2,143 research outputs found
Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS
FePS is a van der Waals compound with a honeycomb lattice that is a good
example of a two-dimensional antiferromagnet with Ising-like anisotropy.
Neutron spectroscopy data from FePS3 were previously analysed using a
straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The
analysis captured most of the elements of the data, however some significant
discrepancies remained. The discrepancies were most obvious at the Brillouin
zone boundaries. The data are subsequently reanalysed allowing for unequal
exchange between nominally equivalent nearest-neighbours, which resolves the
discrepancies. The source of the unequal exchange is attributed to a
biquadratic exchange term in the Hamiltonian which most probably arises from a
strong magnetolattice coupling. The new parameters show that there are features
consistent with Dirac magnon nodal lines along certain Brillouin zone
boundaries.Comment: 8 pages, 4 figures. The following article has been accepted by the
Journal of Applied Physics. After it is published, it will be found at
(https://publishing.aip.org/resources/librarians/products/journals/). The
article was submitted as part of a special topic edition
(https://publishing.aip.org/publications/journals/special-topics/jap/2d-quantum-materials-magnetism-and-superconductivity/
Integrated J- and H-band spectra of globular clusters in the LMC: implications for stellar population models and galaxy age dating
(Abridged) The rest-frame near-IR spectra of intermediate age (1-2 Gyr)
stellar populations are dominated by carbon based absorption features offering
a wealth of information. Yet, spectral libraries that include the near-IR
wavelength range do not sample a sufficiently broad range of ages and
metallicities to allow for accurate calibration of stellar population models
and thus the interpretation of the observations. In this paper we investigate
the integrated J- and H-band spectra of six intermediate age (1-3 Gyr) and old
(>10 Gyr) globular clusters in the Large Magellanic Cloud, using observations
obtained with the SINFONI IFU at the VLT. H-band C2 and K-band 12CO(2-0)
feature strengths are compared to the models of Maraston (2005). C2 is
reasonably well reproduced by the models at all ages, while 12CO(2-0) shows
good agreement for older (age>2 Gyr) populations, but the younger (1.3 Gyr)
globular clusters do not follow the models. We argue that this is due to the
fact that the empirical calibration of the models relies on only a few Milky
Way carbon star spectra, which show different 12CO(2-0) index strengths than
the LMC stars. The C2 absorption feature strength correlates strongly with age.
It is present essentially only in populations that have 1-2 Gyr old stars,
while its value is consistent with zero for older populations. The distinct
spectral energy distribution observed for the intermediate age globular
clusters in the J- and H-bands agrees well with the model predictions of
Maraston for the contribution from the thermally pulsing asymptotic giant
branch phase (TP-AGB). We show that the H-band C2 absorption feature and the
J-, H-band spectral shape can be used as an age indicator for intermediate age
stellar populations in integrated spectra of star clusters and galaxies.Comment: 10 pages, 6 figures, abstract abridged, accepted for publication in
A&
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
Thermophoresis of Brownian particles driven by coloured noise
The Brownian motion of microscopic particles is driven by the collisions with
the molecules of the surrounding fluid. The noise associated with these
collisions is not white, but coloured due, e.g., to the presence of
hydrodynamic memory. The noise characteristic time scale is typically of the
same order as the time over which the particle's kinetic energy is lost due to
friction (inertial time scale). We demonstrate theoretically that, in the
presence of a temperature gradient, the interplay between these two
characteristic time scales can have measurable consequences on the particle
long-time behaviour. Using homogenization theory, we analyse the infinitesimal
generator of the stochastic differential equation describing the system in the
limit where the two characteristic times are taken to zero; from this
generator, we derive the thermophoretic transport coefficient, which, we find,
can vary in both magnitude and sign, as observed in experiments. Furthermore,
studying the long-term stationary particle distribution, we show that particles
can accumulate towards the colder (positive thermophoresis) or the warmer
(negative thermophoresis) regions depending on the dependence of their physical
parameters and, in particular, their mobility on the temperature.Comment: 9 pages, 4 figure
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