Concurrent stochastic methods for global optimization

The global optimization problem, finding the lowest minimizer of a nonlinear function of several variables that has multiple local minimizers, appears well suited to concurrent computation. This paper presents a new parallel algorithm for the global optimization problem. The algorithm is a stochastic method related to the multi-level single-linkage methods of Rinnooy Kan and Timmer for sequential computers. Concurrency is achieved by partitioning the work of each of the three main parts of the algorithm, sampling, local minimization start point selection, and multiple local minimizations, among the processors. This parallelism is of a coarse grain type and is especially well suited to a local memory multiprocessing environment. The paper presents test results of a distributed implementation of this algorithm on a local area network of computer workstations. It also summarizes the theoretical properties of the algorithm

Grids of Stellar Models and Frequencies with CLES + LOSC

We present a grid of stellar models, obtained with the CLES evolution code, following the specification of ESTA-Task1, and the corresponfing seismic properties, computed with the LOSC code. We provide a complete description of the corresponding files that will be available on the ESTA web-pages.Comment: 8 pages, accepted for publication in Astrophys. Space Sci. (CoRoT/ESTA Volume

Bifurcations and chaos in semiconductor superlattices with a tilted magnetic field

We study the effects of dissipation on electron transport in a semiconductor superlattice with an applied bias voltage and a magnetic field that is tilted relative to the superlattice axis. In previous work, we showed that, although the applied fields are stationary, they act like a terahertz plane wave, which strongly couples the Bloch and cyclotron motion of electrons within the lowest miniband. As a consequence, the electrons exhibit a unique type of Hamiltonian chaos, which creates an intricate mesh of conduction channels (a stochastic web) in phase space, leading to a large resonant increase in the current flow at critical values of the applied voltage. This phase-space patterning provides a sensitive mechanism for controlling electrical resistance. In this paper, we investigate the effects of dissipation on the electron dynamics by modifying the semiclassical equations of motion to include a linear damping term. We demonstrate that, even in the presence of dissipation, deterministic chaos plays an important role in the electron transport process. We identify mechanisms for the onset of chaos and explore the associated sequence of bifurcations in the electron trajectories. When the Bloch and cyclotron frequencies are commensurate, complex multistability phenomena occur in the system. In particular, for fixed values of the control parameters several distinct stable regimes can coexist, each corresponding to different initial conditions. We show that this multistability has clear, experimentally observable, signatures in the electron transport characteristics

Using acoustic waves to induce high-frequency current oscillations in superlattices

We show that gigahertz acoustic waves in semiconductor superlattices can induce terahertz (THz) electron dynamics that depend critically on the wave amplitude. Below the threshold amplitude, the acoustic wave drags electrons through the superlattice with a peak drift velocity overshooting that produced by a static electric field. In this regime, single electrons perform drifting orbits with THz frequency components. When the wave amplitude exceeds the critical threshold, an abrupt onset of Bloch-type oscillations causes negative differential velocity. The acoustic wave also affects the collective behavior of the electrons by causing the formation of localized electron accumulation and depletion regions, which propagate through the superlattice, thereby producing self-sustained current oscillations even for very small wave amplitudes. We show that the underlying single-electron dynamics, in particular, the transition between the acoustic wave dragging and Bloch oscillation regimes, strongly influence the spatial distribution of the electrons and the form of the current oscillations. In particular, the amplitude of the current oscillations depends nonmonotonically on the strength of the acoustic wave, reflecting the variation in the single-electron drift velocity

Semiconductor charge transport driven by a picosecond strain pulse

We demonstrate that a picosecond strain pulse can be used to drive an electric current through both thin-film epilayer and heterostructure semiconductor crystals in the absence of an external electric field. By measuring the transient current pulses, we are able to clearly distinguish the effects of the coherent and incoherent components of the acoustic packet. The properties of the strain induced signal suggest a technique for exciting picosecond current pulses, which may be used to probe semiconductor devices

