Showing papers by "International School for Advanced Studies published in 1990"
TL;DR: In this article, the authors derived the subleading terms of an eikonal function up to two loops and for D = 4, they found a finite and positive classical correction to the Einstein deflection, implying a deviation from the external metric picture.
342 citations
TL;DR: A new local decomposition of the observed density gradient is defined and it is shown that the scalar variable defined in the decomposition naturally describes density clumping, and satisfies the standard Bardeen second-order equation.
Abstract: In a previous paper, a second-order propagation equation was derived for covariant and gauge-invariant {ital vector} {ital fields} characterizing density inhomogeneities in an almost-Friedmann-Lemaitre-Robertson-Walker (-FLRW) perfect-fluid universe. However, an error there led to omission of a term representing an effect of vorticity on {ital spatial} {ital density} {ital gradients} at linear level. Here we determine this interaction (leading to an extra term in the second-order propagation equation for the spatial density gradient), and examine its geometrical and physical meaning. We define a new local decomposition of the observed density gradient and we show that the scalar variable defined in the decomposition naturally describes density clumping, and satisfies the standard Bardeen second-order equation. The physical meaning of the other variables defined in the decomposition is discussed, and their propagation equations are presented. Finally, the vorticity-induced time growth of the density gradient is derived in the long-wavelength limit.
172 citations
TL;DR: In this article, a detailed investigation of liquid carbon (l-C) and quenched amorphous carbon at room temperature was carried out using a first-principles molecular-dynamics method.
Abstract: We have carried out a detailed investigation of liquid carbon (l-C) at T\ensuremath{\simeq}5000 K, and quenched amorphous carbon (a-C) at room temperature, using a first-principles molecular-dynamics method. In this paper we report calculations performed for a fixed density corresponding to low pressures. From our results, which agree well with the limited experimental information available, we analyze the short-range order, particularly the fractions of sp, ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ sites, and the electronic properties of the two systems. In addition, we discuss new features of a-C and l-C, predicted by our calculation, and present an analysis of some models proposed in the literature to account for their electronic and bonding properties, as well as of recent controversial experiments.
162 citations
TL;DR: Theoretical models of carbon stars can explain a variety of measured abundances, in particular the range of krypton isotope ratios and the excess 22Ne found in the grains as discussed by the authors.
Abstract: Meteoritic silicon carbide grains, formed in the winds from carbon stars, contain noble gases and other species whose elemental and isotopic abundances are a probe of stellar nucleosynthesis. Theoretical models of carbon stars can explain a variety of measured abundances, in particular the range of krypton isotope ratios and the excess 22Ne found in the grains.
134 citations
TL;DR: In this article, a general notion of G-convergence for sequences of maximal monotone operators of the form h → + ∞, of the solutions u h to the equations and of their momenta a h (x, D u h ).
Abstract: A general notion of G-convergence for sequences of maximal monotone operators of the form is introduced in terms of the asymptotic behavior, as h → + ∞, of the solutions u h to the equations and of their momenta a h ( x , D u h ). The main results of the paper are the local character of the G-convergence and the G-compactness of some classes of nonlinear monotone operators.
127 citations
TL;DR: The dynamical path followed during the reconstruction from the ideal bulk-terminated surface, which is found to be unstable, is described and the phonon spectrum, calculated ab initio for the first time, shows well-defined surface modes at frequencies close to the measured values.
Abstract: Our findings for the equilibrium structure confirm the Pandey (2 × 1) chain model and are in very good agreement with both LEED and medium-energy ion-scattering data, showing in particular a large buckling of the surface chains. We describe the dynamical path followed during the reconstruction from the ideal bulk-terminated surface, which is found to be unstable. The phonon spectrum, which is calculated ab initio for the first time, shows well-defined surface modes at frequencies close to the measured values
115 citations
TL;DR: On considere des surfaces GaAs(110) et GaP(110).
Abstract: On considere des surfaces GaAs(110) et GaP(110). Une tres large contribution a l'anisotropie de la reflectance est liee aux transitions qui n'impliquent pas d'etats de surface. Comparaison avec des donnees experimentales
107 citations
TL;DR: In this paper, the integrated canonical commutation relations for (2 + 1)-dimensional de Sitter/anti-de Sitter gravity in first-order formalism are used to derive generalised spinor representations.
101 citations
TL;DR: In this article, a model based on the sl 2 affine algebra, which is integrable and conformally invariant, is presented, which reduces to the standard Liouville theory or to the sinh-Gordon model under certain limiting conditions.
94 citations
TL;DR: In this article, the authors considered the problem of finding T-periodic solutions for the periodic BVP in the case that the dimension of the space is even, and they proved some results for the solvability of the periodic solution.
90 citations
TL;DR: The results indicate an increase of the diamond melting temperature with pressure, which is opposite to the behavior of silicon and germanium, contrary to long-held assumptions, but agrees with recent experiments, and has important implications for geology and astrophysics.
