Showing papers by "International School for Advanced Studies published in 1993"

On the shape of the light profiles of Early Type galaxies

Abstract: We have obtained the best fit to the light profiles of a luminosity-limited sample of elliptical and S0 galaxies with a power law r 1/n , letting the exponent remain free rather than keeping it fixed at 1/n=1/4 as in the well-known de Vaucouleurs formula. The introduction of a free parameter in the fitting formula (ranging from n=0.5 for =0.3 kpc to n=16 for =25 kpc) is justified by the existence of a good correlation between n and the global galaxian parameters, such as total luminosity and scale-radius. This result seems to be in line with the segregation of properties between the ordinary and bright families of early-type galaxies, and has consequences for the claimed independence of the shape of galaxy profiles with respect to the Fundamental Plane parameters

Transgenic plants expressing a functional single-chain Fv antibody are specifically protected from virus attack

Abstract: EXPRESSION of viral genes in transgenic plants is a very effective tool for attenuating plant viral infection1–3 Nevertheless, the lack of generality and risk issues related to the expression of viral genes in plants4 might limit the exploitation of this strategy Expression in plants of antibodies against essential viral proteins could provide an alternative approach to engineer viral resistance Recently, expression of complete5–7 or engineered7–9 antibodies has been successfully achieved in plants The engineered single-chain Fv antbody scFv (refs 10,11) is particularly suitable for expression in plants because of its small size and the lack of assembly requirements Here we present evidence that constitutive expression in transgenic plants of a scFv antibody, directed against the plant icosahedral tombusvirus artichoke mottled crinkle virus, causes reduction of infection incidence and delay in symptom development

On classification of N =2 supersymmetric theories

Abstract: We find a relation between the spectrum of solitons of massiveN=2 quantum field theories ind=2 and the scaling dimensions of chiral fields at the conformal point. The condition that the scaling dimensions be real imposes restrictions on the soliton numbers and leads to a classification program for symmetricN=2 conformal theories and their massive deformations in terms of a suitable generalization of Dynkin diagrams (which coincides with the A-D-E Dynkin diagrams for minimal models). The Landau-Ginzburg theories are a proper subset of this classification. In the particular case of LG theories we relate the soliton numbers with intersection of vanishing cycles of the corresponding singularity; the relation between soliton numbers and the scaling dimensions in this particular case is a well known application of Picard-Lefschetz theory.

Ab initio lattice dynamics of diamond

Abstract: We present a first-principles calculation of lattice dynamical properties of diamond. Our calculations have been performed using density-functional perturbation theory together with plane-wave expansion and nonlocal pseudopotentials. As a first step we have evaluated the equilibrium structure of diamond via the minimization of the total energy. Then, harmonic phonon dispersion curves and phonon eigenvectors have been evaluated within the linear-response framework. As a by-product of the calculation we have also obtained the internal-strain parameter. Furthermore, we have also tested the validity of the ab initio calculation for describing properties beyond the harmonic approximation. Using the quasiharmonic approximation we have calculated the thermal expansion coefficient and the mode Gr\"uneisen parameter dispersion curves. Where experimental data are available, good agreement is found with our theoretical predictions.

Biophysics of the cochlea: Linear approximation

Abstract: Several deficiencies affecting previous ‘‘box’’ models of the cochlea are overcome in this paper. Both mechanical and hydrodynamical aspects are treated at a level adequate to the complexity of realistic cochlear structures. The dynamics of the cochlea as a passive physical system, in the linear approximation, is described by an integral equation. It is further shown that this equation describes the properties of the working cochlea, provided a force term that accounts for hair cell motility is included. Numerical solutions for different degrees of outer hair cells activity, obtained by matrix methods in the frequency domain, are presented. Amplitudes and phases of the computer‐simulated traveling waves are in fair agreement with basilar membrane responses to tones measured in various experimental conditions.

