1,093 research outputs found
Interactive real-time physics: an intuitive approach to form-finding and structural analysis for design and education
Real-time physics simulation has been extensively used in computer games, but its potential has yet to be fully realised in design and education. We present an interactive 3D physics engine with a wide variety of applications. In common with traditional FEM, the use of a local element stiffness matrix is retained. However, unlike typical non-linear FEM routines, elements forces, moments and inertia are appropriately lumped at nodes following the Dynamic Relaxation Method. A semi-implicit time integration scheme updates linear and angular momentum, and subsequently the local coordinate frames of the nodes. The Co-Rotational formulation is used to compute the resultant field of displacements in global coordinates including large deformations. The results obtained compare well against established commercial software. We demonstrate that the method presented allows the making of interactive structural models that can be used in teaching to develop an intuitive understanding of structural behaviour. We also show that the same interactive physics framework allows real-time optimization that can be used for geometric and structural design application
How heavy can the Fermions in Split Susy be? A study on Gravitino and Extradimensional LSP
In recently introduced Split Susy theories, in which the scale of Susy
breaking is very high, the requirement that the relic abundance of the Lightest
SuperPartner (LSP) provides the Dark Matter of the Universe leads to the
prediction of fermionic superpartners around the weak scale. This is no longer
obviously the case if the LSP is a hidden sector field, such as a Gravitino or
an other hidden sector fermion, so, it is interesting to study this scenario.
We consider the case in which the Next-Lightest SuperPartner (NLSP) freezes out
with its thermal relic abundance, and then it decays to the LSP. We use the
constraints from BBN and CMB, together with the requirement of attaining Gauge
Coupling Unification and that the LSP abundance provides the Dark Matter of the
Universe, to infer the allowed superpartner spectrum. As very good news for a
possible detaction of Split Susy at LHC, we find that if the Gravitino is the
LSP, than the only allowed NLSP has to be very purely photino like. In this
case, a photino from 700 GeV to 5 TeV is allowed, which is difficult to test at
LHC. We also study the case where the LSP is given by a light fermion in the
hidden sector which is naturally present in Susy breaking in Extra Dimensions.
We find that, in this case, a generic NLSP is allowed to be in the range 1-20
TeV, while a Bino NLSP can be as light as tens of GeV.Comment: 29 pages, 12 figures. v2: modified conclusions for bino NLSP. v3:
corrected small mistake in Gauge Coupling Unification, conclusions unchange
Spin Coulomb drag beyond the random phase approximation
We study the spin Coulomb drag in a quasi-two-dimensional electron gas beyond
the random phase approximation (RPA). We find that the finite transverse width
of the electron gas causes a significant reduction of the spin Coulomb drag.
This reduction, however, is largely compensated by the enhancement coming from
the inclusion of many-body local field effects beyond the RPA, thereby
restoring good agreement with the experimental observations by C. P. Weber
\textit{et al.}, Nature, \textbf{437}, 1330 (2005).Comment: 3 figures, accepted for publication in Phys. Rev. Let
Harmonics of the AC susceptibility as probes to differentiate the various creep models
We measured the temperature dependence of the 1st and the 3rd harmonics of
the AC magnetic susceptibility on some type II superconducting samples at
different AC field amplitudes, hAC. In order to interpret the measurements, we
computed the harmonics of the AC susceptibility as function of the temperature
T, by integrating the non-linear diffusion equation for the magnetic field with
different creep models, namely the vortex glass-collective creep
(single-vortex, small bundle and large bundle) and Kim-Anderson model. We also
computed them by using a non-linear phenomenological I-V characteristics,
including a power law dependence of the pinning potential on hAC. Our
experimental results were compared with the numerically computed ones, by the
analysis of the Cole-Cole plots. This method results more sensitive than the
separate component analysis, giving the possibility to obtain detailed
information about the contribution of the flux dynamic regimes in the magnetic
response of the analysed samples.Comment: 9 pages, 6 figures, submitted to Physica
Exchange and correlation effects on the plasmon dispersions and the Coulomb drag in low-density electron bilayers
We investigate the effect of exchange and correlation (xc) on the plasmon
spectrum and the Coulomb drag between spatially separated low-density
two-dimensional electron layers. We adopt a new approach, which employs dynamic
xc kernels in the calculation of the bi-layer plasmon spectra and of the
plasmon-mediated drag, and static many-body local field factors in the
calculation of the particle-hole contribution to the drag. The spectrum of
bi-layer plasmons and the drag resistivity are calculated in a broad range of
temperatures taking into account both intra- and inter-layer correlation
effects. We observe that both plasmon modes are strongly affected by xc
corrections. After the inclusion of the complex dynamic xc kernels, a decrease
of the electron density induces shifts of the plasmon branches in opposite
directions. And this is in stark contrast to the tendency obtained within the
RPA that both optical and acoustical plasmons move away from the boundary of
the particle-hole continuum with a decrease in the electron density. We find
that the introduction of xc corrections results in a significant enhancement of
the transresistivity and qualitative changes in its temperature dependence. In
particular, the large high-temperature plasmon peak that is present in the
random phase approximation is found to disappear when the xc corrections are
included. Our numerical results at low temperatures are in good agreement with
the results of recent experiments by M. Kellogg {\it et al.}, Solid State
Commun. \textbf{123}, 515 (2002).Comment: 28 pages, 15 figure
Comparative study of screened inter-layer interactions in the Coulomb drag effect in bilayer electron systems
Coulomb drag experiments in which the inter-layer resistivity is measured are
important as they provide information on the Coulomb interactions in bilayer
systems. When the layer densities are low correlation effects become
significant to account for the quantitative description of experimental
results. We investigate systematically various models of effective inter-layer
interactions in a bilayer system and compare our results with recent
experiments. In the low density regime, the correlation effects are included
via the intra- and inter-layer local-field corrections. We employ several
theoretical approaches to construct static local-field corrections. Our
comparative study demonstrates the importance of including the correlation
effects accurately in the calculation of drag resistivity. Recent experiments
performed at low layer densities are adequately described by effective
inter-layer interactions incorporating static correlations.Comment: Final Version. To appear in Phys. Rev.