Abstract: Melting of diamond at high pressure and the properties of liquid carbon at pressures greater than 1 megabar were investigated with a first-principles molecular dynamics technique. The results indicate an increase of the diamond melting temperature with pressure, which is opposite to the behavior of silicon and germanium. This is contrary to long-held assumptions, but agrees with recent experiments, and has important implications for geology and astrophysics. As is the case for the solid phase of carbon at low temperature, which changes greatly with pressure from graphite to diamond, the structural and bonding properties of liquid carbon vary strongly with pressure.
TL;DR: In this paper, the chaotic behavior of the Bianchi IX cosmological models was investigated using techniques developed in the study of dynamical systems and chaotic behaviour. And they showed that instead of converging to a constant value, it decreases steadily.
Abstract: In this paper we investigate the chaotic behaviour of the Bianchi IX cosmological models using techniques developed in the study of dynamical systems and chaotic behaviour. We numerically calculate the Lyapunov exponent, λ, and show that instead of converging to a constant value, it decreases steadily. We study this effect further by studying the Lyapunov exponent using short-time averages. We show that the usual method of calculating λ is invalid in the case of a cosmological model.
TL;DR: The large-distance behavior of the spin-spin correlation function of the one-dimensional repulsive Hubbard model is evaluated analytically in the strong-coupling regime at quarter filling and it is found that this behavior is generally valid at any nonzero doping, although the argument is not mathematically rigorous away from quarter filling.
Abstract: The large-distance behavior of the spin-spin correlation function of the one-dimensional repulsive Hubbard model is evaluated analytically in the strong-coupling regime at quarter filling. In this case, its power-law decay is characterized by an exponent \ensuremath{\gamma}=3/2. We have found that this behavior is generally valid at any nonzero doping, although our argument is not mathematically rigorous away from quarter filling. These results strongly suggest that the renormalization-group scaling to the Tomonaga-Luttinger model is exact in the U\ensuremath{\rightarrow}\ensuremath{\infty} Hubbard model.
TL;DR: In this paper, a transect along the longitudinal axis of the Alps seems to indicate that relevant lateral variations in density are present in the lithospheric part of the mantle, which seem to characterize many orogenic belts.
TL;DR: It is found that some degree of cationic mixing must also affect inner planes in the ultrathin regime in order to explain experimental findings.
Abstract: Phonon spectra of ultrathin (GaAs${)}_{\mathrm{n}}$(AlAs${)}_{\mathrm{n}}$ (001) superlattices are studied theoretically using linear-response density-functional techniques. Results are presented for n=1,2,3 superlattices, along with prototype supercell calculations aimed at simulating a completely disordered (alloy) as well as some partially disordered superlattices. Besides interfacial disorder, which modifies the effective confinement length of low-order longitudinal-optic phonons, we find that\char22{}in the ultrathin regime\char22{}some degree of cationic mixing must also affect inner planes in order to explain experimental findings.
TL;DR: In this paper, the vibrational properties of GaAs-AlAs alloys have been studied using large supercells to simulate the disorder and ab initio interatomic force constants.
Abstract: The vibrational properties of ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As alloys have been studied using large supercells to simulate the disorder and ab initio interatomic force constants. In agreement with recent experimental evidence, our results indicate that well defined GaAs-like and AlAs-like phonon dispersions exist for any concentration. Besides broadening phonon states with definite wave vector, alloying narrows the optic branches and lowers the longitudinal modes more than the transverse ones, thus reducing the LO-TO splitting. The acoustic bands are instead rather insensitive to the composition.
TL;DR: In this article, the existence of periodic solutions of q+V'(q)=0 such that 1/2|q'| 2 +V(q) =h where q∈R N, h is a given number, V∈C 2 (R N \{0},R) has a singularity at x=0, and V' denotes the gradient of V
Abstract: This paper deals with the existence of periodic solutions of q″+V'(q)=0 such that 1/2|q'| 2 +V(q)=h where q∈R N , h is a given number, V∈C 2 (R N \{0},R) has a singularity at x=0, and V' denotes the gradient of V
TL;DR: In this article, the existence of a T-periodic solution for a class of Hamiltonian systems which includes the N-body one was shown to be true for any given T. The authors also proved that the solution they find is not a simultaneous collision one.
Abstract: In this paper we prove the existence of a T-periodic solution (for any given T) for a class of Hamiltonian systems which includes the N-body one. We also prove that the solution we find is not a simultaneous collision one.
TL;DR: In this article, it was shown that the higher level SU(2)-coset models can be represented by projected tensor products of the Virasoro models, and the modular covariance of the prescription, construct the monodromy-invariant correlation functions for arbitrary level fields and calculate some of the structure constants.
TL;DR: In this article, a general method for constructing interesting topological gauge theories in arbitrary dimensions is presented, based on the geometrical data of the universal bundle with connection of Atiyah and Singer.
