Showing papers by "International School for Advanced Studies published in 1995"
TL;DR: The average of the bias, under certain conditions, is calculated as an asymptotic expansion in the inverse of the size of the data sample, which agrees with numerical simulations, and is applicable, as an additive correction term, to measurements obtained under such conditions.
Abstract: Extracting information measures from limited experimental samples, such as those normally available when using data recorded in vivo from mammalian cortical neurons, is known to be plagued by a systematic error, which tends to bias the estimate upward. We calculate here the average of the bias, under certain conditions, as an asymptotic expansion in the inverse of the size of the data sample. The result agrees with numerical simulations, and is applicable, as an additive correction term, to measurements obtained under such conditions. Moreover, we discuss the implications for measurements obtained through other usual procedures.
368 citations
TL;DR: It is shown that fractional master equations are contained as a special case within the traditional theory of continuous time random walks for fractional time derivatives of order 0.1.
Abstract: Fractional master equations containing fractional time derivatives of order 0\ensuremath{\le}1 are introduced on the basis of a recent classification of time generators in ergodic theory. It is shown that fractional master equations are contained as a special case within the traditional theory of continuous time random walks. The corresponding waiting time density \ensuremath{\psi}(t) is obtained exactly as \ensuremath{\psi}(t)=(${\mathit{t}}^{\mathrm{\ensuremath{\omega}}\mathrm{\ensuremath{-}}1}$/C)${\mathit{E}}_{\mathrm{\ensuremath{\omega}},\mathrm{\ensuremath{\omega}}}$(-${\mathit{t}}^{\mathrm{\ensuremath{\omega}}}$/C), where ${\mathit{E}}_{\mathrm{\ensuremath{\omega}},\mathrm{\ensuremath{\omega}}}$(x) is the generalized Mittag-Leffler function. This waiting time distribution is singular both in the long time as well as in the short time limit.
338 citations
TL;DR: The basic ingredients are by-products of a standard linear-response calculation of phonon dispersions in the harmonic approximation, resulting in a similarly good agreement with experiments.
Abstract: The anharmonic lifetimes of zone-center optical phonons in C, Si, and Ge are calculated along with their temperature and pressure dependences, using third-order density-functional perturbation theory. Our basic ingredients are by-products of a standard linear-response calculation of phonon dispersions in the harmonic approximation, resulting in a similarly good agreement with experiments. The microscopic mechanisms responsible for the decay are revealed and shown to be different for different materials and to depend sensitively on the applied pressure.
274 citations
TL;DR: In this article, the ground-state energy of integrable 1 + 1 quantum field theories with boundaries is studied, where the boundary is represented by a boundary state, and the thermodynamics involves evaluating scalar products of boundary states with all the states of the theory.
200 citations
TL;DR: In this paper, the density-functional perturbation theory approach to lattice-dynamical calculations is extended to metallic systems and the smearing technique is used to deal with the Fermi surface and its variational formulation is restated.
Abstract: The density-functional perturbation theory approach to lattice-dynamical calculations is extended to metallic systems. The smearing technique is used to deal with the Fermi surface and its variational formulation is restated. First-principles phonon dispersions of Al, Pb and of the transition metal Nb are in good agreement with available experimental data. In particular an accurate description of the anomalies observed in lead and niobium is obtained.
196 citations
TL;DR: In this article, the elementary and solitonic supersymmetric p-brane solutions to supergravity theories form families related by dimensional reduction, each headed by a maximal (stainless) member that cannot be isotropically dimensionally oxidized into higher dimensions.
193 citations
TL;DR: The targeting of ScFv fragments to mitochondria is engineered and it is demonstrated that this can occurr efficiently and extend the range of subcellular compartments where antibody domains can be targeted in order to interfere with the action of the corresponding antigen.
