1,557 research outputs found
Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice
Ever since the experiments which founded the field of highly frustrated
magnetism, the kagome Heisenberg antiferromagnet has been the archetypical
setting for the study of fluctuation induced exotic ordering. To this day the
nature of its classical low-temperature state has remained a mystery: the
non-linear nature of the fluctuations around the exponentially numerous
harmonically degenerate ground states has not permitted a controlled theory,
while its complex energy landscape has precluded numerical simulations at low
temperature. Here we present an efficient Monte Carlo algorithm which removes
the latter obstacle. Our simulations detect a low-temperature regime in which
correlations saturate at a remarkably small value. Feeding these results into
an effective model and analyzing the results in the framework of an appropriate
field theory implies the presence of long-range dipolar spin order with a
tripled unit cell.Comment: 5 pages, 4 figure
Partial order from disorder in a classical pyrochlore antiferromagnet
We investigate theoretically the phase diagram of a classical Heisenberg
antiferromagnet on the pyrochlore lattice perturbed by a weak second-neighbor
interaction J_2. The huge ground state degeneracy of the nearest-neighbor
Heisenberg spins is lifted by J_2 and a magnetically ordered ground state sets
in upon approaching zero temperature. We have found a new, partially ordered
phase with collinear spins at finite temperatures for a ferromagnetic J_2. In
addition to a large nematic order parameter, this intermediate phase also
exhibits a layered structure and a bond order that breaks the sublattice
symmetry. Thermodynamic phase boundaries separating it from the fully
disordered and magnetically ordered states scale as 1.87 J_2 S^2 and 0.26 J_2
S^2 in the limit of small J_2. The phase transitions are discontinuous. We
analytically examine the local stability of the collinear state and obtain a
boundary T ~ J_2^2/J_1 in agreement with Monte Carlo simulations.Comment: 14 pages revtex, revised phase diagram, references adde
Raman Scattering Signatures of Kitaev Spin Liquids in A IrO Iridates
We study theoretically the Raman scattering response in the
gapless quantum spin liquid phase of the Kitaev-Heisenberg model. The dominant
polarization-independent contribution reflects the density of
states of the emergent Majorana fermions in the ground-state flux-sector. The
integrability-breaking Heisenberg exchange generates a second contribution,
whose dominant part has the form of a quantum quench
corresponding to an abrupt insertion of four gauge fluxes. This results
in a weakly polarization dependent response with a sharp peak at the energy of
the flux excitation accompanied by broad features, which can be related to
Majorana fermions in the presence of the perturbed gauge field. We discuss the
experimental situation and explore more generally the influence of
integrability breaking for Kitaev spin liquid response functions.Comment: 9 pages including supp. ma
The structure of Gelfand-Levitan-Marchenko type equations for Delsarte transmutation operators of linear multi-dimensional differential operators and operator pencils. Part 1
An analog of Gelfand-Levitan-Marchenko integral equations for multi-
dimensional Delsarte transmutation operators is constructed by means of
studying their differential-geometric structure based on the classical Lagrange
identity for a formally conjugated pair of differential operators. An extension
of the method for the case of affine pencils of differential operators is
suggested.Comment: 12 page
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Generalized Kasha's Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems
Quantization and Corrections of Adiabatic Particle Transport in a Periodic Ratchet Potential
We study the transport of an overdamped particle adiabatically driven by an
asymmetric potential which is periodic in both space and time. We develop an
adiabatic perturbation theory after transforming the Fokker-Planck equation
into a time-dependent hermitian problem, and reveal an analogy with quantum
adiabatic particle transport. An analytical expression is obtained for the
ensemble average of the particle velocity in terms of the Berry phase of the
Bloch states. Its time average is shown to be quantized as a Chern number in
the deterministic or tight-binding limit, with exponentially small corrections.
In the opposite limit, where the thermal energy dominates the ratchet
potential, a formula for the average velocity is also obtained, showing a
second order dependence on the potential.Comment: 8 page
On the reliability of mean-field methods in polymer statistical mechanics
The reliability of the mean-field approach to polymer statistical mechanics
is investigated by comparing results from a recently developed lattice
mean-field theory (LMFT) method to statistically exact results from two
independent numerical Monte Carlo simulations for the problems of a polymer
chain moving in a spherical cavity and a polymer chain partitioning between two
confining spheres of different radii. It is shown that in some cases the
agreement between the LMFT and the simulation results is excellent, while in
others, such as the case of strongly fluctuating monomer repulsion fields, the
LMFT results agree with the simulations only qualitatively. Various
approximations of the LMFT method are systematically estimated, and the
quantitative discrepancy between the two sets of results is explained with the
diminished accuracy of the saddle-point approximation, implicit in the
mean-field method, in the case of strongly fluctuating fields.Comment: 27 pages, 9 figure
Two universal results for Wilson loops at strong coupling
We present results for Wilson loops in strongly coupled gauge theories. The
loops may be taken around an arbitrarily shaped contour and in any field theory
with a dual IIB geometry of the form M x S^5. No assumptions about
supersymmetry are made. The first result uses D5 branes to show how the loop in
any antisymmetric representation is computed in terms of the loop in the
fundamental representation. The second result uses D3 branes to observe that
each loop defines a rich sequence of operators associated with minimal surfaces
in S^5. The action of these configurations are all computable. Both results
have features suggesting a connection with integrability.Comment: 1+12 pages. LaTeX. No figure
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