1,104 research outputs found
Dipolar spin correlations in classical pyrochlore magnets
We study spin correlations for the highly frustrated classical pyrochlore
lattice antiferromagnets with O(N) symmetry in the limit T->0. We conjecture
that a local constraint obeyed by the extensively degenerate ground states
dictates a dipolar form for the asymptotic spin correlations, at all N 2
for which the system is paramagnetic down to T=0. We verify this conjecture in
the cases N=1 and N=3 by simulations and to all orders in the 1/N expansion
about the solvable N=infinity limit. Remarkably, the N=infinity formulae are an
excellent fit, at all distances, to the correlators at N=3 and even at N=1.
Thus we obtain a simple analytical expression also for the correlations of the
equivalent models of spin ice and cubic water ice, I_h.Comment: 4 pages revtex
Diluted planar ferromagnets: nonlinear excitations on a non-simply connected manifold
We study the behavior of magnetic vortices on a two-dimensional support
manifold being not simply connected. It is done by considering the continuum
approach of the XY-model on a plane with two disks removed from it. We argue
that an effective attractive interaction between the two disks may exist due to
the presence of a vortex. The results can be applied to diluted planar
ferromagnets with easy-plane anisotropy, where the disks can be seen as
nonmagnetic impurities. Simulations are also used to test the predictions of
the continuum limit.Comment: 5 pages, 6 figure
Measurability of the tidal polarizability of neutron stars in late-inspiral gravitational-wave signals
The gravitational wave signal from a binary neutron star inspiral contains
information on the nuclear equation of state. This information is contained in
a combination of the tidal polarizability parameters of the two neutron stars
and is clearest in the late inspiral, just before merger. We use the recently
defined tidal extension of the effective one-body formalism to construct a
controlled analytical description of the frequency-domain phasing of neutron
star inspirals up to merger. Exploiting this analytical description we find
that the tidal polarizability parameters of neutron stars can be measured by
the advanced LIGO-Virgo detector network from gravitational wave signals having
a reasonable signal-to-noise ratio of . This measurability result
seems to hold for all the nuclear equations of state leading to a maximum mass
larger than . We also propose a promising new way of extracting
information on the nuclear equation of state from a coherent analysis of an
ensemble of gravitational wave observations of separate binary merger events.Comment: 28 pages, 6 figures. Submitted to Phys. Rev.
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
Order induced by dipolar interactions in a geometrically frustrated antiferromagnet
We study the classical Heisenberg model for spins on a pyrochlore lattice
interacting via long range dipole-dipole forces and nearest neighbor exchange.
Antiferromagnetic exchange alone is known not to induce ordering in this
system. We analyze low temperature order resulting from the combined
interactions, both by using a mean-field approach and by examining the energy
cost of fluctuations about an ordered state. We discuss behavior as a function
of the ratio of the dipolar and exchange interaction strengths and find two
types of ordered phase. We relate our results to the recent experimental work
and reproduce and extend the theoretical calculations on the pyrochlore
compound, GdTiO, by Raju \textit{et al.}, Phys. Rev. B {\bf 59},
14489 (1999).Comment: 5 pages, 5 figures, AMSLaTe
Valence-bond crystal in a {111} slice of the pyrochlore antiferromagnet
We investigate theoretically the ordering effect of quantum spin fluctuations
in a Heisenberg antiferromagnet on a two-dimensional network of corner sharing
tetrahedra. This network is obtained as a {111} slice of the highly frustrated
pyrochlore lattice, from which it inherits the equivalence of all three pairs
of opposite bonds of each tetrahedron. The lowest-order (in 1/S) quantum
corrections partially lift the huge degeneracy of the classical ground state
and select an ensemble of states with long-range valence-bond order.Comment: 4 pages, 2 EPS figures. Minor revision: clarifications in response to
referee comments, additional reference
Two geometrically frustrated magnets studied by neutron diffraction
In the pyrochlore compounds, TbTiO and TbSnO, only
the Tb ions are magnetic. They exhibit quite abnormal -- and, in view of
their chemical similarity, strikingly different -- magnetic behaviour, as
probed by neutron diffraction at ambient and applied pressure.
TbTiO is a cooperative paramagnet (`spin liquid'), without long
range order at ambient pressure; however, it does become ordered under
pressure. By contrast, TbSnO enters an "ordered spin ice" state
already at ambient pressure. We analyse a simple model which already clearly
exhibits some of the qualitative features observed experimentally. Overall,
comparing these two compounds emphasizes the power of small perturbations in
selecting low-temperature states in geometrically frustrated systems.Comment: 6 pages, 4 figures, International Conference on Neutron Scattering
Sydney(2005
The Lower Critical Dimension of the XY Spin Glass
We investigate the XY spin-glass model in two and three dimensions using the
domain-wall renormalization-group method. The results for systems of linear
sizes up to L=12 (2D) and L=8 (3D) strongly suggest that the lower critical
dimension for spin-glass ordering may be rather than four as
is commonly believed. Our 3D data favor the scenario of a low but finite
spin-glass ordering temperature below the chiral transition but they are also
compatible with the system being at or slightly below its lower critical
dimension.Comment: 4 pages, 3 ps figures. Typos have been corrected, one reference has
been added and the concluding paragraph has been expanded. To appear in Phys.
Rev. Let
Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth
We use analytic formulae obtained from a simple model of crystal growth by
molecular--beam epitaxy to determine step--edge barriers to interlayer
transport. The method is based on information about the surface morphology at
the onset of nucleation on top of first--layer islands in the submonolayer
coverage regime of homoepitaxial growth. The formulae are tested using kinetic
Monte Carlo simulations of a solid--on--solid model and applied to estimate
step--edge barriers from scanning--tunneling microscopy data on initial stages
of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review
B, Rapid Communications, in press
Quantum Monte Carlo simulations of a particle in a random potential
In this paper we carry out Quantum Monte Carlo simulations of a quantum
particle in a one-dimensional random potential (plus a fixed harmonic
potential) at a finite temperature. This is the simplest model of an interface
in a disordered medium and may also pertain to an electron in a dirty metal. We
compare with previous analytical results, and also derive an expression for the
sample to sample fluctuations of the mean square displacement from the origin
which is a measure of the glassiness of the system. This quantity as well as
the mean square displacement of the particle are measured in the simulation.
The similarity to the quantum spin glass in a transverse field is noted. The
effect of quantum fluctuations on the glassy behavior is discussed.Comment: 23 pages, 7 figures included as eps files, uses RevTeX. Accepted for
publication in J. of Physics A: Mathematical and Genera
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