Abstract: In this paper we have engineered the targeting of ScFv fragments to mitochondria and demonstrated that this can occur efficiently This extends the range of subcellular compartments where antibody domains can be targeted in order to interfere with the action of the corresponding antigen Moreover, we have compared the redox state of ScFv fragments targeted to the secretory compartment, the cytosol and the mitochondria, and demonstrated that cysteine residues in ScFv targeted to the secretory compartments and to the mitochondria are oxidized On the contrary, cytosolic antibody domains are expressed in a reduced state, which is probably the reason for their lower expression levels These pitfalls, however, do not prevent their successful utilization for intracellular immunization
189 citations
TL;DR: This work has simulated the conversion of carbon from graphite to diamond under high pressure and found that the transformation path proceeds through sliding of graphite planes into an unusual orthorhombic stacking, from which an abrupt collapse and buckling of the planes leads to both cubic and hexagonal forms of diamond in comparable proportions.
Abstract: Using constant-pressure ab initio molecular dynamics we have simulated the conversion of carbon from graphite to diamond under high pressure. We found that the transformation path proceeds through sliding of graphite planes into an unusual orthorhombic stacking, from which an abrupt collapse and buckling of the planes leads to both cubic and hexagonal forms of diamond in comparable proportions. The mutual orientation of the initial and final phases is in agreement with that of shock-wave experiments.
162 citations
TL;DR: In this article, the form factor bootstrap approach is used to compute the exact contributions in the large-distance expansion of the correlation function of the two-dimensional Ising model in a magnetic field at T = Tc.
142 citations
TL;DR: In this paper, the elementary and solitonic supersymmetric $p$-brane solutions to supergravity theories form families related by dimensional reduction, each headed by a maximal (`stainless') member that cannot be isotropically dimensionally oxidized into higher dimensions.
Abstract: The elementary and solitonic supersymmetric $p$-brane solutions to supergravity theories form families related by dimensional reduction, each headed by a maximal (`stainless') member that cannot be isotropically dimensionally oxidized into higher dimensions. We find several new families, headed by stainless solutions in various dimensions $D\le 9$. In some cases, these occur with dimensions $(D,p)$ that coincide with those of descendants of known families, but since the new solutions are stainless, they are necessarily distinct. The new stainless supersymmetric solutions include a 6-brane and a 5-brane in $D=9$, a string in $D=5$, and particles in all dimensions $5\le D\le 9$.
141 citations
TL;DR: This work investigates the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential, and finds that the melting temperature of a wire with radius R is lower than that of a bulk solid.
Abstract: Recent work has raised considerable interest on the nature of thin metallic wires. We have investigated the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential. We find that---in analogy with cluster melting---the melting temperature $T_m (R)$ of a wire with radius $R$ is lower than that of a bulk solid, $T_m^b$, by $T_m (R) = T_m^b -c/R$. Surface melting effects, with formation of a thin skin of highly diffusive atoms at the wire surface, is observed. The diffusivity is lower where the wire surface has a flat, local (111) orientation, and higher at (110) and (100) rounded areas. The possible relevance to recent results on non-rupturing thin necks between an STM tip and a warm surface is addressed.
TL;DR: In this article, a method for first-principles numerical simulation of solid-solid phase transformation is presented, applied to the study of pressure induced transformations in silicon and carbon.
TL;DR: In this article, a class of two-dimensional globally scale invariant, but not conformally invariant theories is obtained, in the process of discussing global and local scaling properties of models related by duality transformations based on non-semisimple isometry groups.
TL;DR: The results confirm that while the ground state \ensuremath{\alpha} phase is characterized by the notable coexistence of metallic and covalent characters, the other phases are totally metallic.
Abstract: Structural and electronic properties of various solid-state phases of gallium have been studied by means of first-principles total-energy calculations. Our results confirm that while the ground state \ensuremath{\alpha} phase is characterized by the notable coexistence of metallic and covalent characters, the other phases are totally metallic. We predict that a phase transition from GaII to fcc should be observable at pressure \ensuremath{\sim}150 kbar.
TL;DR: Fractal time random walks with generalized Mittag-Leffler functions as waiting time densities are studied in this article, where exact Greens functions corresponding to fractional diffusion are obtained using Mellin transform techniques.
