1,186 research outputs found
Triaxial Angular Momentum Projection and Configuration Mixing calculations with the Gogny force
We present the first implementation in the plane of the
generator coordinate method with full triaxial angular momentum and particle
number projected wave functions using the Gogny force. Technical details about
the performance of the method and the convergence of the results both in the
symmetry restoration and the configuration mixing parts are discussed in
detail. We apply the method to the study of Mg, the calculated energies
of excited states as well as the transition probabilities are compared to the
available experimental data showing a good overall agreement. In addition, we
present the RVAMPIR approach which provides a good description of the ground
and gamma bands in the absence of strong mixing.Comment: 40 pages,14 figure
A study of temperature-related non-linearity at the metal-silicon interface
In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77–300 K IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy “activates” exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities
Status of an automatic Beam Steering for the CLIC Test Facility 3
An automatic beam steering application for CTF 3 is being designed in order to automatize operation of the machine, as well as providing a test-bed for advanced steering algorithms for CLIC. Beam-based correction including dispersion free steering have been investigated. An approach based on a PLACET on-line model has been tested. This paper gives an overview of the current status and the achieved results of the CTF3 automatic steering
The effects of intrinsic noise on the behaviour of bistable cell regulatory systems under quasi-steady state conditions
We analyse the effect of intrinsic fluctuations on the properties of bistable
stochastic systems with time scale separation operating under1 quasi-steady
state conditions. We first formulate a stochastic generalisation of the
quasi-steady state approximation based on the semi-classical approximation of
the partial differential equation for the generating function associated with
the Chemical Master Equation. Such approximation proceeds by optimising an
action functional whose associated set of Euler-Lagrange (Hamilton) equations
provide the most likely fluctuation path. We show that, under appropriate
conditions granting time scale separation, the Hamiltonian can be re-scaled so
that the set of Hamilton equations splits up into slow and fast variables,
whereby the quasi-steady state approximation can be applied. We analyse two
particular examples of systems whose mean-field limit has been shown to exhibit
bi-stability: an enzyme-catalysed system of two mutually-inhibitory proteins
and a gene regulatory circuit with self-activation. Our theory establishes that
the number of molecules of the conserved species are order parameters whose
variation regulates bistable behaviour in the associated systems beyond the
predictions of the mean-field theory. This prediction is fully confirmed by
direct numerical simulations using the stochastic simulation algorithm. This
result allows us to propose strategies whereby, by varying the number of
molecules of the three conserved chemical species, cell properties associated
to bistable behaviour (phenotype, cell-cycle status, etc.) can be controlled.Comment: 33 pages, 9 figures, accepted for publication in the Journal of
Chemical Physic
Examination of the Feynman-Hibbs Approach in the Study of Ne-Coronene Clusters at Low Temperatures
Feynman-Hibbs (FH) effective potentials constitute an appealing approach for
investigations of many-body systems at thermal equilibrium since they allow us
to easily include quantum corrections within standard classical simulations. In
this work we apply the FH formulation to the study of Ne-coronene clusters
( 1-4, 14) in the 2-14 K temperature range. Quadratic (FH2) and quartic
(FH4) contributions to the effective potentials are built upon Ne-Ne and
Ne-coronene analytical potentials. In particular, a new corrected expression
for the FH4 effective potential is reported. FH2 and FH4 cluster energies and
structures -obtained from energy optimization through a basin-hoping algorithm
as well as classical Monte Carlo simulations- are reported and compared with
reference path integral Monte Carlo calculations. For temperatures K,
both FH2 and FH4 potentials are able to correct the purely classical
calculations in a consistent way. However, the FH approach fails at lower
temperatures, especially the quartic correction. It is thus crucial to assess
the range of applicability of this formulation and, in particular, to apply the
FH4 potentials with great caution. A simple model of isotropic harmonic
oscillators allows us to propose a means of estimating the cut-off temperature
for the validity of the method, which is found to increase with the number of
