128 research outputs found
Kagom\'{e} ice state in the dipolar spin ice Dy_{2}Ti_{2}O_{7}
We have investigated the kagom\'{e} ice behavior of the dipolar spin-ice
compound Dy_{2}Ti_{2}O_{7} in magnetic field along a [111] direction using
neutron scattering and Monte Carlo simulations. The spin correlations show that
the kagom\'{e} ice behavior predicted for the nearest-neighbor (NN) interacting
model, where the field induces dimensional reduction and spins are frustrated
in each two-dimensional kagom\'{e} lattice, occurs in the dipole interacting
system. The spins freeze at low temperatures within the macroscopically
degenerate ground states of the NN model.Comment: 5 pages, 3 figures, submitted to PR
Haydeeite: a spin-1/2 kagome ferromagnet
The mineral haydeeite, alpha-MgCu3(OD)6Cl2, is a S=1/2 kagome ferromagnet
that displays long-range magnetic order below TC=4.2 K with a strongly reduced
moment. Our inelastic neutron scattering data show clear spin-wave excitations
that are well described by a Heisenberg Hamiltonian with ferromagnetic
nearest-neighbor exchange J1=-38 K and antiferromagnetic exchange Jd=+11 K
across the hexagons of the kagome lattice. These values place haydeeite very
close to the quantum phase transition between ferromagnetic order and
non-coplanar twelve-sublattice cuboc2 antiferromagnetic order. Diffuse dynamic
short-range ferromagnetic correlations observed above TC persist well into the
ferromagnetically ordered phase with a behavior distinct from critical
scattering
Magnetic phase diagram of spatially anisotropic, frustrated spin-1/2 Heisenberg antiferromagnet on a stacked square lattice
Magnetic phase diagram of a spatially anisotropic, frustrated spin-1/2
Heisenberg antiferromagnet on a stacked square lattice is investigated using
second-order spin-wave expansion. The effects of interlayer coupling and the
spatial anisotropy on the magnetic ordering of two ordered ground states are
explicitly studied. It is shown that with increase in next nearest neighbor
frustration the second-order corrections play a significant role in stabilizing
the magnetization. We obtain two ordered magnetic phases (Neel and stripe)
separated by a paramagnetic disordered phase. Within second-order spin-wave
expansion we find that the width of the disordered phase diminishes with
increase in the interlayer coupling or with decrease in spatial anisotropy but
it does not disappear. Our obtained phase diagram differs significantly from
the phase diagram obtained using linear spin-wave theory.Comment: 22 pages, 6 figures, minor changes from previous versio
The dispersion relation of Landau elementary excitations and the thermodynamic properties of superfluid He
The dispersion relation of the elementary excitations of
superfluid He has been measured at very low temperatures, from saturated
vapor pressure up to solidification, using a high flux time-of-flight neutron
scattering spectrometer equipped with a high spatial resolution detector
(10 'pixels'). A complete determination of is achieved, from
very low wave-vectors up to the end of Pitaeskii's plateau. The results compare
favorably in the whole the wave-vector range with the predictions of the
dynamic many-body theory (DMBT). At low wave-vectors, bridging the gap between
ultrasonic data and former neutron measurements, the evolution with the
pressure from anomalous to normal dispersion, as well as the peculiar
wave-vector dependence of the phase and group velocities, are accurately
characterized. The thermodynamic properties have been calculated analytically,
developing Landau's model, using the measured dispersion curve. A good
agreement is found below 0.85 K between direct heat capacity measurements and
the calculated specific heat, if thermodynamically consistent power series
expansions are used. The thermodynamic properties have also been calculated
numerically; in this case, the results are applicable with excellent accuracy
up to 1.3 K, a temperature above which the dispersion relation itself becomes
temperature dependent.Comment: 35 pages, 59 figures; Supplemental Material: Text, Tables, data file
Low-frequency excitations in glassy selenium: A comparison of neutron-scattering and molecular-dynamics results
The microscopic low-frequency dynamics of glassy selenium is investigated by means of the concurrent use of neutron inelastic scattering and computer simulations. A separation of the dynamic response in terms of intra- and interchain processes is achieved from the analysis of the simulation results. The S(Q,E) dynamic structure factors are analyzed in terms of the frequency moments or from a model scattering law, and the wave-vector dependence of the relevant quantities is established. Finally, the anomalous behavior of the heat capacity at moderately low temperatures is shown to be originated by mostly interchain interactions.Dirección General de Investigación Científica y Técnica PB89-0037-C
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