11,871 research outputs found
Supersymmetry of Noncompact MQCD-like Membrane Instantons and Heat Kernel Asymptotics
We perform a heat kernel asymptotics analysis of the nonperturbative
superpotential obtained from wrapping of an M2-brane around a supersymmetric
noncompact three-fold embedded in a (noncompact) G_2-manifold as obtained in
[1], the three-fold being the one relevant to domain walls in Witten's MQCD
[2], in the limit of small "zeta", a complex constant that appears in the
Riemann surfaces relevant to defining the boundary conditions for the domain
wall in MQCD. The MQCD-like configuration is interpretable, for small but
non-zero zeta as a noncompact/"large" open membrane instanton, and for
vanishing zeta, as the type IIA D0-brane (for vanishing M-theory cicle radius).
We find that the eta-function Seeley de-Witt coefficients vanish, and we get a
perfect match between the zeta-function Seeley de-Witt coefficients (up to
terms quadratic in zeta) between the Dirac-type operator and one of the two
Laplace-type operators figuring in the superpotential. This is an extremely
strong signature of residual supersymmetry for the nonperturbative
configurations in M-theory considered in this work.Comment: 21 pages, LaTeX; v3: several clarifying remarks added, to appear in
JHE
The influence of atmosphere on the performance of pure-phase WZ and ZB InAs nanowire transistors
We compare the characteristics of phase-pure MOCVD grown ZB and WZ InAs
nanowire transistors in several atmospheres: air, dry pure N and O, and
N bubbled through liquid HO and alcohols to identify whether
phase-related structural/surface differences affect their response. Both WZ and
ZB give poor gate characteristics in dry state. Adsorption of polar species
reduces off-current by 2-3 orders of magnitude, increases on-off ratio and
significantly reduces sub-threshold slope. The key difference is the greater
sensitivity of WZ to low adsorbate level. We attribute this to facet structure
and its influence on the separation between conduction electrons and surface
adsorption sites. We highlight the important role adsorbed species play in
nanowire device characterisation. WZ is commonly thought superior to ZB in InAs
nanowire transistors. We show this is an artefact of the moderate humidity
found in ambient laboratory conditions: WZ and ZB perform equally poorly in the
dry gas limit yet equally well in the wet gas limit. We also highlight the
vital role density-lowering disorder has in improving gate characteristics, be
it stacking faults in mixed-phase WZ or surface adsorbates in pure-phase
nanowires.Comment: Accepted for publication in Nanotechnolog
Asteroseismology of red-clump stars with CoRoT and Kepler
The availability of asteroseismic constraints for a large number of red
giants with CoRoT and in the near future with Kepler, paves the way for
detailed studies of populations of galactic-disk red giants. We investigate
which information on the observed population can be recovered by the
distribution of the observed seismic constraints: the frequency of maximum
power of solar-like oscillations () and the large frequency
separation (). We use the distribution of and of
observed by CoRoT in nearly 800 red giants in the first long
observational run, as a tool to investigate the properties of galactic
red-giant stars through the comparison with simulated distributions based on
synthetic stellar populations.
We can clearly identify the bulk of the red giants observed by CoRoT as
red-clump stars, i.e. post-flash core-He-burning stars. The distribution of
and of give us access to the distribution of the
stellar radius and mass, and thus represent a most promising probe of the age
and star formation rate of the disk, and of the mass-loss rate during the
red-giant branch.
This approach will be of great utility also in the interpretation of
forthcoming surveys of variability of red giants with CoRoT and Kepler. In
particular, an asteroseismic mass estimate of clump stars in the old-open
clusters observed by Kepler, would represent a most valuable observational test
of the poorly known mass-loss rate on the giant branch, and of its dependence
on metallicity.Comment: 5 pages, 6 figures, proceeding for "Stellar Pulsation: Challenges for
Theory and Observation", Santa Fe 200
Light Curve Patterns and Seismology of a White Dwarf with Complex Pulsation
The ZZ Ceti star KUV 02464+3239 was observed over a whole season at the
mountain station of Konkoly Observatory. A rigorous frequency analysis revealed
6 certain periods between 619 and 1250 seconds, with no shorter period modes
present. We use the observed periods, published effective temperature and
surface gravity, along with the model grid code of Bischoff-Kim, Montgomery and
Winget (2008) to perform a seismological analysis. We find acceptable model
fits with masses between 0.60 and 0.70 M_Sun. The hydrogen layer mass of the
acceptable models are almost always between 10^-4 and 10^-6 M_*. In addition to
our seismological results, we also show our analysis of individual light curve
segments. Considering the non-sinusoidal shape of the light curve and the
Fourier spectra of segments showing large amplitude variations, the importance
of non-linear effects in the pulsation is clearly seen.Comment: 5 pages, 6 figures, in "Stellar Pulsation: Challenges for Theory and
Observation", Eds. J. Guzik and P. A. Bradley, AIP
The role of initial conditions in the ageing of the long-range spherical model
The kinetics of the long-range spherical model evolving from various initial
states is studied. In particular, the large-time auto-correlation and -response
functions are obtained, for classes of long-range correlated initial states,
and for magnetized initial states. The ageing exponents can depend on certain
qualitative features of initial states. We explicitly find the conditions for
the system to cross over from ageing classes that depend on initial conditions
to those that do not.Comment: 15 pages; corrected some typo
Integrating Dynamics and Wear Modelling to Predict Railway Wheel Profile Evolution
The aim of the work described was to predict wheel
profile evolution by integrating multi-body dynamics
simulations of a wheelset with a wear model.