Abstract: Fractal time random walks with generalized Mittag-Leffler functions as waiting time densities are studied. This class of fractal time processes is characterized by a dynamical critical exponent 0<ω≤1, and is equivalently described by a fractional master equation with time derivative of noninteger order ω. Exact Greens functions corresponding to fractional diffusion are obtained using Mellin transform techniques. The Greens functions are expressible in terms of general H-functions. For ω<1 they are singular at the origin and exhibit a stretched Gaussian form at infinity. Changing the order ω interpolates smoothly between ordinary diffusion ω=1 and completely localized behavior in the ω→0 limit.
TL;DR: In this article, a review article is presented to give a comprehensive description of the scaling properties detected for the distribution of cosmic structures, like galaxies and galaxy clusters, and more details about the description of galaxy clustering in terms of fractal concepts.
TL;DR: In this paper, the authors reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk, and they calculate here spectra from a set of thin and slim, optically thick accretion disks.
Abstract: We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate, the spectra of the high-accretion-rate disks extend into the soft X-rays. A comparison with observations shows that such disks could produce the soft X-ray excesses claimed in some AGNs. We show also that the sequence of our models with fixed mass and different accretion rates can explain the time evolution of the observed spectra in Fairall 9.
TL;DR: In this article, a variational calculus for discontinuous solutions of systems of conservation laws is proposed, which is based on the variational integral calculus for partial differential equations. But the calculus is not suitable for non-convex solutions.
Abstract: (1995). A variational calculus for discontinuous solutions of systems of conservation laws. Communications in Partial Differential Equations: Vol. 20, No. 9-10, pp. 1491-1552.
TL;DR: In this article, a canonical formalism in the radial variable r that is timelike inside the Schwarzschild horizon was developed. But this formalism is restricted to the case where the mass is represented by a canonical function that commutes with the r-Hamiltonian.
Abstract: Starting from the Lagrangian formulation of the Einstein equations for the vacuum static spherically symmetric metric, we develop a canonical formalism in the radial variable r that is timelike inside the Schwarzschild horizon. The Schwarzschild mass turns out to be represented by a canonical function that commutes with the r-Hamiltonian. We investigate the Wheeler-DeWitt quantization and give the general representation for the solution as superposition of eigenfunctions of the mass operator.
TL;DR: In this article, the authors used a model Hamiltonian to investigate polarization in a strongly correlated, and partially ionic, insulator, and evaluated it as a Berry phase, where the static ionic charge is continuous across the transition, whereas the polarization is discontinuous.
Abstract: In a mixed ionic/covalent insulator the dynamical ionic charges are much larger than the static ones. The phenomenon is particularly relevant in ferroelectric perovskites, and is well understood at the mean-field level. Here we use a model Hamiltonian to investigate polarization in a strongly correlated, and partially ionic, insulator. Charge transport is evaluated as a Berry phase. At a critical value of the electron-electron interaction, the model has a transition from a band insulator to a Mott insulator: the static ionic charge is continuous across the transition, whereas the polarization is discontinuous. Above the transition, the anion transports a positive charge.
TL;DR: It is shown that there exists a range of parameters in SU(5) theory for which the grand unified theory symmetry remains broken at high temperature, thus avoiding the phase transition that gives rise to the overproduction of monopoles.
Abstract: We show that there exists a range of parameters in SU(5) theory for which the grand unified theory symmetry remains broken at high temperature, thus avoiding the phase transition that gives rise to the overproduction of monopoles. The thermal production of monopoles can be naturally suppressed, keeping their number density below the cosmological limits. {copyright} {ital 1995 The American Physical Society.}
TL;DR: In this paper, a nonperturbative method based on the form factor bootstrap approach is proposed for the analysis of correlation functions of two-dimensional massless integrable theories and applied to the massless flow between the tricritical Ising and the critical Ising models.
Abstract: A nonperturbative method based on the form factor bootstrap approach is proposed for the analysis of correlation functions of two-dimensional massless integrable theories and applied to the massless flow between the tricritical Ising and the critical Ising models.
TL;DR: A suitably realistic formal model of the relevant circuitry is defined, and its performance in the sense of information theory is evaluated, showing how nearly all the stored information can be preserved if the model Schaffer collaterals are endowed with an optimal degree of Hebbian plasticity, matching that of the CA3 recurrent collateral.