Nonlinear surface waves in left-handed materials

We study both linear and nonlinear surface waves localized at the interface separating a left-handed medium (i.e. the medium with both negative dielectric permittivity and negative magnetic permeability) and a conventional (or right-handed) dielectric medium. We demonstrate that the interface can support both TE- and TM-polarized surface waves - surface polaritons, and we study their properties. We describe the intensity-dependent properties of nonlinear surface waves in three different cases, i.e. when both the LH and RH media are nonlinear and when either of the media is nonlinear. In the case when both media are nonlinear, we find two types of nonlinear surface waves, one with the maximum amplitude at the interface, and the other one with two humps. In the case when one medium is nonlinear, only one type of surface wave exists, which has the maximum electric field at the interface, unlike waves in right-handed materials where the surface-wave maximum is usually shifted into a self-focussing nonlinear medium. We discus the possibility of tuning the wave group velocity in both the linear and nonlinear cases, and show that group-velocity dispersion, which leads to pulse broadening, can be balanced by the nonlinearity of the media, so resulting in soliton propagation.Comment: 9 pages, 10 figure

Black Holes from Nucleating Strings

We evaluate the probability that a loop of string that has spontaneously nucleated during inflation will form a black hole upon collapse, after the end of inflation. We then use the observational bounds on the density of primordial black holes to put constraints on the parameters of the model. Other constraints from the distortions of the microwave background and emission of gravitational radiation by the loops are considered. Also, observational constraints on domain wall nucleation and monopole pair production during inflation are briefly discussed.Comment: 27 pages, tutp-92-

Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations

The approach of the elastic continuum limit in small amorphous bodies formed by weakly polydisperse Lennard-Jones beads is investigated in a systematic finite-size study. We show that classical continuum elasticity breaks down when the wavelength of the sollicitation is smaller than a characteristic length of approximately 30 molecular sizes. Due to this surprisingly large effect ensembles containing up to N=40,000 particles have been required in two dimensions to yield a convincing match with the classical continuum predictions for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk systems. The existence of an effective length scale \xi is confirmed by the analysis of the (non-gaussian) noisy part of the low frequency vibrational eigenmodes. Moreover, we relate it to the {\em non-affine} part of the displacement fields under imposed elongation and shear. Similar correlations (vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table

Program transformation landscapes for automated program modification using Gin

Automated program modification underlies two successful research areas — genetic improvement and program repair. Under the generate-and-validate strategy, automated program modification transforms a program, then validates the result against a test suite. Much work has focused on the search space of application of single fine-grained operators — COPY, DELETE, REPLACE, and SWAP at both line and statement granularity. This work explores the limits of this strategy. We scale up existing findings an order of magnitude from small corpora to 10 real-world Java programs comprising up to 500k LoC. We decisively show that the grammar-specificity of statement granular edits pays off: its pass rate triples that of line edits and uses 10% less computational resources. We confirm previous findings that DELETE is the most effective operator for creating test-suite equivalent program variants. We go farther than prior work by exploring the limits of DELETE ’s effectiveness by exhaustively applying it. We show this strategy is too costly in practice to be used to search for improved software variants. We further find that pass rates drop from 12–34% for single statement edits to 2–6% for 5-edit sequences, which implies that further progress will need human-inspired operators that target specific faults or improvements. A program is amenable to automated modification to the extent to which automatically editing it is likely to produce test-suite passing variants. We are the first to systematically search for a code measure that correlates with a program’s amenability to automated modification. We found no strong correlations, leaving the question open

Interpreting Helioseismic Structure Inversion Results of Solar Active Regions

Helioseismic techniques such as ring-diagram analysis have often been used to determine the subsurface structural differences between solar active and quiet regions. Results obtained by inverting the frequency differences between the regions are usually interpreted as the sound-speed differences between them. These in turn are used as a measure of temperature and magnetic-field strength differences between the two regions. In this paper we first show that the "sound-speed" difference obtained from inversions is actually a combination of sound-speed difference and a magnetic component. Hence, the inversion result is not directly related to the thermal structure. Next, using solar models that include magnetic fields, we develop a formulation to use the inversion results to infer the differences in the magnetic and thermal structures between active and quiet regions. We then apply our technique to existing structure inversion results for different pairs of active and quiet regions. We find that the effect of magnetic fields is strongest in a shallow region above 0.985R_sun and that the strengths of magnetic-field effects at the surface and in the deeper (r < 0.98R_sun) layers are inversely related, i.e., the stronger the surface magnetic field the smaller the magnetic effects in the deeper layers, and vice versa. We also find that the magnetic effects in the deeper layers are the strongest in the quiet regions, consistent with the fact that these are basically regions with weakest magnetic fields at the surface. Because the quiet regions were selected to precede or follow their companion active regions, the results could have implications about the evolution of magnetic fields under active regions.Comment: Accepted for publication in Solar Physic