219 research outputs found
Influence of annealing parameters on the ferromagnetic properties of optimally passivated (Ga,Mn)As epilayers
The influence of annealing parameters - temperature and time - on the
magnetic properties of As-capped (Ga,Mn)As epitaxial thin films have been
investigated. The dependence of the transition temperature (Tc) on annealing
time marks out two regions. The Tc peak behavior, characteristic of the first
region, is more pronounced for thick samples, while for the second
(`saturated') region the effect of the annealing time is more pronounced for
thin samples. A right choice of the passivation medium, growth conditions along
with optimal annealing parameters routinely yield Tc-values of ~ 150 K and
above, regardless of the thickness of the epilayers.Comment: 5 pages, 3 figure
Case Study of a Water Tank Behaviour on an Improved Collapsible Soil
The geotechnical report performed for the design stage of a water tank revealed a soil profile consisting in a thick layer of collapsible/loessial soil. The paper firstly presents the complex characterization of the natural ground conditions before and after the soil cushion performance, during the water filling tests of the tank. Specific charts are presented to emphasize the physical and mechanical parameter differences of the natural and improved ground by the soil cushion. The prediction of the supplementary settlement profile on the construction site of the water tank has been performed due to a significant water leakage from the tank during the filling tests, and thus endangering the tank stability and serviceability. Charts presenting the soil-tank interaction during service are included together with settlement diagrams related to potential water leakage from the tank. The paper presents in the second part the stress and strain states that have been comparatively analyzed for various moistening hypotheses with different risk level, according to the settlement increase based on the up going of the moistening front
The Energy Operator for a Model with a Multiparametric Infinite Statistics
In this paper we consider energy operator (a free Hamiltonian), in the
second-quantized approach, for the multiparameter quon algebras:
with
any hermitian matrix of deformation parameters. We obtain
an elegant formula for normally ordered (sometimes called Wick-ordered) series
expansions of number operators (which determine a free Hamiltonian). As a main
result (see Theorem 1) we prove that the number operators are given, with
respect to a basis formed by "generalized Lie elements", by certain normally
ordered quadratic expressions with coefficients given precisely by the entries
of the inverses of Gram matrices of multiparticle weight spaces. (This settles
a conjecture of two of the authors (S.M and A.P), stated in [8]). These Gram
matrices are hermitian generalizations of the Varchenko's matrices, associated
to a quantum (symmetric) bilinear form of diagonal arrangements of hyperplanes
(see [12]). The solution of the inversion problem of such matrices in [9]
(Theorem 2.2.17), leads to an effective formula for the number operators
studied in this paper. The one parameter case, in the monomial basis, was
studied by Zagier [15], Stanciu [11] and M{\o}ller [6].Comment: 24 pages. accepted in J. Phys. A. Math. Ge
Boundary states in the Nappi-Witten model
We investigate D-branes in the Nappi-Witten model. Classically symmetric
D-branes are classified by the (twisted) conjugacy classes of the Nappi-Witten
group, which specify the geometry of the corresponding D-branes. Quantum
description of the D-branes is given by boundary states, and we need one point
functions of closed strings to construct the boundary states. We compute the
one point functions solving conformal bootstrap constraints, and check that the
classical limit of the boundary states reproduces the geometry of D-branes.Comment: 19 pages, no figure; minor changes, references adde
Computing joint action costs: co-actors minimize the aggregate individual costs in an action sequence
Successful performance in cooperative activities relies on efficient task distribution between co-actors. Previous research found that people often forgo individual efficiency in favor of co-efficiency (i.e., joint-cost minimization) when planning a joint action. The present study investigated the cost computations underlying co-efficient decisions. We report a series of experiments that tested the hypothesis that people compute the joint costs of a cooperative action sequence by summing the individual action costs of their co-actor and themselves. We independently manipulated the parameters quantifying individual and joint action costs and tested their effects on decision-making by fitting and comparing Bayesian logistic regression models. Our hypothesis was confirmed: people weighed their own and their partner’s costs similarly to estimate the joint action costs as the sum of the two individual parameters. Participants minimized the aggregate cost to ensure co-efficiency. The results provide empirical support for behavioral economics and computational approaches that formalize cooperation as joint utility maximization based on a weighted sum of individual action costs
Isometric Embeddings and Noncommutative Branes in Homogeneous Gravitational Waves
We characterize the worldvolume theories on symmetric D-branes in a
six-dimensional Cahen-Wallach pp-wave supported by a constant Neveu-Schwarz
three-form flux. We find a class of flat noncommutative euclidean D3-branes
analogous to branes in a constant magnetic field, as well as curved
noncommutative lorentzian D3-branes analogous to branes in an electric
background. In the former case the noncommutative field theory on the branes is
constructed from first principles, related to dynamics of fuzzy spheres in the
worldvolumes, and used to analyse the flat space limits of the string theory.
The worldvolume theories on all other symmetric branes in the background are
local field theories. The physical origins of all these theories are described
through the interplay between isometric embeddings of branes in the spacetime
and the Penrose-Gueven limit of AdS3 x S3 with Neveu-Schwarz three-form flux.
The noncommutative field theory of a non-symmetric spacetime-filling D-brane is
also constructed, giving a spatially varying but time-independent
noncommutativity analogous to that of the Dolan-Nappi model.Comment: 52 pages; v2: References adde
Properties of branes in curved spacetimes
A generic property of curved manifolds is the existence of focal points. We
show that branes located at focal points of the geometry satisfy special
properties. Examples of backgrounds to which our discussion applies are AdS_m x
S^n and plane wave backgrounds. As an example, we show that a pair of AdS_2
branes located at the north and south pole of the S^5 in AdS_5 x S^5 are half
supersymmetric and that they are dual to a two-monopole solution of N=4 SU(N)
SYM theory. Our second example involves spacelike branes in the (Lorentzian)
plane wave. We develop a modified lightcone gauge for the open string channel,
analyze in detail the cylinder diagram and establish open-closed duality. When
the branes are located at focal points of the geometry the amplitude acquires
most of the characteristics of flat space amplitudes. In the open string
channel the special properties are due to stringy modes that become massless.Comment: 41 pages; v2:typos corrected, ref adde
Non-local string theories on AdS_3 times S^3 and stable non-supersymmetric backgrounds
We exhibit a simple class of exactly marginal "double-trace" deformations of
two dimensional CFTs which have AdS_3 duals, in which the deformation is given
by a product of left and right-moving U(1) currents. In this special case the
deformation on AdS_3 is generated by a local boundary term in three dimensions,
which changes the physics also in the bulk via bulk-boundary propagators.
However, the deformation is non-local in six dimensions and on the string
worldsheet, like generic non-local string theories (NLSTs). Due to the
simplicity of the deformation we can explicitly make computations in the
non-local string theory and compare them to CFT computations, and we obtain
precise agreement. We discuss the effect of the deformation on closed strings
and on D-branes. The examples we analyze include a supersymmetry-breaking but
exactly marginal "double-trace" deformation, which is dual to a string theory
in which no destabilizing tadpoles are generated for moduli nonperturbatively
in all couplings, despite the absence of supersymmetry. We explain how this
cancellation works on the gravity side in string perturbation theory, and also
non-perturbatively at leading order in the deformation parameter. We also
discuss possible flat space limits of our construction.Comment: 40 pages, 6 figures, harvma
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