30,484 research outputs found
Converging shocks in elastic-plastic solids
We present an approximate description of the behavior of an elastic-plastic material processed by a cylindrically or spherically symmetric converging shock, following Whitham's shock dynamics theory. Originally applied with success to various gas dynamics problems, this theory is presently derived for solid media, in both elastic and plastic regimes. The exact solutions of the shock dynamics equations obtained reproduce well the results obtained by high-resolution numerical simulations. The examined constitutive laws share a compressible neo-Hookean structure for the internal energy e = e_(s)(I_1)+e_(h)(Ï,Ï), where e_(s) accounts for shear through the first invariant of the CauchyâGreen tensor, and e_(h) represents the hydrostatic contribution as a function of the density Ï and entropy Ï. In the strong-shock limit, reached as the shock approaches the axis or origin r=0, we show that compression effects are dominant over shear deformations. For an isothermal constitutive law, i.e., e_(h) = e_(h)(Ï), with a power-law dependence e_(h) â Ï_(α), shock dynamics predicts that for a converging shock located at r=R(t) at time t, the Mach number increases as M â [log(1/R)]^α, independently of the space index s, where s=2 in cylindrical geometry and 3 in spherical geometry. An alternative isothermal constitutive law with p(Ï) of the arctanh type, which enforces a finite density in the strong-shock limit, leads to M â R^(â(sâ1)) for strong shocks. A nonisothermal constitutive law, whose hydrostatic part eh is that of an ideal gas, is also tested, recovering the strong-shock limit MâR^(â(sâ1)/n(Îł)) originally derived by Whitham for perfect gases, where Îł is inherently related to the maximum compression ratio that the material can reach, (Îł+1)/(Îłâ1). From these strong-shock limits, we also estimate analytically the density, radial velocity, pressure, and sound speed immediately behind the shock. While the hydrostatic part of the energy essentially commands the strong-shock behavior, the shear modulus and yield stress modify the compression ratio and velocity of the shock far from the axis or origin. A characterization of the elastic-plastic transition in converging shocks, which involves an elastic precursor and a plastic compression region, is finally exposed
Proposal for a Supersymmetric Standard Model
The fact that neutrinos are massive suggests that the minimal supersymmetric
standard model (MSSM) might be extended in order to include three gauge-singlet
neutrino superfields with Yukawa couplings of the type . We
propose to use these superfields to solve the problem of the MSSM without
having to introduce an extra singlet superfield as in the case of the
next-to-MSSM (NMSSM). In particular, terms of the type in the
superpotential may carry out this task spontaneously through sneutrino vacuum
expectation values. In addition, terms of the type avoid the
presence of axions and generate effective Majorana masses for neutrinos at the
electroweak scale. On the other hand, these terms break lepton number and
R-parity explicitly implying that the phenomenology of this model is very
different from the one of the MSSM or NMSSM. For example, the usual neutralinos
are now mixed with the neutrinos. For Dirac masses of the latter of order
GeV, eigenvalues reproducing the correct scale of neutrino masses are
obtained.Comment: 9 pages, latex, title modified. Final version published in PR
Continuum variational and diffusion quantum Monte Carlo calculations
This topical review describes the methodology of continuum variational and
diffusion quantum Monte Carlo calculations. These stochastic methods are based
on many-body wave functions and are capable of achieving very high accuracy.
The algorithms are intrinsically parallel and well-suited to petascale
computers, and the computational cost scales as a polynomial of the number of
particles. A guide to the systems and topics which have been investigated using
these methods is given. The bulk of the article is devoted to an overview of
the basic quantum Monte Carlo methods, the forms and optimisation of wave
functions, performing calculations within periodic boundary conditions, using
pseudopotentials, excited-state calculations, sources of calculational
inaccuracy, and calculating energy differences and forces
Anisotropic response of the moving vortex lattice in superconducting MoGe amorphous films
We have performed magnetic susceptibility measurements in MoGe
amorphous thin films biased with an electrical current using anisotropic coils.
We tested the symmetry of the vortex response changing the relative orientation
between the bias current and the susceptibility coils. We found a region in the
DC current - temperature phase diagram where the dynamical vortex structures
behave anisotropically. In this region the shielding capability of the
superconducting currents measured by the susceptibility coils is less effective
along the direction of vortex motion compared to the transverse direction. This
anisotropic response is found in the same region where the peak effect in the
critical current is developed. On rising temperature the isotropic behavior is
recovered.Comment: 10 pages, 4 figure
Experimental and theoretical studies of sequence effects on the fluctuation and melting of short DNA molecules
Understanding the melting of short DNA sequences probes DNA at the scale of
the genetic code and raises questions which are very different from those posed
by very long sequences, which have been extensively studied. We investigate
this problem by combining experiments and theory. A new experimental method
allows us to make a mapping of the opening of the guanines along the sequence
as a function of temperature. The results indicate that non-local effects may
be important in DNA because an AT-rich region is able to influence the opening
of a base pair which is about 10 base pairs away. An earlier mesoscopic model
of DNA is modified to correctly describe the time scales associated to the
opening of individual base pairs well below melting, and to properly take into
account the sequence. Using this model to analyze some characteristic sequences
for which detailed experimental data on the melting is available [Montrichok et
al. 2003 Europhys. Lett. {\bf 62} 452], we show that we have to introduce
non-local effects of AT-rich regions to get acceptable results. This brings a
second indication that the influence of these highly fluctuating regions of DNA
on their neighborhood can extend to some distance.Comment: To be published in J. Phys. Condensed Matte
Real-time Monocular Object SLAM
We present a real-time object-based SLAM system that leverages the largest
object database to date. Our approach comprises two main components: 1) a
monocular SLAM algorithm that exploits object rigidity constraints to improve
the map and find its real scale, and 2) a novel object recognition algorithm
based on bags of binary words, which provides live detections with a database
of 500 3D objects. The two components work together and benefit each other: the
SLAM algorithm accumulates information from the observations of the objects,
anchors object features to especial map landmarks and sets constrains on the
optimization. At the same time, objects partially or fully located within the
map are used as a prior to guide the recognition algorithm, achieving higher
recall. We evaluate our proposal on five real environments showing improvements
on the accuracy of the map and efficiency with respect to other
state-of-the-art techniques
Clustering transition in a system of particles self-consistently driven by a shear flow
We introduce a simple model of active transport for an ensemble of particles
driven by an external shear flow. Active refers to the fact that the flow of
the particles is modified by the distribution of particles itself. The model
consists in that the effective velocity of every particle is given by the
average of the external flow velocities felt by the particles located at a
distance less than a typical radius, . Numerical analysis reveals the
existence of a transition to clustering depending on the parameters of the
external flow and on . A continuum description in terms of the number
density of particles is derived, and a linear stability analysis of the density
equation is performed in order to characterize the transitions observed in the
model of interacting particles.Comment: 11 pages, 2 figures. To appear in PR
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