Spin-Polarization transition in the two dimensional electron gas
We present a numerical study of magnetic phases of the 2D electron gas near
freezing. The calculations are performed by diffusion Monte Carlo in the fixed
node approximation. At variance with the 3D case we find no evidence for the
stability of a partially polarized phase. With plane wave nodes in the trial
function, the polarization transition takes place at Rs=20, whereas the best
available estimates locate Wigner crystallization around Rs=35. Using an
improved nodal structure, featuring optimized backflow correlations, we confirm
the existence of a stability range for the polarized phase, although somewhat
shrunk, at densities achievable nowadays in 2 dimensional hole gases in
semiconductor heterostructures . The spin susceptibility of the unpolarized
phase at the magnetic transition is approximately 30 times the Pauli
susceptibility.Comment: 7 pages, 4 figure
Exploring the application domain of adaptive structures
Using a previously developed design methodology it was shown that optimal material distribution in combination with strategic integration of the actuation system lead to significant whole-life energy savings when the design is governed by rare but strong loading events. The whole-life energy of the structure is made of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, the actuation system redirects the internal load-path to homogenise the stresses and change the shape of the structure to keep deflections within limits. This paper presents a systematic exploration of the domain in which adaptive two-dimensional pin-jo inted structures are beneficial in terms of whole-life energy and monetary costs savings. Two case studies are considered: a vertical cantilever truss representative of a multi-storey building supported by an exoskeleton structure and a simply supported truss beam which is part of a roof system. This exploration takes five directions studying the influence of: (1) the structural topology (2) the characteristics of the load probability distribution (3) the ratio of live load over dead load (4) the aspect ratio of the structure (e.g. height-to-depth) (5) the material energy intensity factor. Results from the main five strands are combined with those from the monetary cost analysis to identify an optimal region where adaptive structures are most effective in terms of both energy and monetary savings. It was found that the optimal region is broadly that of stiffness-governed structures. For the cantilever case, the optimal region covers most of the application domain and it is not very sensitive to either live-to-dead-load or height-to-depth ratios thus showing a wide range of applicability, including ordinary loading scenarios and relatively deep structures
Electron Correlation and Charge Transfer Instability in Bilayered Two Dimensional Electron Gas
We prove that the predicted charge transfer state in symmetric bilayers of
two dimensional electron gases is always unstable at zero bias voltage, due to
interlayer correlation and/or tunneling. This is most easily seen by resorting
to a pseudospin formalism and considering coherent states obtained from the
charge transfer state through rotations of the pseudospins. Evidently, the
charge transfer state is stabilized by a sufficiently strong gate voltage, as
found in recent experiments. We show that a simple model, in which the layers
are strictly two dimensional, is able to account quantitatively for such
experimental findings, when correlation is properly included.Comment: 5 pages, 3 figures. Subm. to Europhys. Let
Disorder effect on the spin susceptibility of the two-dimensional one-valley electron gas
Starting from the quantum Monte Carlo (QMC) prediction for the ground-state
energy of a clean two-dimensional one-valley (2D1V) electron gas, we estimate
the energy correction due to scattering sources present in actual devices such
as AlAs quantum wells and GaAs heterostructures. We find that the effect of
uncorrelated disorder, in the lowest (second) order in perturbation theory, is
to enhance the spin susceptibility leading to its eventual divergence. In the
density region where the Born approximation is able to reproduce the
experimental mobility, the prediction for the spin susceptibility yielded by
perturbation theory is in very good agreement with the available experimental
evidence.Comment: 9 pages, 3 figures, special issue article for the SCCS2008 conference
(Camerino, Italy
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