Showing papers by "International School for Advanced Studies published in 2001"

Phonons and related crystal properties from density-functional perturbation theory

Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

Dynamical and correlation properties of the Internet

Abstract: The description of the Internet topology is an important open problem, recently tackled with the introduction of scale-free networks. We focus on the topological and dynamical properties of real Internet maps in a three-year time interval. We study higher order correlation functions as well as the dynamics of several quantities. We find that the Internet is characterized by nontrivial correlations among nodes and different dynamical regimes. We point out the importance of node hierarchy and aging in the Internet structure and growth. Our results provide hints towards the realistic modeling of the Internet evolution. Complex networks play an important role in the under- standing of many natural systems (1,2). A network is a set of nodes and links, representing individuals and the interactions among them, respectively. Despite this simple definition, growing networks can exhibit a high degree of complexity, due to the inherent wiring entanglement occur- ring during their growth. The Internet is a capital example of growing network with technological and economical relevance; however, the recollection of router-level maps of the Internet has received the attention of the research community only very recently (3-5). The statistical analysis performed so far has revealed that the Internet ex- hibits several nontrivial topological properties (wiring redundancy, clustering, etc.). Among them, the presence of a power-law connectivity distribution (6,7) makes the Internet an example of the recently identified class of scale-free networks (8). In this Letter, we focus on the dynamical properties of the Internet. We shall consider the evolution of real In- ternet maps from 1997 to 2000, collected by the National Laboratory for Applied Network Research (NLANR) (3). In particular, we will inspect the correlation properties of nodes' connectivity, as well as the time behavior of several quantities related to the growth dynamics of new nodes. Our analysis shows dynamical behavior with dif- ferent growth regimes depending on the node's age and connectivity. The analysis points out two distinct wiring processes: the first one concerns newly added nodes, while the second is related to already existing nodes increasing their interconnections. A feature introduced in this pa- per refers to the Internet hierarchical structure, reflected in a nontrivial scale-free connectivity correlation function. Finally, we discuss recent models for the generation of scale-free networks in the light of the present analysis of real Internet maps. The results presented in this Letter could help develop more accurate models of the Internet.

Abundance Gradients and the Formation of the Milky Way

Abstract: In this paper we adopt a chemical evolution model, which is an improved version of the Chiappini, Matteucci, & Gratton model, assuming two main accretion episodes for the formation of the Galaxy, the first forming the halo and bulge in a short timescale and the second one forming the thin disk, with a timescale that is an increasing function of the Galactocentric distance (being of the order of 7 Gyrs at the solar neighborhood). The present model takes into account in more detail than previously the halo density distribution and explores the effects of a threshold density in the star formation process during both the halo and disk phases. The model also includes the most recent nucleosynthesis prescriptions concerning supernovae of all types, novae, and single stars dying as white dwarfs. In the comparison between model predictions and available data, we have focused our attention on abundance gradients as well as gas, stellar, and star formation rate distributions along the disk, since this kind of model has already proven to be quite successful in reproducing the solar neighborhood characteristics. We suggest that the mechanism for the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially contribute to building up the outer disk. We also show that the existence of a threshold density for the star formation rate, both in the halo and disk phase, is necessary to reproduce the majority of observational data in the solar vicinity and in the whole disk. In particular, a threshold in the star formation implies the occurrence of a gap in the star formation at the halo-disk transition phase, in agreement with recent data. We conclude that a relatively short halo formation timescale (0.8 Gyr), in agreement with recent estimates for the age differences among Galactic globular clusters, coupled with an inside-out formation of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents, at the moment, the most likely explanation for the formation of the Milky Way. This scenario allows us to predict abundance gradients and other radial properties of the Galactic disk in very good agreement with observations. Moreover, as a consequence of the adopted inside-out scenario for the disk, we predict that the abundance gradients along the Galactic disk must have increased with time and that the average ratios in stars (halo plus disk) slightly decrease going from 4 to 10 kpcs from the Galactic center. We also show that the same ratios increase substantially toward the outermost disk regions and the expected scatter in the stellar ages decreases, because the outermost regions are dominated by halo stars. More observations at large Galactocentric distances are needed to test these predictions.

