number of puzzling numerical observations of the
properties of this supersolid. Moreover, we predict that the nearby quantum
phase transition from this supersolid to the Mott insulator is in the
recently-discovered non-compact CP^1 critical universality class (PRB 70,
075104 (2004)). A description of a broad range of other Mott and supersolid
states, and a diverse set of quantum critical points between them, is also
provided.Comment: 24 pages, 14 figure
On the relationship between topological and geometric defects
The study of topology in solids is undergoing a renaissance following renewed
interest in the properties of ferroic domain walls as well as recent
discoveries regarding topological insulators and skyrmionic lattices. Each of
these systems possess a property that is `protected' in a symmetry sense, and
is defined rigorously using a branch of mathematics known as topology. In this
article we review the formal definition of topological defects as they are
classified in terms of homotopy theory, and discuss the precise
symmetry-breaking conditions that lead to their formation. We distinguish
topological defects from geometric defects, which arise from the details of the
stacking or structure of the material but are not protected by symmetry. We
provide simple material examples of both topological and geometric defect
types, and discuss the implications of the classification on the resulting
material properties
Magneto-elastic effects and magnetization plateaus in two dimensional systems
We show the importance of both strong frustration and spin-lattice coupling
for the stabilization of magnetization plateaus in translationally invariant
two-dimensional systems. We consider a frustrated spin-1/2 Heisenberg model
coupled to adiabatic phonons under an external magnetic field. At zero
magnetization, simple structures with two or at most four spins per unit cell
are stabilized, forming dimers or plaquettes, respectively. A much
richer scenario is found in the case of magnetization , where larger
unit cells are formed with non-trivial spin textures and an analogy with the
corresponding classical Ising model is detectable. Specific predictions on
lattice distortions and local spin values can be directly measured by X-rays
and Nuclear Magnetic Resonance experiments.Comment: 4 pages and 4 figure
Topological stripelike coreless textures with inner incommensurability in two-dimensional Heisenberg antiferromagnet
For two-dimensional Heisenberg antiferromagnet we present an analysis of
topological coreless excitations having a stripe form. These textures are
characterized by singularities at boundaries. A detailed classification of the
stripe textures results in a certain analogy with the coreless excitations in
phase: Mermin-Ho and Anderson-Toulouse coreless vortices. The
excitations of the last type may have a low bulk energy. The stripe textures
may be observed as an occurrence of short-range incommensurate order in the
antiferromagnetic environment
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