1,126 research outputs found
Momentum dependent light scattering in insulating cuprates
We investigate the problem of inelastic x-ray scattering in the spin
Heisenberg model on the square lattice. We first derive a momentum dependent
scattering operator for the and polarization geometries. On
the basis of a spin-wave analysis, including magnon-magnon interactions and
exact-diagonalizations, we determine the qualitative shape of the spectra. We
argue that our results may be relevant to help interpret inelastic x-ray
scattering experiments in the antiferromagnetic phase of the cuprates.Comment: 5 pages, 3 figures, to appear in PR
Magnetic charge and ordering in kagome spin ice
We present a numerical study of magnetic ordering in spin ice on kagome, a
two-dimensional lattice of corner-sharing triangles. The magnet has six ground
states and the ordering occurs in two stages, as one might expect for a
six-state clock model. In spin ice with short-range interactions up to second
neighbors, there is an intermediate critical phase separated from the
paramagnetic and ordered phases by Kosterlitz-Thouless transitions. In dipolar
spin ice, the intermediate phase has long-range order of staggered magnetic
charges. The high and low-temperature phase transitions are of the Ising and
3-state Potts universality classes, respectively. Freeze-out of defects in the
charge order produces a very large spin correlation length in the intermediate
phase. As a result of that, the lower-temperature transition appears to be of
the Kosterlitz-Thouless type.Comment: 20 pages, 12 figures, accepted version with minor change
Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice
Magnetic frustration effects in artificial kagome arrays of nanomagnets are
investigated using x-ray photoemission electron microscopy and Monte Carlo
simulations. Spin configurations of demagnetized networks reveal unambiguous
signatures of long range, dipolar interaction between the nanomagnets. As soon
as the system enters the spin ice manifold, the kagome dipolar spin ice model
captures the observed physics, while the short range kagome spin ice model
fails.Comment: 4 pages, 4 figures, 1 tabl
Ferroelectric and Dipolar Glass Phases of Non-Crystalline Systems
In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly
discussed the existence and nature of ferroelectric order in positionally
disordered dipolar materials. Here we report further results and give a
complete description of our work. Simulations of randomly frozen and
dynamically disordered dipolar soft spheres are used to study ferroelectric
ordering in non-crystalline systems. We also give a physical interpretation of
the simulation results in terms of short- and long-range interactions. Cases
where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models,
respectively) are considered. It is found that the Ising model displays
ferroelectric phases in frozen amorphous systems, while the XY and XYZ models
form dipolar glass phases at low temperatures. In the dynamically disordered
model the equations of motion are decoupled such that particle translation is
completely independent of the dipolar forces. These systems spontaneously
develop long-range ferroelectric order at nonzero temperature despite the
absence of any fined-tuned short-range spatial correlations favoring dipolar
order. Furthermore, since this is a nonequilibrium model we find that the
paraelectric to ferroelectric transition depends on the particle mass. For the
XY and XYZ models, the critical temperatures extrapolate to zero as the mass of
the particle becomes infinite, whereas, for the Ising model the critical
temperature is almost independent of mass and coincides with the ferroelectric
transition found for the randomly frozen system at the same density. Thus in
the infinite mass limit the results of the frozen amorphous systems are
recovered.Comment: 25 pages (LATEX, no macros). 11 POSTSCRIPT figures enclosed.
Submitted to Phisical Review E. Contact: [email protected]
Experimental Predictions of The Functional Response of A Freshwater Fish
The functional response is the relationship between the feeding rate of an animal and its food density. It is reliant on two basic parameters; the volume searched for prey per unit time (searching rate) and the time taken to consume each prey item (handling time). As fish functional responses can be difficult to determine directly, it may be more feasible to measure their underlying behavioural parameters in controlled conditions and use these to predict the functional response. Here, we tested how accurately a Type II functional response model predicted the observed functional response of roach Rutilus rutilus, a visually foraging fish, and compared it with Type I functional response. Foraging experiments were performed by exposing fish in tank aquaria to a range of food densities, with their response captured using a two-camera videography system. This system was validated and was able to accurately measure fish behaviour in the aquaria, and enabled estimates of fish reaction distance, swimming speed (from which searching rate was calculated) and handling time to be measured. The parameterised Type II functional response model accurately predicted the observed functional response and was superior to the Type I model. These outputs suggest it will be possible to accurately measure behavioural parameters in other animal species and use these to predict the functional response in situations where it cannot be observed directly
Random site dilution properties of frustrated magnets on a hierarchical lattice
We present a method to analyze magnetic properties of frustrated Ising spin
models on specific hierarchical lattices with random dilution. Disorder is
induced by dilution and geometrical frustration rather than randomness in the
internal couplings of the original Hamiltonian. The two-dimensional model
presented here possesses a macroscopic entropy at zero temperature in the large
size limit, very close to the Pauling estimate for spin-ice on pyrochlore
lattice, and a crossover towards a paramagnetic phase. The disorder due to
dilution is taken into account by considering a replicated version of the
recursion equations between partition functions at different lattice sizes. An
analysis at first order in replica number allows for a systematic
reorganization of the disorder configurations, leading to a recurrence scheme.
This method is numerically implemented to evaluate the thermodynamical
quantities such as specific heat and susceptibility in an external field.Comment: 26 pages, 11 figure
Glassiness Vs. Order in Densely Frustrated Josephson Arrays
We carry out extensive Monte Carlo simulations on the Coulomb gas dual to the
uniformly frustrated two dimensional XY model, for a sequence of frustrations f
converging to the irraltional (3-sqrt 5)/2. We find in these systems a sharp
first order equilibrium phase transition to an ordered vortex structure at a
T_c which varies only slightly with f. This ordered vortex structure remains in
general phase incoherent until a lower pinning transition T_p(f) that varies
with f. We argue that the glassy behaviors reported for this model in earlier
simulations are dynamic effects.Comment: 4 pages, 4 eps figure
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