Stacks and D-brane bundles

In this paper we describe explicitly how the twisted “bundles” on a D-brane worldvolume in the presence of a nontrivial B-field, can be understood in terms of sheaves on stacks. We also take this opportunity to provide the physics community with a readable introduction to stacks and generalized spaces.     

Acoustic Analyses of Developmental Changes and Emotional Expression in the Preverbal Vocalizations of Infants

The nonverbal vocal utterances of seven normally hearing infants were studied within their first year of life with respect to age- and emotion-related changes. Supported by a multiparametric acoustic analysis it was possible to distinguish one inspiratory and eleven expiratory call types. Most of the call types appeared within the first two months; some emerged in the majority of infants not until the 5th (“laugh”) or 7th month (“babble”). Age-related changes in acoustic structure were found in only 4 call types (“discomfort cry,” “short discomfort cry,” “wail,” “moan”). The acoustic changes were characterized mainly by an increase in harmonic-to-noise ratio and homogeneity of the call, a decrease in frequency range and a downward shift of acoustic energy from higher to lower frequencies. Emotion-related differences were found in the acoustic structure of single call types as well as in the frequency of occurrence of different call types. A change from positive to negative emotional state was accompanied by an increase in call duration, frequency range, and peak frequency (frequency with the highest amplitude within the power spectrum). Negative emotions, in addition, were characterized by a significantly higher rate of “crying,” “hic” and “ingressive vocalizations” than positive emotions, while positive emotions showed a significantly higher rate of “babble,” “laugh,” and “raspberry.”     

Completeness of “good” Bethe ansatz solutions of a quantum-group-invariant Heisenberg model

The sl_q(2) quantum-group-invariant Heisenberg model with open boundary conditions is investigated by means of the Bethe ansatz. As is well known, quantum groups for q equal to a root of unity possess a finite number of “good” representations with non-zero q-dimension and “bad” ones with vanishing q-dimension. Correspondingly, the state space of an invariant Heisenberg chain decomposes into “good” and “bad” states. A “good” state may be described by a path of only “good” representations. It is shown that the “good” states are given by all “good” Bethe ansatz solutions with roots restricted to the first periodicity strip, i.e. only positive-parity strings (in the language of Takahashi) are allowed. Applying Bethe's string-counting technique completeness of the “good” Bethe states is proven, i.e. the same number of states is found as the number of all restricted paths on the sl_q(2) Bratteli diagram. It is the first time that a “completeness” proof for an anisotropic quantum-invariant reduced Heisenberg model is performed.     

Methane activation over Ag-exchanged ZSM-5 zeolites: A theoretical study

Methane activation catalyzed over Ag-exchanged ZSM-5 zeolites was investigated by using the density functional theory (DFT) with a cluster model. Two different pathways were taken into account in this work: the “alkyl” and the “carbenium” pathways. The activation barriers obtained are 34.09 and 66.63 kcal/mol for the “alkyl” and the “carbenium” pathway, respectively. The calculated results show that the activation barrier of the “alkyl” pathway is smaller than that of “carbenium” pathway. Consequently, the “alkyl” pathway is the preferential reaction pathway. A new mechanism of methane conversion in the presence of ethene was proposed. In the catalytic cycle, the initial step of methane activation proceeds with the “alkyl” pathway and the Ag⁺ cation acts as an acceptor of the methyl group, then ethene reacts with the Ag⁺CH₃⁻ group to form propene. In addition, it is found that the Ag⁺ cations play an important role in the methane activation, compared with the reaction of methane activation over H-ZSM-5.     

Detection threshold in muscovite mica: Influence of the observation tool

For the observation of nuclear tracks, Atomic Force Microscopy (AFM) is a useful technique. In our study, we have irradiated a muscovite mica sample using 600 keV oxygen ions. This ion's energy is well below the detection threshold predicted by the existing models. The samples were visualized at high resolution with an AFM device. Before chemical etching no tracks were visualized on the surface at an atomic level. However, defects must have been initiated because tracks became observable after a brief etching time. Our results confirm that the detection threshold is influenced by the observation tool. In this article we provide information concerning the “observability” and “etchability” of latent tracks.     

Divisors on elliptic Calabi-Yau 4-folds and the superpotential in F-theory — I

Each smooth elliptic Calabi-Yau 4-fold determines both a three-dimensional physical theory (a compactification of “M-theory”) and a four-dimensional physical theory (using the “F-theory” construction). A key issue in both theories is the calculation of the “superpotential” of the theory, which by a result of Witten is determined by the divisors D on the 4-fold satisfying X( _D = 1. We propose a systematic approach to identify these divisors, and derive some criteria to determine whether a given divisor indeed contributes. We then apply our techniques in explicit examples, in particular, when the base B of the elliptic fibration is a toric variety or a Fano 3-fold.

