Showing papers by "University of California, Santa Barbara published in 1987"
TL;DR: In this article, a functional-integral approach to the dynamics of a two-state system coupled to a dissipative environment is presented, and an exact and general prescription for the reduction, under appropriate circumstances, of the problem of a system tunneling between two wells in the presence of dissipative environments to the spin-boson problem is given.
Abstract: This paper presents the results of a functional-integral approach to the dynamics of a two-state system coupled to a dissipative environment. It is primarily an extended account of results obtained over the last four years by the authors; while they try to provide some background for orientation, it is emphatically not intended as a comprehensive review of the literature on the subject. Its contents include (1) an exact and general prescription for the reduction, under appropriate circumstances, of the problem of a system tunneling between two wells in the presence of a dissipative environment to the "spin-boson" problem; (2) the derivation of an exact formula for the dynamics of the latter problem; (3) the demonstration that there exists a simple approximation to this exact formula which is controlled, in the sense that we can put explicit bounds on the errors incurred in it, and that for almost all regions of the parameter space these errors are either very small in the limit of interest to us (the "slow-tunneling" limit) or can themselves be evaluated with satisfactory accuracy; (4) use of these results to obtain quantitative expressions for the dynamics of the system as a function of the spectral density $J(\ensuremath{\omega})$ of its coupling to the environment. If $J(\ensuremath{\omega})$ behaves as ${\ensuremath{\omega}}^{s}$ for frequencies of the order of the tunneling frequency or smaller, the authors find for the "unbiased" case the following results: For $sl1$ the system is localized at zero temperature, and at finite $T$ relaxes incoherently at a rate proportional to $\mathrm{exp}\ensuremath{-}{(\frac{{T}_{0}}{T})}^{1\ensuremath{-}s}$. For $sg2$ it undergoes underdamped coherent oscillations for all relevant temperatures, while for $1lsl2$ there is a crossover from coherent oscillation to overdamped relaxation as $T$ increases. Exact expressions for the oscillation and/or relaxation rates are presented in all these cases. For the "ohmic" case, $s=1$, the qualitative nature of the behavior depends critically on the dimensionless coupling strength $\ensuremath{\alpha}$ as well as the temperature $T$: over most of the ($\ensuremath{\alpha}$,$T$) plane (including the whole region $\ensuremath{\alpha}g1$) the behavior is an incoherent relaxation at a rate proportional to ${T}^{2\ensuremath{\alpha}\ensuremath{-}1}$, but for low $T$ and $0l\ensuremath{\alpha}l\frac{1}{2}$ the authors predict a combination of damped coherent oscillation and incoherent background which appears to disagree with the results of all previous approximations. The case of finite bias is also discussed.
4,047 citations
TL;DR: Two experiments establish links between desired knowledge about objects and hand movements during haptic object exploration, and establish that in free exploration, a procedure is generally used to acquire information about an object property, not because it is merely sufficient, butBecause it is optimal or even necessary.
1,723 citations
TL;DR: An important feature of the new form of constraints is the natural embedding of the constraint surface of the Einstein phase space into that of Yang-Mills phase space, which provides new tools to analyze a number of issues in both classical and quantum gravity.
Abstract: The phase space of general relativity is first extended in a standard manner to incorporate spinors. New coordinates are then introduced on this enlarged phase space to simplify the structure of constraint equations. Now, the basic variables, satisfying the canonical Poisson-brackets relations, are the (density-valued) soldering forms \ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{} $^{a}$${\mathrm{}}_{A}$${\mathrm{}}^{B}$ and certain spin-connection one-forms ${A}_{\mathrm{aA}}$${\mathrm{}}^{B}$. Constraints of Einstein's theory simply state that \ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{} $^{a}$ satisfies the Gauss law constraint with respect to ${A}_{a}$ and that the curvature tensor ${F}_{\mathrm{abA}}$${\mathrm{}}^{B}$ and ${A}_{a}$ satisfies certain purely algebraic conditions (involving \ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{} $^{a}$). In particular, the constraints are at worst quadratic in the new variables \ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{} $^{a}$ and ${A}_{a}$. This is in striking contrast with the situation with traditional variables, where constraints contain nonpolynomial functions of the three-metric. Simplification occurs because ${A}_{a}$ has information about both the three-metric and its conjugate momentum. In the four-dimensional space-time picture, ${A}_{a}$ turns out to be a potential for the self-dual part of Weyl curvature. An important feature of the new form of constraints is that it provides a natural embedding of the constraint surface of the Einstein phase space into that of Yang-Mills phase space. This embedding provides new tools to analyze a number of issues in both classical and quantum gravity. Some illustrative applications are discussed. Finally, the (Poisson-bracket) algebra of new constraints is computed. The framework sets the stage for another approach to canonical quantum gravity, discussed in forthcoming papers also by Jacobson, Lee, Renteln, and Smolin.