Abstract: A general method for constructing interesting topological gauge theories in arbitrary dimensions is presented. The basic framework upon which these models are built is given by the geometrical data of the "universal bundle with connection" of Atiyah and Singer. The models considered include theories which represent the moduli spaces of flat connections and solutions to the Yang-Mills equations. The former theories correspond to supersymmetric versions of the recently introduced BF systems. In all cases we show explicitly that the quantization can be carried out through the construction of an off-shell nilpotent BRST operator, thus guaranteeing the metric independence of these models.
TL;DR: In this article, the energy differences between icosahedral and cuboctahedral structures for 13-, 19-, and 55-atom clusters are studied theoretically via the Car-Parrinello (CP) method.
TL;DR: In this paper, the authors examined the possibility that new exotic quarks, as for example the Q=− 1 3 charged quarks predicted by E6 models, are stable and showed that their cosmological consequences combined with the bounds that arise from superheavy element searches, together with the bound recently derived by requiring that heavy particles captured by neutron stars do not induce their collapse into a black hole, exclude that possibility.
TL;DR: It is found that liquid GaAs is metallic, with an average coordination number between 5 and 6, and bonds between like atoms are very frequent; three-atom clusters of a single species have a relative probability of 6%, while larger clusters occur only occasionally.
Abstract: The structural and electronic properties of liquid GaAs are investigated using ab initio molecular-dynamics methods. In agreement with experiments, we find that liquid GaAs is metallic, with an average coordination number between 5 and 6. Analysis of the electronic-charge distribution shows that weakly ionic bonds, similar to those of crystalline GaAs, are still present in the liquid. At variance with the crystal, these bonds are not permanent, but are continuously formed and disrupted by atomic diffusion. On the average, however, almost 50% of the atoms of the first coordination shell are bonded to the central atom. Bonds between like atoms are very frequent (\ensuremath{\sim}27%); three-atom clusters of a single species have a relative probability of 6%, while larger clusters occur only occasionally.
TL;DR: In this article, a geometrical framework suggests a re-formulation of the Gepner conjecture about (2,2) superconformal theories as the solution to the Schottky problem for algebraic complex manifolds having trivial canonical bundle.
Abstract: The geometry ofN=2 supergravity is related to the variations of Hodge structure for “formal” Calabi-Yau spaces. All known results in this branch of algebraic geometry are easily recovered from supersymmetry arguments. This identification has a physical meaning for a type IIB superstring compactified on a Calabi-Yau 3-fold. We give exact (non-perturbative) results for the string effective lagrangian. Our geometrical framework suggests a re-formulation of the Gepner conjecture about (2,2) superconformal theories as the solution to theSchottky problem for algebraic complex manifolds having trivial canonical bundle.
TL;DR: In this article, the singularity of the momentum distribution at kF and the power law decay of the spin and charge correlations in real space were determined through finite-size scaling, which confirmed the non-Fermi-liquid nature of the system.
Abstract: We have applied newly numerical schemes to study the ground-state properties of the 1D Hubbard model. Through finite-size scaling, we determine the singularity of the momentum distribution at kF and the power law decay of the spin and charge correlations in real space. Our results confirm the non-Fermi-liquid nature of the system. The model is shown to scale to the Tomonaga-Luttinger model even in the large-U regime where perturbative results are invalid.
TL;DR: In this paper, the level of functional failure of the Palais-Smale condition in the Sobolev space V(Ω) is characterized, and an existence theorem for positive solutions to the mixed boundary problem (0.1) is deduced.
Abstract: In this paper we characterise the levels of the functional (0.3) at which the Palais-Smale condition fails in the Sobolev space V(Ω) defined below. From this result we deduce an existence theorem for positive solutions to the mixed boundary problem (0.1)–(0.2) under geometrical assumptions on the domain Ω and the part of the boundary of Ω where a Neumann condition is prescribed.
TL;DR: It is shown for a wide class of systems in the framework of the total Hamiltonian procedure that all first-class constraints generate canonical transformations connecting physically equivalent states.
Abstract: It is shown for a wide class of systems in the framework of the total Hamiltonian procedure that all first-class constraints generate canonical transformations connecting physically equivalent states. It occurs whenever the constraints arising in the Dirac algorithm are effective when considered in the functional form as they appear in the consistency conditions. The property of hereditary separation between first- and second-class constraints also follows from the above condition. General Poisson-brackets relations among constraints in the representation used here are also obtained. The sources of anomalies in the hereditary property reported in the literature are identified.
TL;DR: Presentation des premiers resultats exacts de l'energie de l'?etat fondamental, dans le modele de Hubbard bidimensionnel sur un amas 4×4 pour plusieurs valeurs de V et de remplissage.
Abstract: We present the first exact results of the ground-state energy in the two-dimensional Hubbard model on a 4\ifmmode\times\else\texttimes\fi{}4 cluster for several values of U and fillings. The behavior of the binding energy of two holes as a function of U is discussed together with some analytical results in limiting cases. The analysis of the hole-hole correlation function allows one to determine the range of values of U where binding might be seen.