When B is Fano, we show how divisors contributing to the superpotential are always “exceptional” (in some sense) for the Calabi-Yau 4-fold X. This naturally leads to certain transitions of X, i.e., birational tranformations to a singular model (where the image of D no longer contributes) as well as certain smoothings of the singular model. The singularities which occur are “canonical”, the same type of singularities of a (singular) Weierstrass model. We work out the transitions. If a smoothing exists, then the Hodge numbers change.

We speculate that divisors contributing to the superpotential are always “exceptional” (in some sense) for X, also in M-theory. In fact we show that this is a consequence of the (log)-minimal model algorithm in dimension 4, which is still conjectural in its generality, but it has been worked out in various cases, among which are toric varieties.     

Physics of protein folding

Protein physics is grounded on three fundamental experimental facts: protein, this long heteropolymer, has a well defined compact three-dimensional structure; this structure can spontaneously arise from the unfolded protein chain in appropriate environment; and this structure is separated from the unfolded state of the chain by the “all-or-none” phase transition, which ensures robustness of protein structure and therefore of its action. The aim of this review is to consider modern understanding of physical principles of self-organization of protein structures and to overview such important features of this process, as finding out the unique protein structure among zillions alternatives, nucleation of the folding process and metastable folding intermediates. Towards this end we will consider the main experimental facts and simple, mostly phenomenological theoretical models. We will concentrate on relatively small (single-domain) water-soluble globular proteins (whose structure and especially folding are much better studied and understood than those of large or membrane and fibrous proteins) and consider kinetic and structural aspects of transition of initially unfolded protein chains into their final solid (“native”) 3D structures.     

Self-organization of surfactant molecules on solid surface: an STM study of sodium alkyl sulfonates

Adsorption and self-organization of sodium alkyl sulfonates (STS and SHS) on HOPG have been studied by using in situ scanning tunneling microscopy (STM). Both SHS and STS molecules adsorb on HOPG surface and form long-range well-ordered monolayers. The molecular rows and the axes of alkyl chain of the molecules cross each other at angles of 60° and 90° in the STS and SHS layers, respectively. Molecular details such as sulfonate functional group (head) and alkyl chain are clearly imaged. The neighboring molecules in different rows form a “head to head” configuration. Structural models for the molecular arrangement of the two adlayers are proposed.     

On the low-frequency limit of the Schönfeld pulse 1/f law

On the basis of propositions of the common fluctuation theory, peculiarities of small fluctuations in real physical systems with limited sizes are analyzed. It is established that small fluctuations should necessarily be divided into two types of fluctuations: “small” and “very small”. It is shown that the damping process of “small” fluctuations has relaxation character, while the damping process of “very small” fluctuations is of random character, i.e., it represents a random rectangular signal. The probability density of “very small” fluctuations is shown to be Gaussian. The agreement of the obtained results with experimental data acquired from semiconductor-based devices is analyzed. A relation between the generation–recombination noise and phonon number fluctuations in semiconductors is studied. On the basis of this consideration it is shown that the Schönfeld pulse spectrum preserves its well-known 1/f form only in the range of intermediate frequencies; at lower frequencies the spectrum gets saturated. An expression for the low-frequency limit of Schönfeld pulse 1/f law is obtained.     

Instability of oscillatory shock profile solutions of the generalized Burgers-KdV equation

Oscillatory shock profile solutions of the Burgers-KdV equation are studied in the limit as the viscosity → 0. A rigorous proof of their instability is obtained by showing that the linearized operator about such a solution has many unstable eigenvalues for sufficiently small . The result is obtained by applying a topological method introduced by Alexander, Gardner and Jones to extract spectral information about the perturbed wave with slightly positive viscosity from particular “pieces” of the underlying wave which are approximated by a solitary wave solution of the reduced equation with no viscous dissipation.     

Zigzag wall dynamics in uniaxial in-plane magnetic thin films

An unusual right/left zigzag wall propagation in the bias (transverse) and ac fields applied simultaneously is observed. A dynamic model based on the variable coercivity, effective transverse field, transformation of the walls from asymmetric Bloch into asymmetric Néel type (and vice versa) is proposed. Degeneration of two segments of one “zig” (or “zag”) which “sense” differently the transverse field is one of the crucial point of the zigzag dynamics.     

