353 research outputs found
Order by disorder and gauge-like degeneracy in quantum pyrochlore antiferromagnet
The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg
spins of large spin length is a highly frustrated model with an macroscopic
degeneracy of classical ground states. The zero-point energy of (harmonic
order) spin wave fluctuations distinguishes a subset of these states. I derive
an approximate but illuminating {\it effective Hamiltonian}, acting within the
subspace of Ising spin configurations representing the {\it collinear} ground
states. It consists of products of Ising spins around loops, i.e has the form
of a lattice gauge theory. The remaining ground state entropy is still
infinite but not extensive, being for system size . All these
ground states have unit cells bigger than those considered previously.Comment: 4pp, one figur
Lattice gas description of pyrochlore and checkerboard antiferromagnets in a strong magnetic field
Quantum Heisenberg antiferromagnets on pyrochlore and checkerboard lattices
in a strong external magnetic field are mapped onto hard-core lattice gases
with an extended exclusion region. The effective models are studied by the
exchange Monte Carlo simulations and by the transfer matrix method. The
transition point and the critical exponents are obtained numerically for a
square-lattice gas of particles with the second-neighbor exclusion, which
describes a checkerboard antiferromagnet. The exact structure of the magnon
crystal state is determined for a pyrochlore antiferromagnet.Comment: 11 pages, accepted versio
BaCu3O4: High Temperature Magnetic Order in One-Dimensional S=1/2 Diamond-Chains
The magnetic properties of the alkaline earth oxocuprate BaCu3O4 are
investigated. We show that the characteristic Cu3O4 layers of this material can
be described with diamond chains of antiferromagnetically coupled Cu 1/2 spins
with only a weak coupling between two adjacent chains. These Cu3O4 layers seem
to represent a so far unique system of weakly coupled one-dimensional magnetic
objects where the local AF ordering of the Cu2+ ions leads to an actual net
magnetic moment of an isolated diamond chain. We demonstrate a magnetic
transition at a high N\'eel temperature T_{N}=336 K
Quantum order by disorder and accidental soft mode Er2Ti2O7
Motivated by recent neutron scattering experiments, we derive and study an
effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet
Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any
continuous symmetry, it does lead to a one-parameter `accidental' classical
degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the
non-coplanar spin structure observed experimentally -- a rare experimental
instance of quantum order by disorder. A non-Goldstone low-energy mode is
present in the excitation spectrum in accordance with inelastic neutron
scattering data. Our theory also resolves the puzzle of the experimentally
observed continuous ordering transition, absent from previous models.Comment: 5 pages, 4 figures, final versio
Semiclassical degeneracies and ordering for highly frustrated magnets in a field
We discuss ground state selection by quantum fluctuations in frustrated
magnets in a strong magnetic field. We show that there exist dynamical
symmetries -- one a generalisation of Henley's gauge-like symmetry for
collinear spins, the other the quantum relict of non-collinear weathervane
modes -- which ensure a partial survival of the classical degeneracies. We
illustrate these for the case of the kagome magnet, where we find zero-point
energy differences to be rather small everywhere except near the collinear
`up-up-down` configurations, where there is rotational but not translational
symmetry breaking. In the effective Hamiltonian, we demonstrate the presence of
a term sensitive to a topological `flux'. We discuss the connection of such
problems to gauge theories by casting the frustrated lattices as medial
lattices of appropriately chosen simplex lattices, and in particular we show
how the magnetic field can be used to tune the physical sector of the resulting
gauge theories.Comment: 10 pages, 8 figure
Claims by Non-Citizens under the Americans with Disabilities Act: Proper Extraterritorial Application in Torrico v. International Business Machines
Three dimensional generalization of the - Heisenberg model on a square lattice and role of the interlayer coupling
A possibility to describe magnetism in the iron pnictide parent compounds in
terms of the two-dimensional frustrated Heisenberg - model has been
actively discussed recently. However, recent neutron scattering data has shown
that the pnictides have a relatively large spin wave dispersion in the
direction perpendicular to the planes. This indicates that the third dimension
is very important. Motivated by this observation we study the --
model that is the three dimensional generalization of the -
Heisenberg model for and S = 1. Using self-consistent spin wave
theory we present a detailed description of the staggered magnetization and
magnetic excitations in the collinear state. We find that the introduction of
the interlayer coupling suppresses the quantum fluctuations and
strengthens the long range ordering. In the -- model, we find
two qualitatively distinct scenarios for how the collinear phase becomes
unstable upon increasing . Either the magnetization or one of the spin
wave velocities vanishes. For renormalization due to quantum
fluctuations is significantly stronger than for S=1, in particular close to the
quantum phase transition. Our findings for the -- model are of
general theoretical interest, however, the results show that it is unlikely
that the model is relevant to undoped pnictides.Comment: 11 pages, 10 figures. Updated version, several references adde
Habitat Management to Reduce Human Exposure to Trypanosoma cruzi and Western Conenose Bugs (Triatoma protracta).
Chagas disease, which manifests as cardiomyopathy and severe gastrointestinal dysfunction, is caused by Trypanosoma cruzi, a vector-borne parasite. In California, the vector Triatoma protracta frequently colonizes woodrat (Neotoma spp.) lodges, but may also invade nearby residences, feeding upon humans and creating the dual risk of bite-induced anaphylaxis and T. cruzi transmission. Our research aimed to assess T. cruzi presence in woodrats in a previously unstudied northern California area, statistically evaluate woodrat microhabitat use with respect to vegetation parameters, and provide guidance for habitat modifications to mitigate public health risks associated with Tr. protracta exposure. Blood samples from big-eared woodrats (N. macrotis) trapped on rural private properties yielded a T. cruzi prevalence of 14.3%. Microhabitat analyses suggest that modifying vegetation to reduce understory density within a 40Â meter radius of human residences might minimize woodrat lodge construction within this buffer area, potentially decreasing human exposure to Tr. protracta
Neutron Scattering and magnetization studies of BaCuCoOCl: A decorated two-dimensional antiferromagnet
BaCuOCl has two inter-penetrating square Cu sublattices, one
with square root 2 times the in-plane spacing of the other. Isotropic magnetic
interactions between the two sublattices are completely frustrated. Quantum
fluctuations resolve the intrinsic degeneracy in the ordering direction of the
more weakly coupled sublattice in favor of collinear ordering. We present
neutron scattering and magnetization studies of the magnetic structure when the
Cu ions are substituted with Co. The Co spins create new magnetic interactions
between the two sublattices. The ordering behavior of both Cu sublattices is
retained largely unmodified. Between the phase transitions of the two
sublattices spin-glass behavior is observed. Magnetization results show a
strong enhancement to the ferromagnetic aspect of the magnetic structure. The
combination of glassy behavior and large moments strongly suggest that the Co
moments induce the formation of local canted states.Comment: 4 figure
Quantum stabilization of 1/3-magnetization plateau in Cs_2CuBr_4
We consider the phase diagram of a spatially anisotropic 2D triangular
antiferromagnet in a magnetic field. Classically, the ground state is
umbrella-like for all fields, but we show that the quantum phase diagram is
much richer and contains a 1/3 magnetization plateau, two commensurate planar
states, two incommensurate chiral umbrella phases, and, possibly, a planar
state separating the two chiral phases. Our analysis sheds light on several
recent experimental findings for the spin-1/2 system Cs_2CuBr_4.Comment: 4+ pages, 2 figure
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