16,107 research outputs found
Magnetic Structure and Spin Waves in the Kagom\'{e} Jarosite compound
We present a detailed study of the magnetic structure and spin waves in the
Fe jarosite compound for the most general
Hamiltonian involving one- and two-spin interactions which are allowed by
symmetry. We compare the calculated spin-wave spectrum with the recent neutron
scattering data of Matan {\it et al.} for various model Hamiltonians which
include, in addition to isotropic Heisenberg exchange interactions between
nearest () and next-nearest () neighbors, single ion anisotropy and
Dzyaloshinskii-Moriya (DM) interactions. We concluded that DM interactions are
the dominant anisotropic interaction, which not only fits all the splittings in
the spin-wave spectrum but also reproduces the small canting of the spins out
of the Kagom\'e plane. A brief discussion of how representation theory
restricts the allowed magnetic structure is also given.Comment: 23 pages, 17 figures, submitted to Phys. Rev. B (March 2006
Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7
The magnetocaloric effect of polycrystalline samples of pure and Y-doped
dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K
to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic
interest, it is proposed that the magnetocaloric effect may be used as an
appropriate tool for the qualitative study of slow relaxation processes in the
spin ice regime. In the high temperature regime the temperature change on
adiabatic demagnetization was found to be consistent with previously published
entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by
adiabatic demagnetization was followed by an irreversible rise in temperature
that persisted after the removal of the applied field. The relaxation time
derived from this temperature rise was found to increase rapidly down to 0.3 K.
The data near to 0.3 K indicated a transition into a metastable state with much
slower relaxation, supporting recent neutron scattering results. In addition,
magnetic dilution of 50 % concentration was found to significantly prolong the
dynamical response in the milikelvin temperature range, in contrast with
results reported for higher temperatures at which the spin correlations are
suppressed. These observations are discussed in terms of defects and loop
correlations in the spin ice state.Comment: 9 figures, submitted to Phys. Rev.
The detailed chemical composition of the terrestrial planet host Kepler-10
Chemical abundance studies of the Sun and solar twins have demonstrated that
the solar composition of refractory elements is depleted when compared to
volatile elements, which could be due to the formation of terrestrial planets.
In order to further examine this scenario, we conducted a line-by-line
differential chemical abundance analysis of the terrestrial planet host
Kepler-10 and fourteen of its stellar twins. Stellar parameters and elemental
abundances of Kepler-10 and its stellar twins were obtained with very high
precision using a strictly differential analysis of high quality CFHT, HET and
Magellan spectra. When compared to the majority of thick disc twins, Kepler-10
shows a depletion in the refractory elements relative to the volatile elements,
which could be due to the formation of terrestrial planets in the Kepler-10
system. The average abundance pattern corresponds to ~ 13 Earth masses, while
the two known planets in Kepler-10 system have a combined ~ 20 Earth masses.
For two of the eight thick disc twins, however, no depletion patterns are
found. Although our results demonstrate that several factors (e.g., planet
signature, stellar age, stellar birth location and Galactic chemical evolution)
could lead to or affect abundance trends with condensation temperature, we find
that the trends give further support for the planetary signature hypothesis.Comment: 12 pages, 11 figures, accepted for publication in MNRA
Zooming in on local level statistics by supersymmetric extension of free probability
We consider unitary ensembles of Hermitian NxN matrices H with a confining
potential NV where V is analytic and uniformly convex. From work by
Zinn-Justin, Collins, and Guionnet and Maida it is known that the large-N limit
of the characteristic function for a finite-rank Fourier variable K is
determined by the Voiculescu R-transform, a key object in free probability
theory. Going beyond these results, we argue that the same holds true when the
finite-rank operator K has the form that is required by the Wegner-Efetov
supersymmetry method of integration over commuting and anti-commuting
variables. This insight leads to a potent new technique for the study of local
statistics, e.g., level correlations. We illustrate the new technique by
demonstrating universality in a random matrix model of stochastic scattering.Comment: 38 pages, 3 figures, published version, minor changes in Section
Surface phase transitions in one-dimensional channels arranged in a triangular cross-sectional structure: Theory and Monte Carlo simulations