Abstract: Within the theory that describes the hippocampus as a device for the on-line storage of complex memories, the crucial autoassociative operations are ascribed mainly to the recurrent CA3 network. The CA3-to-CA1 connections may still be important, both in completing information retrieval and in re-expanding, with minimal information loss, the highly compressed representation retrieved in CA3. To quantify these effects, I have defined a suitably realistic formal model of the relevant circuitry, and evaluated its performance in the sense of information theory. Analytical estimates, calculated with mean-field, replica and saddle-point techniques, of the amount of information present in the model CA1 output, reveal how such performance depends on different parameters characterising these connections. In particular, nearly all the stored information can be preserved if the model Schaffer collaterals are endowed with an optimal degree of Hebbian plasticity, matching that of the CA3 recurrent collaterals.
TL;DR: In this article, the acceleration and Fermi-Walker transport for the circular motion of particles, photons and gyroscopes in stationary, axially symmetric spacetimes in terms of the optical reference geometry are discussed.
Abstract: We discuss the acceleration and Fermi-Walker transport for the circular motion of particles, photons and gyroscopes in stationary, axially symmetric spacetimes in terms of the optical reference geometry.
TL;DR: In this paper, a method of quantization which produces a nontrivial effective potential for the conformal factor of the metric was discussed, both in the case of matter fields propagating in a fixed background geometry, and in a case of pure gravity.
TL;DR: Due to the running of the VEV of the dilaton all particles that would naively seem to have masses larger than Planck's mass, may actually not propagate and solve the problem of unitarity in these theories.
Abstract: We consider a scalar-metric gauge theory of gravity with an independent, metric, connection, and dilaton. The role of the dilaton is to provide the scale of all masses, via its vacuum expectation value. In this theory, we study the renormalization-group flow of the dilaton potential, taking into account threshold effects at the Planck scale. Because of the running of the VEV of the dilaton all particles that would naively seem to have masses larger than Planck's mass may actually not propagate. This could solve the problem of unitarity in these theories.
TL;DR: It is suggested that this new receptor type, which mediates bicuculline-insensitive responses with slow kinetics, may potentiate the depolarizing action of GABA during a critical period of postnatal development and therefore play a crucial role in synaptogenesis.
Abstract: 1. The patch-clamp technique was used to characterize, in acutely dissociated CA3 rat hippocampal neurons, the whole cell and single channel properties of a novel response to gamma-aminobutyric acid (GABA) present only during a restricted period of postnatal development. 2. At postnatal days 0-10 (P0-P10), both GABA (100 microM) and isoguvacine (50 microM) evoked at a holding potential of -50 mV, in symmetrical chloride solution, whole cell inward currents. Bicuculline blocked the response to isoguvacine but only reduced the response to GABA (from 512 +/- 137 pA to 60 +/- 13 pA, mean +/- SE). After P12, bicuculline abolished the response to GABA. 3. The bicuculline-insensitive GABA currents were Cl- mediated and antagonized by picrotoxin. The desensitization rate was slower than the conventional bicuculline-sensitive response. The peak to plateau ratio induced by 0.1 or 1 mM of GABA shifted from 4.6 +/- 0.4 and 17.7 +/- 2.6 to 1.5 +/- 0.1 and 3.1 +/- 0.5 in the absence or in the presence of bicuculline, respectively. The recovery from desensitization was significantly faster for the bicuculline-insensitive responses. 4. In excised outside-out patches, GABA (20 microM) activated, in the presence of bicuculline (100 microM), single channel currents having conductances of 14, 22, and 31 pS. These values were similar to those obtained in the same preparation, in the absence of bicuculline. 5. These findings suggest that this new receptor type, which mediates bicuculline-insensitive responses with slow kinetics, may potentiate the depolarizing action of GABA during a critical period of postnatal development and therefore play a crucial role in synaptogenesis.
TL;DR: In this article, it was shown that the well known N = 1 NLS equation possesses N = 2 supersymmetry and thus it is actually the n = 2 NLS equations.