973 citations
TL;DR: In this paper, the complete low-energy effective action to quartic order in the bosonic fields for the heterotic string was calculated and shown to reduce to the supergravity sector of the superstring when the gauge connection is equal to the generalized spin connection.
943 citations
759 citations
TL;DR: Cross-validation as mentioned in this paper is a statistical procedure that produces an estimate of forecast skill which is less biased than the usual hindcast skill estimates, which can also provide important diagnostic information about influential cases in the dataset and the stability of the model.
Abstract: Cross-validation is a statistical procedure that produces an estimate of forecast skill which is less biased than the usual hindcast skill estimates. The cross-validation method systematically deletes one or more cases in a dataset, derives a forecast model from the remaining cases, and tests it on the deleted case or cases. The procedure is nonparametric and can be applied to any automated model building technique. It can also provide important diagnostic information about influential cases in the dataset and the stability of the model. Two experiments were conducted using cross-validation to estimate forecast skill in different predictive models of North Pacific sea surface temperatures (SSTs). The results indicate that bias, or artificial predictability (defined here as the difference between the usual hindcast skill and the forecast skill estimated by cross-validation), increases with each decision—either screening of potential predictors or fixing the value of a coefficient—drawn from the da...
757 citations
TL;DR: In this paper, an evolutionary sequence from protostars to pre-main sequence stars is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984).
Abstract: An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.
750 citations
TL;DR: Variations in the axion field can give rise to peculiar distributions of charge and current, and these effects provide a simple understanding of the fractional electric charge on dyons and of some recently discovered oddities in the electrodynamics of antiphase boundaries in PbTe.
Abstract: The equations of axion electrodynamics are studied. Variations in the axion field can give rise to peculiar distributions of charge and current. These effects provide a simple understanding of the fractional electric charge on dyons and of some recently discovered oddities in the electrodynamics of antiphase boundaries in PbTe. Some speculations regarding the possible occurrence of related phenomena in other solids are presented.
743 citations
TL;DR: The authors reviewed emerging findings applying self-efficacy theory to career-relevant behaviors; examines a number of conceptual and methodological issues arising from this work; and offers several directions for future research and theory on the career selfefficacy construct.
678 citations
TL;DR: A calculus for computing Filippov's differential inclusion is developed which simplifies the analysis of dynamical systems described by differential equations with a discontinuous right-hand side and the rigorous stability analysis of variable structure systems is routine.
Abstract: This paper develops a calculus for computing Filippov's differential inclusion which simplifies the analysis of dynamical systems described by differential equations with a discontinuous right-hand side. In particular, when a slightly generalized Lyapunov theory is used, the rigorous stability analysis of variable structure systems is routine. As an example, a variable structure control law for rigid-link robot manipulators is described and its stability is proved.
648 citations
TL;DR: It is shown that a similar approach may be taken, involving the generalized singular value decomposition of a certain product of matrices without explicitly forming the product, to the classical simultaneous diagonalization problem.
Abstract: An algorithm is presented in this paper for computing state-space balancing transformations directly from a state-space realization. The algorithm requires no "squaring up" or unnecessary matrix products. Various algorithmic aspects are discussed in detail. A key feature of the algorithm is the determination of a contragredient transformation through computing the singular value decomposition of a certain product of matrices without explicitly forming the product. Other contragredient transformation applications are also described. It is further shown that a similar approach may be taken, involving the generalized singular value decomposition, to the classical simultaneous diagonalization problem. These SVD-based simultaneous diagonalization algorithms provide a computational alternative to existing methods for solving certain classes of symmetric positive definite generalized eigenvalue problems.
TL;DR: In this paper, it was shown that epithermal mineralization is localized in dilational jogs between en echelon fault segments, as fissure veins or as hydrothermally cemented, high-dilation wall-rock breccias.