Monte Carlo simulations and finite-size scaling analysis have been carried
out to study the critical behavior in a submonolayer lattice-gas of interacting
monomers adsorbed on one-dimensional channels arranged in a triangular
cross-sectional structure. The model mimics a nanoporous environment, where
each nanotube or unit cell is represented by a one-dimensional array. Two kinds
of lateral interaction energies have been considered: , interaction
energy between nearest-neighbor particles adsorbed along a single channel and
, interaction energy between particles adsorbed across
nearest-neighbor channels. For and , successive planes are
uncorrelated, the system is equivalent to the triangular lattice and the
well-known
ordered phase is found at low temperatures and a coverage, , of 1/3
. In the more general case ( and ), a
competition between interactions along a single channel and a transverse
coupling between sites in neighboring channels allows to evolve to a
three-dimensional adsorbed layer. Consequently, the and structures "propagate" along the
channels and new ordered phases appear in the adlayer. The Monte Carlo
technique was combined with the recently reported Free Energy Minimization
Criterion Approach (FEMCA), to predict the critical temperatures of the
order-disorder transformation. The excellent qualitative agreement between
simulated data and FEMCA results allow us to interpret the physical meaning of
the mechanisms underlying the observed transitions.Comment: 24 pages, 6 figure
Properties of a classical spin liquid: the Heisenberg pyrochlore antiferromagnet
We study the low-temperature behaviour of the classical Heisenberg
antiferromagnet with nearest neighbour interactions on the pyrochlore lattice.
Because of geometrical frustration, the ground state of this model has an
extensive number of degrees of freedom. We show, by analysing the effects of
small fluctuations around the ground-state manifold, and from the results of
Monte Carlo and molecular dynamics simulations, that the system is disordered
at all temperatures, T, and has a finite relaxation time, which varies as 1/T
for small T.Comment: 4 pages revtex; 3 figures automatically include
Magnetic susceptibility of diluted pyrochlore and SCGO antiferromagnets
We investigate the magnetic susceptibility of the classical Heisenberg
antiferromagnet with nearest-neighbour interactions on the geometrically
frustrated pyrochlore lattice, for a pure system and in the presence of
dilution with nonmagnetic ions. Using the fact that the correlation length in
this system for small dilution is always short, we obtain an approximate but
accurate expression for the magnetic susceptibility at all temperatures. We
extend this theory to the compound SrCr_{9-9x}Ga_{3+9x}O_{19} (SCGO) and
provide an explanation of the phenomenological model recently proposed by
Schiffer and Daruka [Phys. Rev. B56, 13712 (1997)].Comment: 4 pages, Latex, 4 postscript figures automatically include
Corrigendum to "De-escalation therapy among bacteraemic patients with community-acquired pneumonia" [Clin Microbiol Infect 21 (2015) 936.e11-936.e18]
Competition Between Exchange and Anisotropy in a Pyrochlore Ferromagnet
The Ising-like spin ice model, with a macroscopically degenerate ground
state, has been shown to be approximated by several real materials. Here we
investigate a model related to spin ice, in which the Ising spins are replaced
by classical Heisenberg spins. These populate a cubic pyrochlore lattice and
are coupled to nearest neighbours by a ferromagnetic exchange term J and to the
local axes by a single-ion anisotropy term D. The near neighbour spin
ice model corresponds to the case D/J infinite. For finite D/J we find that the
macroscopic degeneracy of spin ice is broken and the ground state is
magnetically ordered into a four-sublattice structure. The transition to this
state is first-order for D/J > 5 and second-order for D/J < 5 with the two
regions separated by a tricritical point. We investigate the magnetic phase
diagram with an applied field along [1,0,0] and show that it can be considered
analogous to that of a ferroelectric.Comment: 7 pages, 4 figure
Electron Spin Resonance of defects in the Haldane System Y(2)BaNiO(5)
We calculate the electron paramagnetic resonance (EPR) spectra of the
antiferromagnetic spin-1 chain compound Y(2)BaNi(1-x)Mg(x)O(5) for different
values of x and temperature T much lower than the Haldane gap (~100K). The
low-energy spectrum of an anisotropic Heisenberg Hamiltonian, with all
parameters determined from experiment, has been solved using DMRG. The observed
EPR spectra are quantitatively reproduced by this model. The presence of
end-chain S=1/2 states is clearly observed as the main peak in the spectrum and
the remaining structure is completely understood.Comment: 5 pages, 4 figures include
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