Discrete Solitons and Breathers with Dilute Bose-Einstein Condensates

Abstract: We study the dynamical phase diagram of a dilute Bose-Einstein condensate (BEC) trapped in a periodic potential. The dynamics is governed by a discrete nonlinear Schr\"odinger equation: intrinsically localized excitations, including discrete solitons and breathers, can be created even if the BEC's interatomic potential is repulsive. Furthermore, we analyze the Anderson-Kasevich experiment [Science 282, 1686 (1998)], pointing out that mean field effects lead to a coherent destruction of the interwell Bloch oscillations.

Josephson junction arrays with Bose-Einstein condensates.

Abstract: We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.

Internal shocks in the jets of radio-loud quasars

Abstract: The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.

The Effects of Photoionization on Galaxy Formation - II: Satellites in the Local Group

Abstract: We use a self-consistent model of galaxy formation and the evolution of the intergalactic medium to study the effects of the reionization of the universe at high redshift on the properties of satellite galaxies like those seen around the Milky Way. Photoionization suppresses the formation of small galaxies, so that surviving satellites are preferentially those that formed before the universe reionized. As a result, the number of satellites expected today is about an order of magnitude smaller than the number inferred by identifying satellites with subhalos in high-resolution simulations of the dark matter. The resulting satellite population has an abundance and a distribution of circular velocities similar to those observed in the Local Group. We explore many other properties of satellite galaxies, including their gas content, metallicity and star formation rate, and find generally good agreement with available data. Our model predicts the existence of many as yet undetected satellites in the Local Group. We quantify their observability in terms of their apparent magnitude and surface brightness and also in terms of their constituent stars. A near-complete census of the Milky Way's satellites would require imaging to V~20 and to a surface brightness fainter than 26 V-band magnitudes per square arcsecond. Satellites with integrated luminosity V=15 should contain of order 100 stars brighter than B=26, with central stellar densities of a few tens per square arcminute. Discovery of a large population of faint satellites would provide a strong test of current models of galaxy formation.

Large-scale jets in active galactic nuclei: multiwavelength mapping

Abstract: ABSTRA C T X-ray emission from large-scale extragalactic jets is likely to be as a result of inverse Compton scattering of relativistic particles off seed photons of both the cosmic microwave background field and the blazar nucleus. The first process dominates the observed highenergy emission of large-scale jets if the plasma is moving at highly relativistic speeds and if the jet is aligned with the line of sight, i.e. in powerful flat radio spectrum quasars. The second process is relevant when the plasma is moving at mildly bulk relativistic speeds, and can dominate the high-energy emission in misaligned sources, i.e. in radio galaxies. We show that this scenario satisfactorily accounts for the spectral energy distribution detected by Chandra from the jet and core of PKS 0637‐752.

The dark matter distribution in disc galaxies

Abstract: We use high-quality optical rotation curves of nine low-luminosity disc galaxies to obtain the velocity profiles of the surrounding dark matter haloes. We find that they increase linearly with radius at least out to the edge of the stellar disc, implying that, over the entire stellar region, the density of the dark halo is about constant. The properties of the mass structure of these haloes are similar to those found for a number of dwarf and low surface brightness galaxies, but provide a more substantial evidence of the discrepancy between the halo mass distribution predicted in the cold dark matter scenario and those actually detected around galaxies. We find that the density law proposed by Burkert reproduces the halo rotation curves, with halo central densities (ρ0∼1–4×10−24 g cm−3) and core radii (r0∼5–15 kpc) scaling as ρ0∝r0−2/3.