Abstract: Much fault-hosted epithermal mineralization is localized in dilational jogs between en echelon fault segments, as fissure veins or as hydrothermally cemented, high-dilation wall-rock breccias. Jog widths may range from millimetres to kilometres; vein textures record histories of incremental development. Perturbation or arrest of earthquake ruptures at dilational jogs has been observed and is believed to involve extensional fracturing at the rupture tip, locally reducing fluid pressure and inducing suctions opposing rapid slip transfer across the jog. This forced fissuring leads to brecciation by hydraulic implosion and to a concentrated fluid influx, allowing delayed slip transfer accompanied by aftershock activity. Within the southern San Andreas fault system, major dilational jogs extend throughout the seismogenic regime and form loci for magmatic-hydrothermal systems; they act as vertical pipelike conduits for enhanced fluid flow. Rupture termination at these structures has sometimes been followed by hydrothermal eruptions, suggesting that high-level boiling events are triggered by the arrest mechanism. It thus seems probable that episodic mineral deposition in the top 1–2 km of such jogs is induced by the dynamic effects of rupturing on the flanking strike-slip faults.
TL;DR: The topological order in the resonating valence-bond state is studied, finding neutral spin-1/2 fermions and charge +- e spinless bosons, analogous to the solitons in polyacetylene, form a degenerate Bose gas even at high temperatures.
Abstract: We study the topological order in the resonating valence-bond state. The elementary excitations have reversed charge-statistics relations: There are neutral spin-1/2 fermions and charge \ifmmode\pm\else\textpm\fi{}e spinless bosons, analogous to the solitons in polyacetylene. The charged excitations are very light, and form a degenerate Bose gas even at high temperatures. We discuss this model in the context of the recently discovered oxide superconductors.
TL;DR: In this article, a self-consistent scenario for all stages of planetary accretion which satisfies observational constraints is proposed. But it is not a selfconsistent model for all phases of planetary formation.
TL;DR: In this article, a detailed discussion is given of the mechanism by which this suppression takes place, through a combination of slight misorientation and a high-temperature surface anneal, which leads to the pairing of all Si surface steps into a particular kind of double-height steps.
TL;DR: The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm structure which is metallic and superconducting.
Abstract: ${\mathrm{La}}_{1.85}$${\mathrm{Ba}}_{0.15}$${\mathrm{CuO}}_{4}$ exhibits high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity with an onset temperature near 33 K as measured by resistivity and magnetic susceptibility. The nonsuperconducting end-member compound, ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, which has an orthorhombic, Cmca structure, exhibits semiconducting behavior. The orthorhombic distortion is proposed to result from a Peierls 2${\mathrm{k}}_{\mathrm{F}}$ instability or a soft zone-boundary phonon mode. The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm structure which is metallic and superconducting.
TL;DR: In this article, a speech accommodation theory for the first decade and beyond of the 1990s is presented. But this theory is restricted to speech accomodation theory and is not suitable for non-accomodative speech.
Abstract: (1987). Speech Accommodation Theory: The First Decade and Beyond. Annals of the International Communication Association: Vol. 10, Communication Yearbook 10, pp. 13-48.
TL;DR: Treatment and generalization data demonstrated that manipulation of parameters of natural language interactions and motivational techniques resulted in broadly generalized treatment gains.
Abstract: The purpose of this study was to attempt to improve verbal language acquisition for nonverbal autistic children by manipulating traditional teaching techniques so they incorporated parameters of natural language interactions and motivational techniques. Within a multiple baseline design, treatment was conducted in a baseline condition with trials presented serially in a traditional analogue clinical format where the therapist presented instructions, prompts, and reinforcers for correct responses. Then, these variables were manipulated in the natural language teaching condition such that (a) stimulus items were functional and varied, (b) natural reinforcers were employed, (c) communicative attempts were also reinforced, and (d) trials were conducted within a natural interchange. Treatment and generalization data demonstrated that manipulation of these variables resulted in broadly generalized treatment gains. Implications for language intervention are discussed.
TL;DR: Microelectrode studies demonstrate the existence of persistent oxygen and pH gradients around flocculent, macroscopic marine particles known as marine snow, which may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.
Abstract: Microelectrode studies demonstrate the existence of persistent oxygen and pH gradients around flocculent, macroscopic marine particles known as marine snow. Oxygen is partially, but continuously, depleted within and around marine snow in the dark and can be completely depleted within large fecal pellets. Boundary layers hundreds of micrometers thick are maintained despite advection of fluid past the particles. The existence of chemical microhabitats on the scale of millimeters around macroscopic particles in the pelagic zone may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.
TL;DR: This work considers adding Euler densities to the Einstein action as new gravity interactions in higher-dimensional theories when the appropriate surface terms are included, and discusses the relevance of such interactions in the context of string theory.
Abstract: We consider adding Euler densities to the Einstein action as new gravity interactions in higher-dimensional theories. When the appropriate surface terms are included, the surface geometry is sufficient data for the boundary-value problem associated with the new Lagrangians. We also discuss the relevance of such interactions in the context of string theory.