Recent advances in electron phase microscopy

Electron microscopy utilizing the phase information of an electron beam has recently seen remarkable progress due to the development of a “coherent” field-emission electron beam and the corresponding image-processing techniques. This paper reviews these developments and their applications.     

Discrete gauge symmetry anomalies

It has been recently argued that quantum gravity effects strongly violate all non-gauge symmetries. This would suggest that all low energy discrete symmetries should be gauge symmetries, either continuous or discrete. Acceptable continuous gauge symmetries are constrained by the condition they should be anomaly free. We show here that any discrete gauge symmetry should also obey certain “discrete anomaly cancellation” conditions. These conditions strongly constrains the massles fermion content of the theory and follow from the “parent” cancellation of the usual continuous gauge anomalies. They have interesting applications in model building. As an example we consider the constraints on the Z_N “generalized matter parities” of the supersymmetric standard model. We show that only a few (including the standard R-parity) are “discrete anomaly free” unless the fermion content of the minimal supersymmetric standard model is enlarged.     

Analysis of the critical behaviour of curved regions in equilibrium shapes of in crystals

Crystal shapes near {111} facets have been analyzed on indium crystals in their equilibrium shape. These measurements are compared with two theoretical concepts of the critical behaviour of curved regions: the “Pokrovsky-Talapov transition” and the “mean field theory”. Taking into account, on the one hand, the inaccuracy of the experimental determination of the origin of the curved region and, on the other hand, the “window” of validity of the Pokrovsky-Talapov transition theory, the choice between the two theories is difficult. Nevertheless the analytical expression of the mean field theory reproduces surprisingly well all the points of the experimental profile.     

Comments on “Spectral analysis of thermally stimulated exoelectron emission from glass deposited on metal by argon plasma treatment”

A “spectral analysis” using a fit of a single glow curve only — without kinetic check — does not prove the correctness of the model of Sakurai and Momose [Phys. Lett. A 179 (1993) 55] and thus of parameters estimated on this basis. Contrary to this a “complete analysis” yields three of four parameters directly from the experiment. Two criteria are given to verify the model experimentally.     

A U(1) gauge-Higgs system with a radial degree of freedom

The phase diagram of a lattice U(1) gauge-Higgs model is derived without freezing the Higgs field length. For sufficienly small Higgs self-coupling strength, the “confinement” and “Higgs” phases are separated, in contrast to what is observed in the fixed length model.     

Optical aperture synthesis using successive exposure of a single photograph and spatial-filtering “low-frequency redundancy” suppression

A high-resolution “full spatial-frequency range” optical image may be synthesized by superposing in a single plate a suitable set of low-resolution partial frequency-range photographs, obtained separately or simultaneously, with “low-frequency redundancy” suppressed by spatial filtering.     

Monte Carlo simulations on a rigid-rod model for a langmuir monolayer

A monolayer of amphiphilic molecules on water surface (Langmuir monolayer) at the so-called “solid” phase is studied using Monte Carlo simulations. Each molecule is considered to be a rigid rod, one end (the hydrophilic “head”) of which is assumed to be fixed on a two-dimensional lattice, while the other end (the hydrophobic “tail”) interacts with its nearest and next-nearest neighbours through the Lennard-Jones potential. With increase in temperature, the system undergoes a first-order transition from a low-tilt to a high-tilt phase. With increase in tail-length, the two-phase coexistence region decreases and the transition becomes sharper.     

Learning voice analysis using an interactive multi-media package: Development and preliminary evaluation

An important outcome of education for speech-language pathologypractice is the ability to analyze voices perceptually, a complex task that is often difficult for novices. This article describes an interactive multi-media package, “A Sound Judgement,” that is designed to help students develop skills in perceptual voice analysis and to link their perceptions to laryngeal physiology. The package presents a range of clients with vocal impairments at increasing levels of complexity. Each case has a videoed interview, endoscopic views and animations of the larynx, and case history information. Students make perceptual ratings of clients' voices on a format designed specifically for this package and feedback is provided using ratings made by expert speech-language pathologists. High levels of consensus for the perceptual judgments were achieved among the expert raters. Preliminary evaluations by students have demonstrated that “A Sound Judgement” is likely to be a valuable educational tool.     

Extended conformal symmetries

We discuss various aspects of two-dimensional extended conformal symmetries, better known under the name “W-symmetries”. In particular, we discuss the gauging of W - symmetries and the construction of the so-called “W-gravity” theories.