Generalized Lanczos algorithm for variational quantum Monte Carlo

Abstract: We show that the standard Lanczos algorithm can be efficiently implemented statistically and self-consistently improved, using the stochastic reconfiguration method, which has been recently introduced to stabilize the Monte Carlo sign problem instability. With this scheme a few Lanczos steps over a given variational wave function are possible even for large size as a particular case of a more general and more accurate technique that allows to obtain lower variational energies. This method has been tested extensively for a strongly correlated model like the $t\ensuremath{-}J$ model. With the standard Lanczos technique it is possible to compute any kind of correlation functions, with no particular computational effort. By using the fact that the variance $〈{H}^{2}〉\ensuremath{-}〈H{〉}^{2}$ is zero for an exact eigenstate, we show that the approach to the exact solution with few Lanczos iterations is indeed possible even for $\ensuremath{\sim}100$ electrons for reasonably good initial wave functions. The variational stochastic reconfiguration technique presented here allows in general a many-parameter energy optimization of any computable many-body wave function, including for instance generic long-range Jastrow factors and arbitrary site-dependent orbital determinants. This scheme improves further the accuracy of the calculation, especially for long-distance correlation functions.

Analogue gravity from Bose-Einstein condensates

Abstract: We analyse prospects for the use of Bose–Einstein condensates as condensedmatter systems suitable for generating a generic ‘effective metric’, and for mimicking kinematic aspects of general relativity. We extend the analysis due to Garay et al (2000 Phys. Rev. Lett. 85 4643, 2001 Phys. Rev. A 63 023611). Taking a long-term view, we ask what the ultimate limits of such a system might be. To this end, we consider a very general version of the nonlinear Schr¨ odinger equation (with a 3-tensor position-dependent mass and arbitrary nonlinearity). Such equations can be used, for example, in discussing Bose–Einstein condensates in heterogeneous and highly nonlinear systems. We demonstrate that at low momenta linearized excitations of the phase of the condensate wavefunction obey a ( 3+1 )-dimensional d’Alembertian equation coupling to a ( 3+1 )-dimensional Lorentzian-signature ‘effective metric’ that is generic, and depends algebraically on the background field. Thus at low momenta this system serves as an analogue for the curved spacetime of general relativity. In contrast, at high momenta we demonstrate how one can use the eikonal approximation to extract a well controlled Bogoliubovlike dispersion relation, and (perhaps unexpectedly) recover non-relativistic Newtonian physics at high momenta. Bose–Einstein condensates appear to be an extremely promising analogue system for probing kinematic aspects of general relativity.

Multiplicity results for some nonlinear Schrodinger equations with potentials

Abstract: Multiple semiclassical standing waves for a class of nonlinear Schrodinger equations with potentials are found by means of a perturbative variational method.

Explaining the uneven distribution of numbers in nature: the laws of Benford and Zipf

Abstract: The distribution of first digits in numbers series obtained from very different origins shows a marked asymmetry in favor of small digits that goes under the name of Benford's law. We analyze in detail this property for different data sets and give a general explanation for the origin of the Benford's law in terms of multiplicative processes. We show that this law can be also generalized to series of numbers generated from more complex systems like the catalogs of seismic activity. Finally, we derive a relation between the generalized Benford's law and the popular Zipf's law which characterize the rank order statistics and has been extensively applied to many problems ranging from city population to linguistics.

Dark-matter spike at the galactic center?

Abstract: The past growth of the central black hole (BH) might have enhanced the density of cold dark matter halo particles at the galactic center. We compute this effect in realistic growth models of the present (2-3)×106M⊙ BH from a low-mass seed BH, with special attention to dynamical modeling in a realistic galaxy environment with merger and orbital decay of a seed BH formed generally outside the exact center of the halo. An intriguing “very-dense spike” of dark matter has been claimed in models of Gondolo and Silk with a density high enough to contradict with experimental upper bounds of neutralino annihilation radiation. This “spike” disappears completely or is greatly weakened when we include important dynamical processes neglected in their idealized or restrictive picture with cold particles surrounding an at-the-center zero-seed adiabatically growing BH. For the seed BH to spiral in and settle to the center within a Hubble time by dynamical friction, the seed mass must be at least a significant fraction of the present BH. Any subsequent at-the-center growth of the BH and steepening of the central Keplerian potential well can squeeze the halo density distribution only mildly, whether the squeezing happens adiabatically or instantaneously.