TL;DR: In this article, the thermochromism and solvatochromisms of solutions of poly(3-alkylthienylenes) are reported and the experimental results indicate the presence of two coexisting phases: polymer in solution and polymer in microcrystalline aggregates.
Abstract: Thermochromism and solvatochromism of solutions of poly(3-alkylthienylenes) are reported. The experimental results indicate the presence of two coexisting phases: polymer in solution and polymer in microcrystalline aggregates. From the concentration independence of the thermochromism, it is concluded that the transition is fundamentally driven by a single chain mechanism, and that the aggregation (microcrystallization) of the poly(3-alkylthienylene) macromolecules can occur only after the single chain conformational change has occurred.
TL;DR: In this article, the observational consequences of a vacuum energy which decays in time were examined and it was shown that in both radiation and matter dominated eras, the ratio of the vacuum energy to the total energy density of the universe must be small.
TL;DR: The anchor-point hypothesis of spatial cognition, according to which primary nodes or reference points anchor distinct regions in cognitive space, brings together certain frequently reported apparent properties of mental maps: the regionalization and hierarchical organization of cognitive space as discussed by the authors.
TL;DR: This paper solves a general finite-horizon problem by applying the calculus of variations to derive the optimal trajectory of the vector consisting of the concatenated descriptor, codescriptor, and control vectors, and calculates the optimal feedback gain relating the control and descriptor variable.
Abstract: In this paper we investigate the linear-quadratic optimal regulator problem for the continuous-time descriptor system E\dot{x} = Ax + Bu where E is, in general, a singular matrix. We solve first a general finite-horizon problem by applying the calculus of variations to derive the optimal trajectory of the vector consisting of the concatenated descriptor, codescriptor, and control vectors. From this trajectory the optimal feedback gain relating the control and descriptor variable can be computed. By transforming to a coordinate system which can be computed by performing a singular value decomposition of E we derive several Riccati differential equations, all of which have the same solution; this solution gives the optimal cost. The steady-state optimal feedback gain can be computed by solving an eigenvalue-eigenvector problem formulated from the untransformed system parameters. In general, there does not exist a unique optimal feedback gain but rather the gain is constrained to lie in a linear variety whose dimension is equal to the number of inputs times the rank deficiency of E .
TL;DR: The self-energies themselves, calculated in the Hedin-Lundqvist GW approximation, reproduce the experimental quasiparticle energies accurately, and are also shown to be well modeled by a simple functional form in real space.
Abstract: We examine the trends in the self-energy operators of Si, diamond, GaAs, and AlAs, and in their corresponding exchange-correlation potentials ${V}_{\mathrm{xc}}$ and their discontinuities \ensuremath{\Delta}. The potentials are calculated from the self-energies, thus avoiding use of a local-density approximation (LDA). In each case about 80% of the LDA band-gap error is also present for the true density-functional theory eigenvalue difference derived from ${V}_{\mathrm{xc}}$ and so is caused by \ensuremath{\Delta}. The self-energies themselves, calculated in the Hedin-Lundqvist GW approximation, reproduce the experimental quasiparticle energies accurately, and are also shown to be well modeled by a simple functional form in real space.
TL;DR: In this article, the electrical conductivity of polyacetylene has been investigated with and without the presence of intrinsic electron-photon scattering and sample imperfection, and it has been shown that polyacetylacetylene with fewer sp3 defects exhibits substantially higher electrical conductivities than in material prepared by other methods.
Abstract: The electrical conductivity of conducting polymers results from mobile charge carriers introduced into the π-electron system through doping1,2. Because of the large infra-chain transfer integrals, the transport of charge is believed to be principally along the conjugated chains, with inter-chain hopping as a necessary secondary step. In conducting polymers, as in all metals and semiconductors, charge transport is limited by a combination of intrinsic electron–photon scattering and sample imperfection. Although relatively high conductivities (σ ≈ 1,000 S cm−1) have been reported for partially orientated and heavily doped polyacetylene1–3, the absence of a metal-like temperature dependence implies that the observed values are not intrinsic. In doped polyacetylene, (CH)x, electrical transport can be limited both by microscopic defects (leading to scattering and localization) and by the more macroscopic complex fibrillar morphology12 and associated interfibrillar contacts. Thus, with improvements in material quality, one might anticipate corresponding improvements in the electrical conductivity. Here we report the synthesis of polyacetylene with fewer sp3 defects than in material prepared by other methods. The higher-quality material exhibits substantially higher electrical conductivity; maximum values of >20,000 S cm−1 are obtained after doping with iodine. The conductivity has been measured as a function of temperature and pressure: at 0.48 K and 10 kbar, iodine-doped samples remain highly conducting (σ ≈ 9,000 S cm−1).
