5,298 research outputs found
Structural Relationship between Negative Thermal Expansion and Quartic Anharmonicity of Cubic ScF_3
Cubic scandium trifluoride (ScF_3) has a large negative thermal expansion over a wide range of temperatures. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the densities of states, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO_9 structure of ScF_3, and accounts for phonon stiffening with the temperature and a significant part of the negative thermal expansion
Effect of spin-orbit coupling on the excitation spectrum of Andreev billiards
We consider the effect of spin-orbit coupling on the low energy excitation
spectrum of an Andreev billiard (a quantum dot weakly coupled to a
superconductor), using a dynamical numerical model (the spin Andreev map).
Three effects of spin-orbit coupling are obtained in our simulations: In zero
magnetic field: (1) the narrowing of the distribution of the excitation gap;
(2) the appearance of oscillations in the average density of states. In strong
magnetic field: (3) the appearance of a peak in the average density of states
at zero energy. All three effects have been predicted by random-matrix theory.Comment: 5 pages, 4 figure
Eynard-Mehta theorem, Schur process, and their pfaffian analogs
We give simple linear algebraic proofs of Eynard-Mehta theorem,
Okounkov-Reshetikhin formula for the correlation kernel of the Schur process,
and Pfaffian analogs of these results. We also discuss certain general
properties of the spaces of all determinantal and Pfaffian processes on a given
finite set.Comment: AMSTeX, 21 pages, a new section adde
Airy processes and variational problems
We review the Airy processes; their formulation and how they are conjectured
to govern the large time, large distance spatial fluctuations of one
dimensional random growth models. We also describe formulas which express the
probabilities that they lie below a given curve as Fredholm determinants of
certain boundary value operators, and the several applications of these
formulas to variational problems involving Airy processes that arise in
physical problems, as well as to their local behaviour.Comment: Minor corrections. 41 pages, 4 figures. To appear as chapter in "PASI
Proceedings: Topics in percolative and disordered systems
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
Bee-Friendly Beef: Developing Biodiverse Pastures to Increase Ecosystem Services
The capacity of grasslands to provide ecosystem services, such as pollinator resources, is often limited by lack of plant biodiversity. This is true of grasslands in the eastern US that are dominated by tall fescue (Festuca arundinacea) a non-native, cool-season grass that is typically toxic to cattle. This paper summarizes a research project in Virginia, USA exploring the idea that ecosystem services provided by tall fescue-dominated grasslands can be improved by increasing the plant biodiversity available to beef cattle and bees. Within three 6.5 ha tall fescue grasslands, we established 0.8 ha plots with a 17 species mix of native warm-season grasses (NWSGs) and wildflowers. Beginning in 2018, we measured grass and wildflower establishment, attractiveness of wildflowers to bees, abundance and diversity of bee communities in biodiverse pastures and adjacent tall fescue pastures. Many of the 18 species sown established well expect for NWSGs. Competition from wildflowers likely suppressed native grasses and limited forage availability for beef cattle. Cattle largely ignored the wildflowers. This finding suggests that cattle and pollinators can share this biodiverse grassland as their primary foods are mutually exclusive. The total number of bees was almost double in wildflower-enhanced grasslands compared with more typical tall fescue grasslands. We observed most bee landings on purple coneflower (Echinacea purpurea) and anise hyssop (Agastache foeniculum). Several weedy species such as milkweed (Asclepias syriaca) and musk thistle (Carduus nutans) were also attractive to bees. Preliminary analyses identified at least 28 bee morphospecies and a distinct bee community present in wildflower pastures. While these results were promising, more research is needed on ways to establish biodiverse grasslands so that a more optimal balance of grasses and wildflowers can be sustained to benefit both cattle production and pollinators
Q-Dependent Susceptibilities in Ferromagnetic Quasiperiodic Z-Invariant Ising Models
We study the q-dependent susceptibility chi(q) of a series of quasiperiodic
Ising models on the square lattice. Several different kinds of aperiodic
sequences of couplings are studied, including the Fibonacci and silver-mean
sequences. Some identities and theorems are generalized and simpler derivations
are presented. We find that the q-dependent susceptibilities are periodic, with
the commensurate peaks of chi(q) located at the same positions as for the
regular Ising models. Hence, incommensurate everywhere-dense peaks can only
occur in cases with mixed ferromagnetic-antiferromagnetic interactions or if
the underlying lattice is aperiodic. For mixed-interaction models the positions
of the peaks depend strongly on the aperiodic sequence chosen.Comment: LaTeX2e, 26 pages, 9 figures (27 eps files). v2: Misprints correcte
The Determinant Representation for a Correlation Function in Scaling Lee-Yang Model
We consider the scaling Lee-Yang model. It corresponds to the unique
perturbation of the minimal CFT model M(2,5). This is not a unitary model. We
used known expression for form factors in order to obtain a closed expression
for a correlation function of a trace of energy-momentum tensor. This
expression is a determinant of an integral operator. Similar determinant
representation were proven to be useful not only for quantum correlation
functions but also in matrix models.Comment: 14 pages, LaTeX, no figure
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