12,572 research outputs found
Spin-resolved optical conductivity of two-dimensional group-VIB transition-metal dichalcogenides
We present an ab-initio study of the spin-resolved optical conductivity of
two-dimensional (2D) group-VIB transition-metal dichalcogenides (TMDs). We
carry out fully-relativistic density-functional-theory calculations combined
with maximally localized Wannier functions to obtain band manifolds at
extremely high resolutions and focus on the photo-response of 2D TMDs to
circularly-polarized light in a wide frequency range. We present extensive
numerical results for monolayer TMDs involving molybdenum and tungsten combined
with sulphur and selenium. Our numerical approach allows us to locate with a
high degree of accuracy the positions of the points in the Brillouin zone that
are responsible for van Hove singularities in the optical response.
Surprisingly, some of the saddle points do not occur exactly along
high-symmetry directions in the Brillouin zone, although they happen to be in
their close proximity.Comment: 9 pages, 5 figure
Transport properties of room temperature ionic liquids from classical molecular dynamics
Room Temperature Ionic Liquids (RTILs) have attracted much of the attention
of the scientific community in the past decade due the their novel and highly
customizable properties. Nonetheless their high viscosities pose serious
limitations to the use of RTILs in practical applications. To elucidate some of
the physical aspects behind transport properties of RTILs, extensive classical
molecular dynamics (MD) calculations are reported. Bulk viscosities and ionic
conductivities of butyl-methyl-imidazole based RTILs are presented over a wide
range of temperatures. The dependence of the properties of the liquids on
simulation parameters, e.g. system size effects and choice of the interaction
potential, is analyzed
Accurate polarization within a unified Wannier function formalism
We present an alternative formalism for calculating the maximally localized
Wannier functions in crystalline solids, obtaining an expression which is
extremely simple and general. In particular, our scheme is exactly invariant
under Brillouin zone folding, and therefore it extends trivially to the
Gamma-point case. We study the convergence properties of the Wannier functions,
their quadratic spread and centers as obtained by our simplified technique. We
show how this convergence can be drastically improved by a simple and
inexpensive ``refinement'' step, which allows for very efficient and accurate
calculations of the polarization in zero external field.Comment: 9 pages, 4 figure
Mass spectrum from stochastic Levy-Schroedinger relativistic equations: possible qualitative predictions in QCD
Starting from the relation between the kinetic energy of a free
Levy-Schroedinger particle and the logarithmic characteristic of the underlying
stochastic process, we show that it is possible to get a precise relation
between renormalizable field theories and a specific Levy process. This
subsequently leads to a particular cut-off in the perturbative diagrams and can
produce a phenomenological mass spectrum that allows an interpretation of
quarks and leptons distributed in the three families of the standard model.Comment: 8 pages, no figures. arXiv admin note: substantial text overlap with
arXiv:1008.425
Superconducting quantum point contact with split gates in the two dimensional LaAlO3/SrTiO3 superfluid
One of the hallmark experiments of quantum transport is the observation of
the quantized resistance in a point contact formed with split gates in
GaAs/AlGaAs heterostructures. Being carried out on a single material, they
represent in an ideal manner equilibrium reservoirs which are connected only
through a few electron mode channel with certain transmission coefficients. It
has been a long standing goal to achieve similar experimental conditions also
in superconductors, only reached in atomic scale mechanically tunable break
junctions of conventional superconducting metals, but here the Fermi wavelength
is so short that it leads to a mixing of quantum transport with atomic orbital
physics. Here we demonstrate for the first time the formation of a
superconducting quantum point contact (SQPC) with split gate technology in a
superconductor, utilizing the unique gate tunability of the two dimensional
superfluid at the LaAlO3/SrTiO3 (LAO/STO) interface. When the constriction is
tuned through the action of metallic split gates we identify three regimes of
transport: (i) SQPC for which the supercurrent is carried only by a few quantum
transport channels. (ii) Superconducting island strongly coupled to the
equilibrium reservoirs. (iii) Charge island with a discrete spectrum weakly
coupled to the reservoirs. Our experiments demonstrate the feasibility of a new
generation of mesoscopic all-superconductor quantum transport devices.Comment: 18 page
Sex allocation theory reveals a hidden cost of neonicotinoid exposure in a parasitoid wasp
P.R.W. was funded by the University of Stirling, C.V.B. and S.M.G. were funded by Nuffield Research Placements and N.C., J.G. and D.M.S. were funded by NERC (NE/J024481/1).Sex allocation theory has proved to be one the most successful theories in evolutionary ecology. However, its role in more applied aspects of ecology has been limited. Here we show how sex allocation theory helps uncover an otherwise hidden cost of neonicotinoid exposure in the parasitoid wasp Nasonia vitripennis. Female N. vitripennis allocate the sex of their offspring in line with Local Mate Competition (LMC) theory. Neonicotinoids are an economically important class of insecticides, but their deployment remains controversial, with evidence linking them to the decline of beneficial species. We demonstrate for the first time to our knowledge, that neonicotinoids disrupt the crucial reproductive behaviour of facultative sex allocation at sub-lethal, field-relevant doses in N. vitripennis. The quantitative predictions we can make from LMC theory show that females exposed to neonicotinoids are less able to allocate sex optimally and that this failure imposes a significant fitness cost. Our work highlights that understanding the ecological consequences of neonicotinoid deployment requires not just measures of mortality or even fecundity reduction among non-target species, but also measures that capture broader fitness costs, in this case offspring sex allocation. Our work also highlights new avenues for exploring how females obtain information when allocating sex under LMC.Publisher PDFPeer reviewe
Mercury's lowâdegree geoid and topography controlled by insolationâdriven elastic deformation
©2015. American Geophysical UnionMercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a longâwavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasiâhydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercury's geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation
Lorentz-Lorenz Coefficient, Critical Point Constants, and Coexistence Curve of 1,1-Difluoroethylene
We report measurements of the Lorentz-Lorenz coefficient density dependence,
the critical temperature, and the critical density, of the fluid
1,1-difluoroethylene. Lorentz-Lorenz coefficient data were obtained by
measuring refractive index and density of the same fluid sample independently
of one another. Accurate determination of the Lorentz-Lorenz coefficient is
necessary for transformation of refractive index data into density data from
optics-based experiments on critical phenomena of fluid systems done with
different apparatus, with which independent measurement of the refractive indes
and density is not possible. Measurements were made along the coexistence curve
of the fluid and span the density range 0.01 to 0.80 g/cc. The Lorentz-Lorenz
coefficient results show a stronger density dependence along the coexistence
curve than previously observed in other fluids, with a monotonic decrease from
a density of about 0.2 g/cc onwards, and an overall variation of about 2.5% in
the density range studied. No anomaly in the Lorentz-Lorenz coefficient was
observed near the critical density. The critical temperature is measured at
Tc=(302.964+-0.002) K (29.814 C) and the measured critical density is
(0.4195+-0.0018)g/cc.Comment: 14 pages, 6 figures, MikTeX 2.4, submitted to Physical Review
Present-day Mars' seismicity predicted from 3-D thermal evolution models of interior dynamics
©2018. American Geophysical UnionThe Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the presentâday amount and distribution of seismicity using 3âD numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two endâmember cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 Ă 1016 and 3.9 Ă 1019 Nm
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