635 research outputs found
Multiferroicity in spin ice: towards a magnetic crystallography of Tb2Ti2O7 in a field
We combine two aspects of magnetic frustration, multiferroicity and emergent
quasi-particles in spin liquids, by studying magneto-electric monopoles. Spin
ice offers to couple these emergent topological defects to external fields, and
to each other, in unusual ways, making possible to lift the degeneracy
underpinning the spin liquid and to potentially stabilize novel forms of charge
crystals, opening the path to a "magnetic crystallography". In developing the
general phase diagram including nearest-neighbour coupling, Zeeman energy,
electric and magnetic dipolar interactions, we uncover the emergence of a
bi-layered crystal of singly-charged monopoles, whose stability, remarkably, is
strengthened by an external [110] magnetic field. Our theory is able to account
for the ordering process of Tb2Ti2O7 in large field for reasonably small
electric energy scales.Comment: 10 pages, 10 figure
Slow relaxation and sensitivity to disorder in trapped lattice fermions after a quench
We consider a system of non-interacting fermions in one dimension subject to
a single-particle potential consisting of (a) a strong optical lattice, (b) a
harmonic trap, and (c) uncorrelated on-site disorder. After a quench, in which
the center of the harmonic trap is displaced, we study the occupation function
of the fermions and the time-evolution of experimental observables.
Specifically, we present numerical and analytical results for the post-quench
occupation function of the fermions, and analyse the time-evolution of the
real-space density profile. Unsurprisingly for a non-interacting (and therefore
integrable) system, the infinite-time limit of the density profile is
non-thermal. However, due to Bragg-localization of the higher-energy
single-particle states, the approach to even this non-thermal state is
extremely slow. We quantify this statement, and show that it implies a
sensitivity to disorder parametrically stronger than that expected from
Anderson localization.Comment: 15 pages, 11 figure
Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7
We study the finite-temperature transition in a model XY antiferromagnet on a
pyrochlore lattice, which describes the pyrochlore material Er2Ti2O7. The
ordered magnetic structure selected by thermal fluctuations is six-fold
degenerate. Nevertheless, our classical Monte Carlo simulations show that the
critical behavior corresponds to the three-dimensional XY universality class.
We determine an additional critical exponent nu_6=0.75>nu characteristic of a
dangerously irrelevant scaling variable. Persistent thermal fluctuations in the
ordered phase are revealed in Monte Carlo simulations by the peculiar
coexistence of Bragg peaks and diffuse magnetic scattering, the feature also
observed in neutron diffraction experiments.Comment: 5+5 pages (including supplemental material
Quantum spin liquids: a large-S route
This paper explores the large-S route to quantum disorder in the Heisenberg
antiferromagnet on the pyrochlore lattice and its homologues in lower
dimensions. It is shown that zero-point fluctuations of spins shape up a
valence-bond solid at low temperatures for one two-dimensional lattice and a
liquid with very short-range valence-bond correlations for another. A
one-dimensional model demonstrates potential significance of quantum
interference effects (as in Haldane's gap): the quantum melting of a
valence-bond order yields different valence-bond liquids for integer and
half-integer values of S.Comment: Proceedings of Highly Frustrated Magnetism 2003 (Grenoble), 6 LaTeX
page
Pocket Monte Carlo algorithm for classical doped dimer models
We study the correlations of classical hardcore dimer models doped with
monomers by Monte Carlo simulation. We introduce an efficient cluster
algorithm, which is applicable in any dimension, for different lattices and
arbitrary doping. We use this algorithm for the dimer model on the square
lattice, where a finite density of monomers destroys the critical confinement
of the two-monomer problem. The monomers form a two-component plasma located in
its high-temperature phase, with the Coulomb interaction screened at finite
densities. On the triangular lattice, a single pair of monomers is not
confined. The monomer correlations are extremely short-ranged and hardly change
with doping.Comment: 6 pages, REVTeX
Spin ice under pressure: symmetry enhancement and infinite order multicriticality
We study the low-temperature behaviour of spin ice when uniaxial pressure
induces a tetragonal distortion. There is a phase transition between a Coulomb
liquid and a fully magnetised phase. Unusually, it combines features of
discontinuous and continuous transitions: the order parameter exhibits a jump,
but this is accompanied by a divergent susceptibility and vanishing domain wall
tension. All these aspects can be understood as a consequence of an emergent
SU(2) symmetry at the critical point. We map out a possible experimental
realisation
Clustering in Hilbert space of a quantum optimization problem
The solution space of many classical optimization problems breaks up into
clusters which are extensively distant from one another in the Hamming metric.
Here, we show that an analogous quantum clustering phenomenon takes place in
the ground state subspace of a certain quantum optimization problem. This
involves extending the notion of clustering to Hilbert space, where the
classical Hamming distance is not immediately useful. Quantum clusters
correspond to macroscopically distinct subspaces of the full quantum ground
state space which grow with the system size. We explicitly demonstrate that
such clusters arise in the solution space of random quantum satisfiability
(3-QSAT) at its satisfiability transition. We estimate both the number of these
clusters and their internal entropy. The former are given by the number of
hardcore dimer coverings of the core of the interaction graph, while the latter
is related to the underconstrained degrees of freedom not touched by the
dimers. We additionally provide new numerical evidence suggesting that the
3-QSAT satisfiability transition may coincide with the product satisfiability
transition, which would imply the absence of an intermediate entangled
satisfiable phase.Comment: 11 pages, 6 figure
A Three Dimensional Kasteleyn Transition: Spin Ice in a [100] Field
We examine the statistical mechanics of spin-ice materials with a [100]
magnetic field. We show that the approach to saturated magnetisation is, in the
low-temperature limit, an example of a 3D Kasteleyn transition, which is
topological in the sense that magnetisation is changed only by excitations that
span the entire system. We study the transition analytically and using a Monte
Carlo cluster algorithm, and compare our results with recent data from
experiments on Dy2Ti2O7.Comment: 4 pages, 5 figure
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