Ab initio calculation of phonon dispersions in II-VI semiconductors

Abstract: The vibrational properties of ZnSe, ZnTe, CdSe, and CdTe are determined by density-functional perturbation theory. To this end we have generalized this method so as to explicitly account for the nonlinear core correction to the exchange and correlation energy of systems treated with pseudopotentials. Furthermore, we have implemented a method to enhance the transferability of pseudopotentials of group-II atoms with shallow d electrons frozen in the core. The accuracy obtained in this way is similar to that previously achieved for elemental and III-V semiconductors.

Macroscopic electric polarization as a geometric quantum phase

Abstract: I outline a modern theory of the macroscopic polarization P in crystalline dielectrics. While P itself is not an observable, the polarization difference ΔP between any two crystal states can be measured, defined and calculated as a bulk material property: the root of this fact lies in a geometric quantum phase. In general, ΔP cannot be recovered from periodic charge densities, where any phase information is deleted: dielectric polarization is in fact a quantum phenomenon, governed by a gauge-invariant phase feature of the crystal wave functions.

Ising Model and N = 2 Supersymmetric Theories

Abstract: We establish a direct link between massive Ising model and arbitrary massiveN=2 supersymmetric QFT's in two dimensions. This explains why the equations which appear in the computation of spin-correlations in the non-critical Ising model are the same as those describing the geometry of vacua inN=2 theories. The tau-function appearing in the Ising model (i.e., the spin correlation function) is reinterpreted in theN=2 context as a new “index”. In special cases this new index is related to the Ray-Singer analytic torsion, and can be viewed as a generalization of that to the loop space of Kahler manifolds.

Spectral variations of LMC X-3 observed with Ginga

Abstract: The prime black hole candidate LMC X-3 was observed over three years with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of about 198 (or possibly about 99) days (Cowley et al., 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above about 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object.

Path integration via summation of perturbation expansions and applications to totally reflecting boundaries, and potential steps

Abstract: The path integral for the propagator is expanded into a perturbation series, which can be exactly summed in the case of \ensuremath{\delta}-function perturbations giving a closed expression for the (energy-dependent) Green function. Making the strength of the \ensuremath{\delta}-function perturbation infinite repulsive produces a totally reflecting boundary, hence giving a path integral solution in half spaces in terms of the corresponding Green function. The example of the Wood-Saxon potential serves by an appropriate limiting procedure to obtain the Green function for the step potential and the finite potential well in the half space, respectively.

Coexistence of Monatomic and Diatomic Molecular Fluid Character in Liquid Gallium

Abstract: We have performed an ab initio molecular-dynamics simulation of liquid Ga at high temperature (1000 K). Our results are in good agreement with scattering data and with Knight-shift experiments. A remarkable feature of our findings is the coexistence in the liquid state of metallic and covalent characters. Covalency manifests itself in the appearance of very short-lived Ga-Ga bonds, which represent remnants in the liquid of the crystalline form α-Ga. We set up a two-fluid scheme which can be used for the analysis of fluids where metallic and covalent characters coexist.

Second-order Raman spectra of diamond from ab initio phonon calculations.

Abstract: The second-order Raman spectra of diamond and silicon have been calculated using ab initio phonons and phenomenological polarizability coefficients. The sharp peak in the spectrum of diamond near the two-phonon cutoff is explained by a maximum in the vibrational density of states; this maximum originates from the uppermost phonon branch whose frequencies are calculated to have a minimum at the Brillouin-zone center. This frequency minimum as well as the sharp Raman peak are unique to diamond and do not occur for the other group-IV semiconductors. In our calculation based on harmonic ab initio lattice dynamics neither two-phonon bound states nor polarizability matrix element effects are needed to explain the peak, and we feel that the long-standing controversy about its origin has been resolved.

Use of living columns to select specific phage antibodies.