TL;DR: There is no hidden order pattern for spin glasses at all T less than T(C), the ordered-phase spin correlations being chaotic functions of spin separation at fixed temperature or of temperature at scale lengths L greater than (T delta T) exp -1/zeta.
Abstract: The microscopic structure of the ordered phase of spin glasses is investigated theoretically in the framework of the T = 0 fixed-point model (McMillan, 1984; Fisher and Huse, 1986; and Bray and Moore, 1986). The sensitivity of the ground state to changes in the interaction strengths at T = 0 is explored, and it is found that for sufficiently large length scales the ground state is unstable against arbitrarily weak perturbations to the bonds. Explicit results are derived for d = 1, and the implications for d = 2 and d = 3 are considered in detail. It is concluded that there is no hidden order pattern for spin glasses at all T less than T(C), the ordered-phase spin correlations being chaotic functions of spin separation at fixed temperature or of temperature (for a given pair of spins) at scale lengths L greater than (T delta T) exp -1/zeta, where zeta = d(s)/2 - y, d(s) is the interfacial fractal dimension, and -y is the thermal eigenvalue at T = 0.
TL;DR: A structural finite-element technique based on Bernoulli-Euler beam theory is presented which will permit the finding of the torques (or forces) that are necessary to apply at one end of a flexible link to produce a desired motion at the other end.
Abstract: A structural finite-element technique based on Bernoulli-Euler beam theory is presented which will permit the finding of the torques (or forces) that are necessary to apply at one end of a flexible link to produce a desired motion at the other end. This technique is suitable for the open loop control of the tip motion. It may also provide a good control law for feedback control. The finite-element method is used to discretize the equations of motion. This method has a major advantage in the fact that different material properties and boundary conditions like hubs, tip loads, changes in cross sections, etc., can be handled in a very simple and straightforward manner. The resulting differential equations are integrated via the frequency domain. This allows for the expansion of the desired end motion into its harmonic components and helps to visualize the complex wave propagation nature of the problem. The performance of the proposed technique is illustrated in the solution of a practical example. Results point out the potential that this technique has in the study of the dynamics and control not only of flexible robots, but also of any other flexible mechanisms like those used in biomechanics, where high precision at the tip of very light flexible arms is required.
TL;DR: Mussels, clams, and tube worms collected in the vicinity of hydrocarbon seeps on the Louisiana slope contain mostly "dead" carbon, indicating that dietary carbon is largely derived from seeping oil and gas.
Abstract: Mussels, clams, and tube worms collected in the vicinity of hydrocarbon seeps on the Louisiana slope contain mostly "dead" carbon, indicating that dietary carbon is largely derived from seeping oil and gas. Enzyme assays, elemental sulfur analysis, and carbon dioxide fixation studies demonstrate that vestimentiferan tube worms and three clam species contain intracellular, autotrophic sulfur bacterial symbionts. Carbon isotopic ratios of 246 individual animal tissues were used to differentiate heterotrophic (8(13)C = -14 to -20 per mil), sulfur-based (8(13)C = -30 to -42 per mil), and methane-based (8(13)C = <-40 per mil) energy sources. Mussels with symbiotic methanotrophic bacteria reflect the carbon isotopic composition of the methane source. Isotopically light nitrogen and sulfur confirm the chemoautotrophic nature of the seep animals. Sulfur-based chemosynthetic animals contain isotopically light sulfur, whereas methane-based symbiotic mussels more closely reflect the heavier oceanic sulfate pool. The nitrogen requirement of some seep animals may be supported by nitrogen-fixing bacteria. Some grazing neogastropods have isotopic values characteristic of chemosynthetic animals, suggesting the transfer of carbon into the background deep-sea fauna.
TL;DR: Two algorithms for the numerical simulation of SU(3) lattice gauge theory with dynamical quarks are discussed, based on the hybrid stochastic method of Duane and Kogut, which allow the simulation of arbitrary numbers of quarks.
Abstract: We discuss two algorithms for the numerical simulation of SU(3) lattice gauge theory with dynamical quarks. Both are based on the hybrid stochastic method of Duane and Kogut. They provide a relatively rapid evolution of the gauge fields through configuration space and good control of errors. One of the algorithms allows the simulation of arbitrary numbers of quarks. Tests of the algorithms are presented as well as initial data from a study of the thermodynamics of quarks and gluons with Kogut-Susskind fermions.