Showing papers by "International School for Advanced Studies published in 1992"
TL;DR: In this paper, the state-of-the-art for induced and spontaneous polarization in real materials is presented, both for spontaneous and induced polarization, and for the latter case, a novel approach is proposed.
Abstract: Contrary to common textbook statements, the macroscopic electric polarization of a crystal cannot be unambiguously defined as the dipole of a unit cell. The only measurable quantities are either derivatives of the polarization (dielectric susceptibility, Born effective charges, piezoelectricity, pyroelectricity) or finite differences (ferroelectricity). The differential concept is a basic one in theoretical work too. I sketch here the basic theory and the state of the art in its implementation to real materials, both for induced and spontaneous polarization; for the latter case, I also propose a novel approach.
374 citations
TL;DR: In this paper, it was shown that Tr(−1) F F e βH is an index for N = 2 supersymmetric theories in two dimensions, in the sense that it is independent of almost all deformations of the theory.
337 citations
TL;DR: Using quantum Monte Carlo and finite-size scaling for the Hubbard model, this paper found evidence of a zero-temperature transition between the nonmagnetic semi-metal and an antiferromagnetic insulator in the 2D honeycomb lattice for a nontrivial value of U/t?=?4.5???0.5.
Abstract: Using quantum Monte Carlo and finite-size scaling for the Hubbard model, we find evidence of a zero-temperature transition between the nonmagnetic semi-metal and an antiferromagnetic insulator in the 2D honeycomb lattice for a nontrivial value of U/t?=?4.5???0.5. The corresponding transition in Hartree-Fock mean field is at U/t?=?2.23, which indicates the importance of quantum fluctuations. This represents the first example of Mott-Hubbard transition in a 2D bipartite lattice. Similar transitions are predicted for special lattices in higher dimensions, in particular for the 3D diamond lattice.
281 citations
TL;DR: In this paper, the authors estimate the baryon mass density of the universe due to the stars in galaxies and the hot gas in clusters and groups of galaxies using the Efstathiou, Ellis & Peterson luminosity function, together with van der Marel and Persic & Salucci's mass-to-light versus luminosity relationships.
Abstract: We estimate the baryon mass density of the Universe due to the stars in galaxies and the hot gas in clusters and groups of galaxies. The galaxy contribution is computed by using the Efstathiou, Ellis & Peterson luminosity function, together with van der Marel and Persic & Salucci’s mass-to-light versus luminosity relationships. We find stars ≃ 0.002. For clusters and groups we use the Edge et al. X -ray luminosity function, and Edge & Stewart and Kriss, Cioffi & Canizares’ (gas mass )-luminosity relations. We find gas ≃ 0.001. The total amount of visible baryons is then b ≃ 0.003, i.e. less than 10 per cent of the lower limit predicted by standard primordial nucleosynthesis, implying that the great majority of baryons in the Universe are unseen.
247 citations
TL;DR: In this paper, the authors analyze those integrable statistical systems which originate from some relevant perturbations of the minimal models of conformal field theories and show that the central charge of the original conformal theories can be recovered from the scattering data.
213 citations
TL;DR: In this article, the integral representation in BV(Ω; R m ) of the functional F arising from the relaxation of F(u) = ∝ Ω f(▽u) dx, u ϵ C 1 (Ω, R m ).
165 citations
146 citations
TL;DR: In this article, the authors present a new approach to density functional theory, which does not require the calculation of Kohn-Sham orbitals, and the computational workload required by their method scales linearly with the volume of the system, thus opening the way to first principles calculations for very large systems.
Abstract: We present a new approach to density functional theory, which does not require the calculation of Kohn-Sham orbitals. The computational workload required by our method—which is based on the calculation of selected elements of the Green's function—scales linearly with the volume of the system, thus opening the way to first-principles calculations for very large systems. Some of the problems which still hinder the achievement of this goal are discussed, and possible solutions are outlined. As an application, we calculate the charge density of a model silicon supercell containing 64 atoms slightly displaced at random from equilibrium.
134 citations
TL;DR: In this paper, the authors compare various approaches giving the same leading eikonal approximation for S-matrix singularities and the physics of black hole formation, and stress the need of going beyond this approximation in order to control impact parameters b≅GE which are crucial for understanding S-mixture singularities.
124 citations
TL;DR: In this paper, the authors investigated the local linear stability properties of two-component galactic disks and found that the stabilizing role of finite-thickness effects can partially counterbalance the cold interstellar gas in linear regimes.
Abstract: The thickness scales and the local parameters relevant to both the equilibrium and stability of two-component galactic disks are studied. The results obtained, as regards the vertical structure at equilibrium of two-component galactic disks, are used to investigate their local linear stability properties. Under reasonable assumptions, finite-thickness corrections to the local dispersion relation can be expressed in terms of two reduction factors weakening the response of the two components or, equivalently, lowering their equilibrium surface densities. An ansatz for such reduction factors, justified by extending the analysis performed for one-component purely stellar disks, is made, and the corresponding two-fluid marginal stability curve is studied in standard star-dominated and peculiar gas-dominated regimes. It is found that the stabilizing role of finite-thickness effects can partially counterbalance the destabilizing role of the cold interstellar gas in linear regimes.