Abstract: Here we demonstrate that it is possible to confront two recombinant microorganisms in order to select one using the other. We have shown that an epitope derived from p21ras expressed within the outer membrane protein, LamB, can be recognized both by the monoclonal antibody Y13-259, as well as the single chain Fv fragment derived from it. This specificity, which is maintained when the Y13-259 single chain Fv is expressed as a fusion protein with the phage fd gene 3 protein, has allowed us to use the living column of LamB-ras to purify Y13-259 phage from a background of non-binding phage, even at dilutions as high as 10 phage in 1010 irrelevant phage.

Kodaira-Spencer Theory of Gravity and Exact Results for Quantum String Amplitudes

Abstract: We develop techniques to compute higher loop string amplitudes for twisted $N=2$ theories with $\hat c=3$ (i.e. the critical case). An important ingredient is the discovery of an anomaly at every genus in decoupling of BRST trivial states, captured to all orders by a master anomaly equation. In a particular realization of the $N=2$ theories, the resulting string field theory is equivalent to a topological theory in six dimensions, the Kodaira--Spencer theory, which may be viewed as the closed string analog of the Chern--Simon theory. Using the mirror map this leads to computation of the `number' of holomorphic curves of higher genus curves in Calabi--Yau manifolds. It is shown that topological amplitudes can also be reinterpreted as computing corrections to superpotential terms appearing in the effective 4d theory resulting from compactification of standard 10d superstrings on the corresponding $N=2$ theory. Relations with $c=1$ strings are also pointed out.

On the pairwise velocity dispersion of galaxies

Abstract: Previous statistical results have shown that the pairwise velocity dispersion σ g (r p ) of galaxies at small separations (r p ≃ 1 h −1 Mpc) is about 300 km s −1 . This value is significantly lower than the predictions of currently favoured models of structure formation, unless a large pairwise velocity bias exists between galaxies and the underlying mass. Using various samples of galaxies (CfA, SSRS, IRAS, CfA slice, samples containing ellipticals or spirals), we show that the results of σ g (r p ) are sensitive to sampling effects, such as the inclusion of dominating clusters, and the correction of the Virgocentric infall. The value of σ g (r p = 1 h −1 Mpc) is as high as ∼ 1000 km s −1 for the CfA slice including Coma, and as low as ∼ 300 km s −1 for the spiral sample and for the CfA sample with a correction of the Virgocentric infall

Cloning and expression of an anti-nerve growth factor (NGF) antibody for studies using the neuroantibody approach.

Abstract: 1. The neuroantibody approach, based on the expression of selected monoclonal antibodies by cells of the nervous system, has recently been described (Cattaneo and Neuberger, 1987; Piccioliet al., 1991). In order to apply this experimental strategy to study the role of nerve growth factor (NGF) in the central nervous system (CNS), we exploited the monoclonal antibody (mAb),αD11, which neutralizes very efficiently the biological activity of NGF, bothin vitro andin vivo (Cattaneoet al., 1988). 2. TheαD11 antibody chains were cloned and expressed in COS cells as rat/human chimaeric proteins. The cloned antibody was shown to display all the properties of the parentalαD11 antibody, including its ability to neutralize NGF biological activity. 3. This will allow us to engineer the expression of recombinantαD11 antibodies in the CNS, to study the role of NGF in the developing and adult nervous system. This approach can be extended to other neurotrophic factors for which neutralizing monoclonal antibodies are available.

X-ray polarization in the two-phase model for AGN and X-ray binaries

Abstract: We calculate the polarization properties of the two-phase model proposed by Haardt & Maraschi to explain the X-ray emission of AGN and X-ray binaries. In this model, hot electrons in an optically thin corona Comptonize the soft photons emitted by the underlying accretion disc to produce the X-rays. We find that the degree of polarization depends strongly on the inclination angle of the system, on the optical depth of the hot corona and on the assumed law for the soft flux. The main result is the strong dependence of the polarization properties on the energy, and in particular the orthogonality between the UV/soft X-ray and the hard X-ray polarizations