String tension and stability of magic tip-suspended nanowires.

Abstract: Multishell helical gold nanowires were recently imaged by electron microscopy. We show theoretically that the contact with the gold tips at either end of the wire plays a crucial role and that local minima in the string tension rather than the total wire free energy determine the nanowire stability. Density functional electronic structure calculations of the simplest and thinnest coaxial gold and silver wires of variable radius and chirality were carried out. We found a string tension minimum for a single-tube gold nanowire that is chiral and consists of seven strands, in striking agreement with observation. In contrast, no such minimum was found for silver, where the s-d competition leading to surface reconstruction is lacking.

Spectral gamma-ray signatures of cosmological dark matter annihilations.

Abstract: We propose a new signature for weakly interacting massive particle (WIMP) dark matter, a spectral feature in the diffuse extragalactic gamma-ray radiation. This feature, a sudden drop of the gamma-ray intensity at an energy corresponding to the WIMP mass, comes from the asymmetric distortion of the line due to WIMP annihilation into two gamma rays caused by the cosmological redshift. Unlike other proposed searches for a line signal, this method is not very sensitive to the exact dark matter density distribution in halos and subhalos.

The dividing line between fr I and fr II radio-galaxies

Abstract: In the radio { host galaxy optical luminosity plane FR I and FR II radio{galaxies are clearly divided. Since the optical luminosity of an elliptical galaxy is an indication of the mass of its central black hole, we propose that the FR I{FR II dividing luminosity is a function of the mass of the black hole powering the active nucleus. Furthermore, as the radio power gives an estimate of the total kinetic power carried by the jet, the FR I{FR II separation can be re{interpreted as occurring at a constant ratio between the jet power and the black hole mass. There is also convincing evidence of a correlation between the radio power and the luminosity in narrow emission lines. As the latter results from photoionization by the radiation produced by accretion, we can estimate the ionizing luminosity and nd that the separation luminosity can be also re{expressed as a constant accretion rate between10 2 {10 3 of the Eddington one. This possibly regulates the accretion mode and the consequent

Extreme synchrotron BL Lac objects - Stretching the blazar sequence

Abstract: We performed an observational program with the X{ray satellite BeppoSAX, to study objects with extreme synchrotron peak frequencies (peak > 1 keV). Of the seven sources observed, ve revealed or conrmed their extreme nature. Four showed peak frequencies in the range 1{5 keV, while one (1ES 1426+428) displayed a flat power law spectrum (x =0 :92 0:04) which locates its synchrotron peak at or above 100 keV. This is the third source of this type ever found, after Mkn 501 and 1ES 2344+514. In the context of the whole blazar class, the broad band properties of these objects conrm the scenario of a synchrotron peak smoothly spanning the IR { X{ray range, which explains the multi{frequency properties of the blazar class. Our data also conrm the large peak variability which seems to characterize this class of sources, compared with lower peak objects. Given the high synchrotron peak energies, which flag the presence of high energy electrons, these extreme BL Lacs are also good candidates for TeV emission, and therefore good probes of the IR background.