123 citations
TL;DR: In this paper, a new covariant and gauge-invariant (GI) treatment of perturbations in a Robertson-Walker universe dominated by a classical scalar field is proposed.
Abstract: The authors propose a new covariant and gauge-invariant (GI) treatment of perturbations in a Robertson-Walker universe dominated by a classical scalar field phi . They first set up the formalism, based on the natural slicing of the problem by the surfaces phi =constant, and introduce a set of covariantly defined GI variables. In their approach the whole inhomogeneity of the matter field is incorporated in the GI spatial fluctuations of the momentum psi of phi ; then the GI density perturbations are simply proportional to the momentum perturbations. The inhomogeneity of the geometry is characterized by GI fluctuations of the 3-curvature scalar of the surfaces phi =constant. The time evolution of the matter and curvature perturbations are coupled by a pair of first-order linear differential equations. Correspondingly, each GI variable satisfies a second-order linear homogeneous differential equation. When the background curvature vanishes, k=0, the curvature variable is conserved for perturbation scales larger than the horizon, but this is no longer true in general if k not=0. They discuss simple examples, including the case when more than one scalar field is present, recovering standard results for inflationary universe models. They also demonstrate that in coasting solutions with k=-1, inhomogeneities are damped out on all scales.
TL;DR: In this paper, a complete sample of Virgo cluster galaxies and a larger set of heterogeneous literature data was used to show that elliptical, early-type dwarfs, and bulges of S0s and of spirals form two distinct families in the plane of the effective parameters, R e and μ e.
Abstract: Using a complete sample of Virgo cluster galaxies and a larger set of heterogeneous literature data, we show that ellipticals, early-type dwarfs, and bulges of S0s and of spirals form two distinct families in the plane of the effective parameters, R e and μ e . The «ordinary» family is composed of all galaxies fainter than M B ≃−19.3. Their effective parameters range over a wide interval for the same total luminosity: R e varies by at least ∼0.7 dex and μ e by 3.5 mag. Moreover, the interval spanned by R e is the same at any luminosity (down to M B ≃−12.0), with a sharp upper boundary at R e ≃3 kpc.
TL;DR: In this paper, the ground-state metric is nonsingular as the size of the manifold shrinks to zero, suggesting that 2D quantum field theory makes sense even beyond zero radius.
Abstract: We show that the metric and Berry's curvature for the ground states of $N=2$ supersymmetric $\ensuremath{\sigma}$ models can be computed exactly as one varies the Kahler structure. For the case of ${\mathrm{CP}}^{n}$ these are related to special solutions of affine Toda equations. This allows us to extract exact results. We find that the ground-state metric is nonsingular as the size of the manifold shrinks to zero, suggesting that 2D quantum field theory makes sense even beyond zero radius. Thus it seems that manifolds with zero size are nonsingular as target spaces for string theory (even when they are not conformal).
TL;DR: In this article, the quantum properties of a system of nonrelativistic anyons, described in the first-quantization framework, are reviewed, in the infinite plane, the torus and the sphere.
TL;DR: The relative stability of relaxed icosahedral and cubotahedral lead clusters has been determined by computing their binding energies, using an empirical many-body Hamiltonian fitted on bulk and surface properties of Pb as mentioned in this paper.
TL;DR: The magnetic field supports and confines the dense clouds in a manner analogous to prominences above the solar photosphere as mentioned in this paper, which suppress much of the primary synchrotron radiation emitted by relativistic electrons in the strong magnetic field which is responsible for accelerating them.
Abstract: Small amounts of very dense matter in the central engine of an active galaxy have an important effect on the emitted spectrum at ultraviolet and longer wavelengths. If only 0.1 per cent of the total matter expected there is in the form of small dense clouds, then much of the infrared and optical spectrum is absorbed and reradiated in the ultraviolet. Such clouds thereby suppress much of the primary synchrotron radiation emitted by relativistic electrons in the strong magnetic field which is responsible for accelerating them. The magnetic field supports and confines the dense clouds in a manner analogous to prominences above the solar photosphere
TL;DR: In this paper, the infinite volume limit of the dynamics of mean-field spin models is obtained through a direct analysis of the equations of motion, in a large class of representations of the spin algebra.
Abstract: The infinite-volume limit of the dynamics of (generalized) mean-field spin models is obtained through a direct analysis of the equations of motion, in a large class of representations of the spin algebra. The resulting dynamics fits into a general framework for systems with long-range interaction: variables at infinity appear in the time evolution of local variables and spontaneous symmetry breaking with an energy gap follows from this mechanism. The independence of the construction of the approximation scheme in finite volume is proven.
TL;DR: In this paper, the authors analyzed K 0 − K 0 and B 0 − B 0 mixing on the basis of the most recent experimental results and theoretical lattice determination of the relevant matrix elements.
TL;DR: There are switchable task-determined pre-cognitive strategies of vision that can be learned and that the distribution of lateral masking may be part of what is learned.