The wear modelling approach is based on a wear
index commonly used in rail wear predictions. This
assumes wear is proportional to Tγ, where T is tractive
force and γ is slip at the wheel/rail interface. Twin disc
testing of rail and wheel materials was carried out to
generate wear coefficients for use in the model.
The modelling code is interfaced with
ADAMS/Rail, which produces multi-body dynamics
simulations of a railway wheelset and contact conditions
at the wheel/rail interface. Simplified theory of rolling
contact is used to discretise the contact patches
produced by ADAMS/Rail and calculate traction and
slip within each.
The wear model combines the simplified theory of
rolling contact, ADAMS/Rail output and the wear
coefficients to predict the wear and hence the change of
wheel profile for given track layouts
Optimal interlayer hopping and high temperature Bose–Einstein condensation of local pairs in quasi 2D superconductors
Both FeSe and cuprate superconductors are quasi 2D materials with high transition temperatures and local fermion pairs. Motivated by such systems, we investigate real space pairing of fermions in an anisotropic lattice model with intersite attraction, V, and strong local Coulomb repulsion, U, leading to a determination of the optimal conditions for superconductivity from Bose–Einstein condensation. Our aim is to gain insight as to why high temperature superconductors tend to be quasi 2D. We make both analytically and numerically exact solutions for two body local pairing applicable to intermediate and strong V. We find that the Bose–Einstein condensation temperature of such local pairs pairs is maximal when hopping between layers is intermediate relative to in-plane hopping, indicating that the quasi 2D nature of unconventional superconductors has an important contribution to their high transition temperatures
A New Source for Electroweak Baryogenesis in the MSSM
One of the most experimentally testable explanations for the origin of the
baryon asymmetry of the universe is that it was created during the electroweak
phase transition, in the minimal supersymmetric standard model. Previous
efforts have focused on the current for the difference of the two Higgsino
fields, , as the source of biasing sphalerons to create the baryon
asymmetry. We point out that the current for the orthogonal linear combination,
, is larger by several orders of magnitude. Although this increases
the efficiency of electroweak baryogenesis, we nevertheless find that large
CP-violating angles are required to get a large enough baryon
asymmetry.Comment: 4 pages, 2 figures; numerical error corrected, which implies that
large CP violation is needed to get observed baryon asymmetry. We improved
solution of diffusion equations, and computed more accurate values for
diffusion coefficient and damping rate
1-d gravity in infinite point distributions
The dynamics of infinite, asymptotically uniform, distributions of
self-gravitating particles in one spatial dimension provides a simple toy model
for the analogous three dimensional problem. We focus here on a limitation of
such models as treated so far in the literature: the force, as it has been
specified, is well defined in infinite point distributions only if there is a
centre of symmetry (i.e. the definition requires explicitly the breaking of
statistical translational invariance). The problem arises because naive
background subtraction (due to expansion, or by "Jeans' swindle" for the static
case), applied as in three dimensions, leaves an unregulated contribution to
the force due to surface mass fluctuations. Following a discussion by
Kiessling, we show that the problem may be resolved by defining the force in
infinite point distributions as the limit of an exponentially screened pair
interaction. We show that this prescription gives a well defined (finite) force
acting on particles in a class of perturbed infinite lattices, which are the
point processes relevant to cosmological N-body simulations. For identical
particles the dynamics of the simplest toy model is equivalent to that of an
infinite set of points with inverted harmonic oscillator potentials which
bounce elastically when they collide. We discuss previous results in the
literature, and present new results for the specific case of this simplest
(static) model starting from "shuffled lattice" initial conditions. These show
qualitative properties (notably its "self-similarity") of the evolution very
similar to those in the analogous simulations in three dimensions, which in
turn resemble those in the expanding universe.Comment: 20 pages, 8 figures, small changes (section II shortened, added
discussion in section IV), matches final version to appear in PR
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