Dynamical and chemical evolution of gas-rich dwarf galaxies

Abstract: We study the effect of a single, instantaneous starburst on the dynamical and chemical evolution of a gas-rich dwarf galaxy, the potential well of which is dominated by a dark matter halo. We follow the dynamical and chemical evolution of the interstellar medium (ISM) by means of an improved two-dimensional hydrodynamical code coupled with detailed chemical yields originating from type II SNe, type Ia SNe and single low- and intermediate-mass stars (IMS). In particular we follow the evolution of the abundances of H, He, C, N, O, Mg, Si and Fe. We find that for a galaxy resembling IZw18, a galactic wind develops as a consequence of the starburst and it carries out of the galaxy mostly the metal-enriched gas. In addition, we find that different metals are lost differentially in the sense that the elements produced by type Ia SNe are lost more efficiently than others. As a consequence of that, we predict larger [α/Fe] ratios for the gas inside the galaxy than for the gas leaving the galaxy. A comparison of our predicted abundances of C, N, O and Si in the case of a burst occurring in a primordial gas shows a very good agreement with the observed abundances in IZw18 as long as the burst has an age of ∼31 Myr and IMS produce some primary nitrogen. However, we cannot exclude that a previous burst of star formation had occurred in IZw18, especially if the pre-enrichment produced by the older burst was lower than Z=0.01 Z⊙. Finally, at variance with previous studies, we find that most of the metals reside in the cold gas phase already after a few Myr. This result is mainly caused by the assumed low SN II heating efficiency, and justifies the generally adopted homogeneous and instantaneous mixing of gas in chemical evolution models.

Raman scattering intensities in α-quartz: A first-principles investigation

Abstract: Using a first-principles density functional approach, we calculate the first-order Raman intensities of alpha -quartz. The dynamical charge tensors, vibrational frequencies and eigenmodes, and polarizability tensors are obtained within a perturbational approach. We calculate Raman intensities by evaluating the variation of the polarizability tensors for finite displacements of the atoms. Calculated intensities agree well with experimental data, showing an average error of 13% for relative intensities. Using our first-principles results as reference, we critically examine simple models for the Raman activity. We first consider a bond polarizability model, for which the parameters are derived from our first-principles results for alpha -quartz. This model reproduces the first-principles intensities with an average error of 15%. In the attempt of reducing this error, we then introduce a model in which the symmetry of the first neighbor shell is accounted for in the most general way. For alpha -quartz, this model extends the bond polarizability model, which is recovered as a special case. The model, which fully accounts for the local symmetry, describes the first-principles results within an average error of 12%, marginally improving upon the bond polarizability model (15%). However, when these models with parameters derived for alpha -quartz are applied to a cristobalite polymorph, only the bond polarizability model shows good transferability properties. These results support the use of the bond polarizability model as a simple scheme for calculating Raman intensities in tetrahedrally bonded SiO2 systems.

Resonating Valence Bond Wave Functions for Strongly Frustrated Spin Systems

Abstract: The resonating-valence-bond (RVB) theory for two-dimensional quantum antiferromagnets is shown to be the correct paradigm for large enough ``quantum frustration.'' This scenario, proposed a long time ago but never confirmed by microscopic calculations, is strongly supported by a new type of variational wave function, which is extremely close to the exact ground state of the ${J}_{1}{\ensuremath{-}J}_{2}$ Heisenberg model for $0.4\ensuremath{\lesssim}{J}_{2}{/J}_{1}\ensuremath{\lesssim}0.5$. This wave function is proposed to represent the generic spin-half RVB ground state in spin liquids.

The dividing line between FR I and FR II radio--galaxies

Abstract: In the radio - host galaxy optical luminosity plane FR I and FR II radio-galaxies are clearly divided. Since the optical luminosity of an elliptical galaxy is an indication of the mass of its central black hole, we propose that the FR I - FR II dividing luminosity is a function of the mass of the black hole powering the active nucleus. Furthermore, as the radio power gives an estimate of the total kinetic power carried by the jet, the FR I - FR II separation can be re-interpreted as occurring at a constant ratio between the jet power and the black hole mass. There is also convincing evidence of a correlation between the radio power and the luminosity in narrow emission lines. As the latter results from photoionization by the radiation produced by accretion, we can estimate the ionizing luminosity and find that the separation luminosity can be also re-expressed as a constant accretion rate between \~0.01-0.001 of the Eddington one. This possibly regulates the accretion mode and the consequent presence and characteristics of nuclear outflows.

