489 research outputs found
Quantum Disordered Ground States in Frustrated Antiferromagnets with Multiple Ring Exchange Interactions
We present a certain class of two-dimensional frustrated quantum Heisenberg
spin systems with multiple ring exchange interactions which are rigorously
demonstrated to have quantum disordered ground states without magnetic
long-range order. The systems considered in this paper are s=1/2
antiferromagnets on a honeycomb and square lattices, and an s=1 antiferromagnet
on a triangular lattice. We find that for a particular set of parameter values,
the ground state is a short-range resonating valence bond state or a valence
bond crystal state. It is shown that these systems are closely related to the
quantum dimer model introduced by Rokhsar and Kivelson as an effective
low-energy theory for valence bond states.Comment: 6 pages, 4 figure
Monte Carlo Calculation of the Spin-Stiffness of the Two-Dimensional Heisenberg Model
Using a collective-mode Monte Carlo method (the Wolff-Swendsen-Wang
algorithm), we compute the spin-stiffness of the two-dimensional classical
Heisenberg model. We show that it is the relevant physical quantity to
investigate the behaviour of the model in the very low temperature range
inaccessible to previous studies based on correlation length and susceptibility
calculations.Comment: 6 pages, latex, 3 postscript figures appended, DIM preprint 93-3
Temperature-Dependent X-Ray Absorption Spectroscopy of Colossal Magnetoresistive Perovskites
The temperature dependence of the O K-edge pre-edge structure in the x-ray
absorption spectra of the perovskites La(1-x)A(x)MnO(3), (A = Ca, Sr; x = 0.3,
0.4) reveals a correlation between the disappearance of the splitting in the
pre-edge region and the presence of Jahn-Teller distortions. The different
magnitudes of the distortions for different compounds is proposed to explain
some dissimilarity in the line shape of the spectra taken above the Curie
temperature.Comment: To appear in Phys. Rev. B, 5 pages, 3 figure
XY checkerboard antiferromagnet in external field
Ordering by thermal fluctuations is studied for the classical XY
antiferromagnet on a checkerboard lattice in zero and finite magnetic fields by
means of analytical and Monte Carlo methods. The model exhibits a variety of
novel broken symmetries including states with nematic ordering in zero field
and with triatic order parameter at high fields.Comment: 6 page
Physical Conditions in Shocked Interstellar Gas Interacting with the Supernova Remnant IC 443
We present the results of a detailed investigation into the physical
conditions in interstellar material interacting with the supernova remnant IC
443. Our analysis is based on a comprehensive examination of high-resolution
far-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph
onboard the Hubble Space Telescope of two stars behind IC 443. One of our
targets (HD 43582) probes gas along the entire line of sight through the
supernova remnant, while the other (HD 254755) samples material located ahead
of the primary supernova shock front. We identify low velocity quiescent gas in
both directions and find that the densities and temperatures in these
components are typical of diffuse atomic and molecular clouds. Numerous high
velocity components are observed in the absorption profiles of neutral and
singly-ionized atomic species toward HD 43582. These components exhibit a
combination of greatly enhanced thermal pressures and significantly reduced
dust-grain depletions. We interpret this material as cooling gas in a
recombination zone far downstream from shocks driven into neutral gas clumps.
The pressures derived for a group of ionized gas components at high positive
velocity toward HD 43582 are lower than those of the other shocked components,
pointing to pressure inhomogeneities across the remnant. A strong very high
velocity component near -620 km/s is seen in the absorption profiles of
highly-ionized species toward HD 43582. The velocity of this material is
consistent with the range of shock velocities implied by observations of soft
thermal X-ray emission from IC 443. Moderately high-velocity gas toward HD
254755 may represent shocked material from a separate foreground supernova
remnant.Comment: 88 pages, 27 figures, accepted for publication in Ap
Classical heisenberg antiferromagnet away from the pyrochlore lattice limit: entropic versus energetic selection
The stability of the disordered ground state of the classical Heisenberg
pyrochlore antiferromagnet is studied within extensive Monte Carlo simulations
by introducing an additional exchange interaction that interpolates
between the pyrochlore lattice () and the face-centered cubic lattice
(). It is found that for as low as , the system is
long range ordered : the disordered ground state of the pyrochlore
antiferromagnet is unstable when introducing very small deviations from the
pure limit. Furthermore, it is found that the selected phase is a
collinear state energetically greater than the incommensurate phase suggested
by a mean field analysis. To our knowledge this is the first example where
entropic selection prevails over the energetic one.Comment: 5 (two-column revtex4) pages, 1 table, 7 ps/eps figures. Submitted to
Phys. Rev.
Vortex ordering in fully-frustrated superconducting systems with dice lattice
The structure and the degenracy of the ground state of a fully-frustrated
XY-model are investigated for the case of a dice lattice geometry.
The results are applicable for the description of Josephson junction arrays
and thin superconducting wire networks in the external magnetic field providing
half-integer number of flux quanta per plaquette. The mechanisms of disordering
of vortex pattern in such systems are briefly discussed.Comment: 10 pages, 3 figure
Superconducting Phase with Fractional Vortices in the Frustrated Kagome Wire Network at f=1/2
In classical XY kagome antiferromagnets, there can be a novel low temperature
phase where has quasi-long-range order but is
disordered, as well as more conventional antiferromagnetic phases where
is ordered in various possible patterns ( is the angle of orientation
of the spin). To investigate when these phases exist in a physical system, we
study superconducting kagome wire networks in a transverse magnetic field when
the magnetic flux through an elementary triangle is a half of a flux quantum.
Within Ginzburg-Landau theory, we calculate the helicity moduli of each phase
to estimate the Kosterlitz-Thouless (KT) transition temperatures. Then at the
KT temperatures, we estimate the barriers to move vortices and effects that
lift the large degeneracy in the possible patterns. The effects we have
considered are inductive couplings, non-zero wire width, and the
order-by-disorder effect due to thermal fluctuations. The first two effects
prefer patterns while the last one selects a
pattern of supercurrents. Using the parameters of recent experiments, we
conclude that at the KT temperature, the non-zero wire width effect dominates,
which stabilizes a conventional superconducting phase with a current
pattern. However, by adjusting the experimental parameters, for example by
bending the wires a little, it appears that the novel superconducting
phase can instead be stabilized. The barriers to vortex motion are low enough
that the system can equilibrate into this phase.Comment: 30 pages including figure
Landau levels in the case of two degenerate coupled bands: kagome lattice tight-binding spectrum
The spectrum of charged particles hopping on a kagome lattice in a uniform
transverse magnetic field shows an unusual set of Landau levels at low field.
They are unusual in two respects: the lowest Landau levels are paramagnetic so
their energies decrease linearly with increasing field magnitude, and the
spacings between the levels are not equal. These features are shown to follow
from the degeneracy of the energy bands in zero magnetic field. We give a
general discussion of Landau levels in the case of two degenerate bands, and
show how the kagome lattice tight-binding model includes one special case of
this more general problem. We also discuss the consequences of this for the
behavior of the critical temperature of a kagome grid superconducting wire
network, which is the experimental system that originally motivated this work.Comment: 18 pages, 8 figure
Long range Neel order in the triangular Heisenberg model
We have studied the Heisenberg model on the triangular lattice using several
Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact
diagonalization (ED) (up to 36 sites). By studying the spin gap as a function
of the system size we have obtained a robust evidence for a gapless spectrum,
confirming the existence of long range Neel order. Our best estimate is that in
the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by
about 59% from its classical value and the ground state energy per site is
e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the
important ground state correlations at short distance.Comment: 4 pages, RevTeX + 4 encapsulated postscript figure
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