atoms adsorbed on the coronene molecule
X-ray detection with Micromegas with background levels below 10 keVcms
Micromegas detectors are an optimum technological choice for the detection of
low energy x-rays. The low background techniques applied to these detectors
yielded remarkable background reductions over the years, being the CAST
experiment beneficiary of these developments. In this document we report on the
latest upgrades towards further background reductions and better understanding
of the detectors' response. The upgrades encompass the readout electronics, a
new detector design and the implementation of a more efficient cosmic muon veto
system. Background levels below 10keVcms have been
obtained at sea level for the first time, demonstrating the feasibility of the
expectations posed by IAXO, the next generation axion helioscope. Some results
obtained with a set of measurements conducted in the x-ray beam of the CAST
Detector Laboratory will be also presented and discussed
Exact SU(2)*U(1) Stringy Black Holes
Extreme magnetic dilaton black holes are promoted to exact solutions of
heterotic string theory with unbroken supersymmetry. With account taken of
alpha' corrections this is accomplished by supplementing the known solutions
with SU(2) Yang-Mills vectors and scalars in addition to the already existing
Abelian U(1) vector field. The solution has a simple analytic form and includes
multi-black-holes. The issue of exactness of other black-hole-type solutions,
including extreme dilaton electrically charged black holes and Taub-NUT
solutions is discussed.Comment: 10 pages, SU-ITP-94-27 and QMW-PH-94-34 (version accepted for
publication in Phys. Rev., contains a discussion of (4.1) supersymmetry of
the black hole sigma model
New Formulations of D=10 Supersymmetry and D8-O8 Domain Walls
We discuss a generalized form of IIA/IIB supergravity depending on all R-R
potentials C^(p) (p=0,1,...,9) as the effective field theory of Type IIA/IIB
superstring theory. For the IIA case we explicitly break this R-R democracy to
either p=5 which allows us to write a new bulk action that can be
coupled to N=1 supersymmetric brane actions.
The case of 8-branes is studied in detail using the new bulk & brane action.
The supersymmetric negative tension branes without matter excitations can be
viewed as orientifolds in the effective action. These D8-branes and O8-planes
are fundamental in Type I' string theory. A BPS 8-brane solution is given which
satisfies the jump conditions on the wall. It implies a quantization of the
mass parameter in string units. Also we find a maximal distance between the two
walls, depending on the string coupling and the mass parameter. We derive the
same results via supersymmetric flow equations.Comment: 30 pages, 2nd version: reference adde
Electrical activation of nitrogen heavily implanted 3C-SiC(100)
A degenerated wide bandgap semiconductor is a rare system. In general, implant levels lie deeper in the band-gap and carrier freeze-out usually takes place at room temperature. Nevertheless, we have observed that heavily doped n-type degenerated 3C-SiC films are achieved by nitrogen implantation level of ∼6x1020cm-3 at 20K. According to temperature dependent Hall measurements, nitrogen activation rates decrease with the doping level from almost 100% (1.5x1019cm-3, donor level 15meV) to ∼12% for 6x1020cm-3. Free donors are found to saturate in 3C-SiC at ∼7x1019cm-3. The implanted film electrical performances are characterized as a function of the dopant doses and post implantation annealing (PIA) conditions by fabricating Van der Pauw structures. A deposited SiO2 layer was used as the surface capping layer during the PIA process to study its effect on the resultant film properties. From the device design point of view, the lowest sheet resistivity (∼1.4mΩ.cm) has been observed for medium doped (4x1019cm-3) sample with PIA 1375 °C 2 h without a SiO2 cap
No chiral truncation of quantum log gravity?
At the classical level, chiral gravity may be constructed as a consistent
truncation of a larger theory called log gravity by requiring that left-moving
charges vanish. In turn, log gravity is the limit of topologically massive
gravity (TMG) at a special value of the coupling (the chiral point). We study
the situation at the level of linearized quantum fields, focussing on a unitary
quantization. While the TMG Hilbert space is continuous at the chiral point,
the left-moving Virasoro generators become ill-defined and cannot be used to
define a chiral truncation. In a sense, the left-moving asymptotic symmetries
are spontaneously broken at the chiral point. In contrast, in a non-unitary
quantization of TMG, both the Hilbert space and charges are continuous at the
chiral point and define a unitary theory of chiral gravity at the linearized
level.Comment: 20 pages, no figures, references adde
- …