PINOCCHIO: pinpointing orbit-crossing collapsed hierarchical objects in a linear density field

Abstract: PINOCCHIO (PINpointing Orbit-Crossing Collapsed Hierarchical Objects) is a new algorithm for identifying dark matter halos in a given numerical realisation of the linear density field in a hierarchical universe (Monaco et al 2001) It is shown that Lagrangian perturbation theory, and in particular its ellipsoidal truncation, is able to predict accurately the collapse, in the orbit-crossing sense, of generic mass elements Some points that have undergone orbit crossing are assigned to the network of filaments and sheets that connects the halos; it is demonstrated that this network resembles closely that found in N-body simulations The code generates a catalogue of dark matter halos with known mass, position, velocity, merging history and angular momentum It is shown that the predictions of the code are very accurate when compared with the results of large N-body simulations that cover a range of cosmological models, box sizes and numerical resolutions The mass function is recovered with an accuracy of better than 10 per cent in number density for halos with at least 30-50 particles A similar accuracy is reached in the estimate of the correlation length r_0 The good agreement is still valid on the object-by-object level, with 70-100 per cent of the objects with more than 50 particles in the simulations also identified by our algorithm For these objects the masses are recovered with an error of 20-40 per cent, and positions and velocities with a root mean square error of ~1-2 Mpc (05-2 grid lengths) and ~100 km/s, respectively The recovery of the angular momentum of halos is considerably noisier and accuracy at the statistical level is achieved only by introducing free parameters The algorithm requires negligible computer time as compared with performing a numerical $N$-body simulation

Stylized facts of financial markets and market crashes in Minority Games

Abstract: We present and study a Minority Game based model of a financial market where adaptive agents—the speculators—interact with deterministic agents—called producers. Speculators trade only if they detect predictable patterns which grant them a positive gain. Indeed the average number of active speculators grows with the amount of information that producers inject into the market. Transitions between equilibrium and out of equilibrium behavior are observed when the relative number of speculators to the complexity of information or to the number of producers are changed. When the system is out of equilibrium, stylized facts arise, such as fat tailed distribution of returns and volatility clustering. Without speculators, the price follows a random walk; this implies that stylized facts arise because of the presence of speculators. Furthermore, if speculators abandon price taking behavior, stylized facts disappear.

The clustering evolution of the galaxy distribution.

Abstract: We follow the evolution of the galaxy population in a ΛCDM cosmology by means of high-resolution N-body simulations in which the formation of galaxies and their observable properties are calculated using a semi-analytic model. We display images of the spatial distribution of galaxies in the simulations that illustrate its evolution and provide a qualitative understanding of the processes responsible for the various biases that develop. We consider three specific statistical measures of clustering at Graphic and Graphic: the correlation length (in both real and redshift space) of galaxies of different luminosity, the morphology–density relation and the genus curve of the topology of galaxy isodensity surfaces. For galaxies with luminosity below L∗, the Graphic correlation length depends very little on the luminosity of the sample, but for brighter galaxies it increases very rapidly, reaching values in excess of 10-h−1Mpc. The ‘accelerated’ dynamical evolution experienced by galaxies in rich clusters, which is partly responsible for this effect, also results in a strong morphology–density relation. Remarkably, this relation is already well-established at Graphic. The genus curves of the galaxies are significantly different from the genus curves of the dark matter, however this is not a result of genuine topological differences but rather of the sparse sampling of the density field provided by galaxies. The predictions of our model at Graphic will be tested by forthcoming data from the 2dF and Sloan galaxy surveys, and those at Graphic by the DEEP and VIRMOS surveys.

Dynamical scaling of the DNA unzipping transition.

Abstract: We report studies of the dynamics of a set of exactly solvable lattice models for the force-induced DNA unzipping transition. Besides yielding the whole equilibrium phase diagram, which reveals a reentrance, these models enable us to characterize the dynamics of the process starting from a nonequilibrium initial condition. The thermal melting of DNA displays a model dependent time evolution. On the contrary, the dynamical mechanism for the unzipping by force is very robust and the scaling behavior is independent of the details of the description and, hence, superuniversal.