TL;DR: In this article, the Toda lattice was introduced and reformulated in operator form for one-matrix models, and a procedure for extracting KdV hierarchies without taking a continuum limit was proposed.
TL;DR: In this paper, the limits on deviation of the lepton and quark weak-couplings from their standard model values in a general class of models where the known fermions are allowed to mix with new heavy particles with exotic SU(2) x U(1) quantum number assignments (left-handed singlets or right-handed doublets).
TL;DR: A qualitative picture of the T=0 phase diagram for two Hubbard chains with interchain hopping t ⊥ and on-site repulsion U is obtained and explicitly allows for renormalization of the Fermi momenta.
Abstract: The T=0 phase diagram for two Hubbard chains with interchain hopping ${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$ and on-site repulsion U is derived within a renormalization-group (RG) approach. We explicitly allow for renormalization of the Fermi momenta. A nonzero ${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$ drives the chains towards strong coupling, and only for large ${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$ is a Luttinger liquid recovered. By use of weak-coupling RG, a qualitative picture of the phase diagram can be obtained. Our calculations show two different strong-coupling regimes: one with nonvanishing effective interchain hopping (〈${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$〉\ensuremath{
e}0), not confining, for small U, and another with 〈${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$〉=0, confining, for large U. Numerical diagonalizations for a small cluster support the RG results.
TL;DR: A structural model of amorphous gallium arsenide is constructed by quenching from the melt, via first-principles molecular-dynamics simulations, and finds that the predominant defects in this system are not wrong bonds, but threefold-coordinated atoms.
Abstract: We have constructed a structural model of amorphous gallium arsenide by quenching from the melt, via first-principles molecular-dynamics simulations. The properties of our structure agree well with the available experimental information. We find that the predominant defects in this system are not wrong bonds, but threefold-coordinated atoms. Because of a relaxation mechanism similar to that occurring on the GaAs(110) surface, these do not yield states in the gap, but yield empty Ga and filled As dangling-bond states near the band edges.
TL;DR: In this article, it was shown that the BRST quantum version of pure D=4 N=2 supergravity can be topologically twisted to yield a formulation of topological gravity in four dimensions.
Abstract: We show that the BRST quantum version of pure D=4 N=2 supergravity can be topologically twisted, to yield a formulation of topological gravity in four dimensions. The topological BRST complex is just a rearrangement of the old BRST complex, that partly modifies the role of physical and ghost fields: indeed, the new ghost number turns out to be the sum of the old ghost number plus the internal U(1) charge. Furthermore, the action of N=2 supergravity is retrieved from topological gravity by choosing a gauge fixing that reduces the space of physical states to the space of gravitational instanton configurations, namely to self-dual spin connections. The descent equations relating the topological observables are explicitly exhibited and discussed. Ours is a first step in a programme that aims at finding the topological sector of matter coupled N=2 supergravity, viewed as the effective Lagrangian of type II superstrings and, as such, already related to 2D topological field-theories. As it stands the theory we discuss may prove useful in describing gravitational instantons moduli-spaces.
TL;DR: In this paper, sufficient conditions for the G-convergence of sequences of quasi-linear monotone operators were determined, together with an asymptotic formula for G-limit.
Abstract: We determine some sufficient conditions for the G-convergence of sequences of quasi-linear monotone operators, together with an asymptotic formula for the G-limit. We then prove a homogenization theorem for quasiperiodic monotone operators and, eventually, extend this result to general almost periodic monotone operators using an approximation result and a closure lemma.
TL;DR: In this article, the authors consider the realization of N = 2 superconformal models in terms of free first-order (b, c, β, γ)-systems, and show that an arbitrary Landau-Ginzburg interaction with quasi-homogeneous potential can be introduced without spoiling the (2, 2)-superconformality invariance.
TL;DR: From a simple resonance S matrix satisfying the «O 3 property», a large class of models possessing resonance factorized S matrices is proposed and investigated, predicting new flows in nonunitary minimal models.
Abstract: We propose and investigate a large class of models possessing resonance factorized S matrices. The associated Casimir energy describes a rich pattern of renormalization-group trajectories related to flows in the coset models based on the simply laced Lie algebras. From a simple resonance S matrix satisfying the «O 3 property» we predict new flows in nonunitary minimal models
TL;DR: In this article, a new mechanism for late cosmological baryon asymmetry in models with first order electroweak phase transition was proposed, which arises through the decay of particles produced out of equilibrium in bubble collision.
TL;DR: In this paper, the Schottky parametrization is used to generalize the one-loop amplitudes to multiloop level, and an explicit expression for the N -gluon h -loop amplitude is given.
TL;DR: In this article, the microscopic aspects of the phase separation connected with the sharp edges recently observed on the equilibrium crystal shape of lead, about twenty degrees below the bulk melting temperature, were studied.
Abstract: We study the microscopic aspects of the phase separation connected with the sharp edges recently observed on the equilibrium crystal shape of lead, about twenty degrees below the bulk melting temperature. We present molecular-dynamics simulations based on a many-body interatomic potential. The high-temperature orientational phase separation can be viewed as a faceting induced by anisotropy of surface melting and should occur quite universally for vicinals of nonmelting crystal surfaces.