Showing papers by "Luciano Pietronero published in 2010"

The complex universe: recent observations and theoretical challenges

Abstract: The large-scale distribution of galaxies in the universe displays a complex pattern of clusters, super-clusters, filaments and voids with sizes limited only by the boundaries of the available samples. A quantitative statistical characterization of these structures shows that galaxy distribution is inhomogeneous in these samples, being characterized by large amplitude fluctuations of large spatial extension. Over a large range of scales, both the average conditional density and its variance show a non-trivial scaling behavior: at small scales, r < 20 Mpc/h, the average (conditional) density scales as r − 1. At larger scales, the density depends only weakly (logarithmically) on the system size and density fluctuations follow the Gumbel distribution of extreme value statistics. These complex behaviors are different from what is expected in a homogeneous distribution with Gaussian fluctuations. The observed density inhomogeneities pose a fundamental challenge to the standard picture of cosmology but they also represent an important opportunity which points to new directions with respect to many cosmological puzzles. Indeed, the fact that matter distribution is not uniform, in the limited range of scales sampled by observations, raises the question of understanding how inhomogeneities affect the large-scale dynamics of the universe. We discuss several attempts which try to model inhomogeneities in cosmology, considering their effects with respect to the role and abundance of dark energy and dark matter.

The complex universe: recent observations and theoretical challenges

Abstract: The large scale distribution of galaxies in the universe displays a complex pattern of clusters, super-clusters, filaments and voids with sizes limited only by the boundaries of the available samples. A quantitative statistical characterization of these structures shows that galaxy distribution is inhomogeneous in these samples, being characterized by large-amplitude fluctuations of large spatial extension. Over a large range of scales, both the average conditional density and its variance show a nontrivial scaling behavior: at small scales, r<20 Mpc/h, the average (conditional) density scales as 1/r. At larger scales, the density depends only weakly (logarithmically) on the system size and density fluctuations follow the Gumbel distribution of extreme value statistics. These complex behaviors are different from what is expected in a homogeneous distribution with Gaussian fluctuations. The observed density inhomogeneities pose a fundamental challenge to the standard picture of cosmology but it also represent an important opportunity which points to new directions with respect to many cosmological puzzles. Indeed, the fact that matter distribution is not uniform, in the limited range of scales sampled by observations, rises the question of understanding how inhomogeneities affect the large-scale dynamics of the universe. We discuss several attempts which try to model inhomogeneities in cosmology, considering their effects with respect to the role and abundance of dark energy and dark matter.

Fermi surface shrinking, band shifts and interband coupling in iron-based pnictides

Abstract: Band shifts are systematically observed in pnictides using de Hass-van Alphen and ARPES techniques. Most interesting, such shifts result to be band-sensitive, so that hole-like bands are shifted downwards while electron-like ones are shifted upwards. In this contribution we present a comprehensive explanation for the origin of such band shifts. Using a four band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V ≈ 0.46 eV which corresponds to a mass enhancement Z ≈ 1.5 .

Statistical physics: Physicists get social

Abstract: The community of statistical physicists meets every three years on a different continent at the series of STATPHYS conferences to define the state-of-the-art in the field and to outline its possible evolution.

Comment on Finite-Size Berezinskii-Kosterlitz- Thouless Transition at Grain Boundaries in Solid 4He and the Role of 3He Impurities . Authors' reply

Abstract: Gaudio et al. [1] seek to explain the strong rounding of the transition in the nonclassical rotational inertia (NCRI) observed in torsion oscillator experiments on solid He. They propose that this rounding is the result of a finite-size Berezinskii-Kosterlitz-Thouless (BKT) transition [2] with the superfluid component residing on grain boundaries in polycrystalline samples. A grain size of approximately 130 Å is required to reproduce the shape of the NCRI signal. This theory, however, is developed for disconnected grains and does not account for the three-dimensional connectivity of the system. Isolated grains of this size within the sample would make unobservably small contributions to the moment of inertia of the macroscopic torsion oscillator. The theory of superflow along grain boundaries in solid He is essentially equivalent to that of superfluid films on the surfaces of a porous material, and a correct understanding of both systems requires that the twodimensional superfluid surfaces are interconnected. A theory of the crossover from finite-size BKT behavior to the critical behavior of the three-dimensional XY model was developed in Ref. [3] and predicts a sharp transition rather than the rounded transition characteristic of a finite system. In order to distinguish between the theoretical pictures presented in Refs. [1,3], we have carried out simulations, using the worm algorithm, of the superfluid density of the XY model in three different geometries. The first geometry is an ‘ ‘ square lattice torus with ‘ 1⁄4 16, similar to the finite-size spherical geometry analyzed in [1]. The second geometry has XY spins on the sites of a simple cubic lattice and serves as a model for the sharp transition in bulk He. The third geometry is a diluted lattice of ‘ ‘ plaquettes of spins, again with ‘ 1⁄4 16. The plaquettes form the faces of a cubic lattice. On each plaquette, the spins are arranged on a square lattice and have four neighbors while spins along edges and at corners connecting plaquettes may have more neighbors. The lattice of plaquettes is randomly diluted so that only 30% of the possible plaquettes are present. Since the site percolation threshold for the 2D square lattice is 59%, only small planar regions are present although the system is three-dimensionally still connected. This plaquette geometry is an approximation to a disordered connected system of grain boundaries. For most simulations, we consider a system of 32 plaquettes. The results are shown in Fig. 1. The ‘ ‘ torus (d) displays a rounded behavior typical for all finite-size systems. The simple cubic lattice (m) displays the expected sharp 3D XY transition. The most interesting curve is for the diluted plaquette geometry ( ). For this geometry, we see a sharp transition, not at all like the behavior of a single isolated plaquette. In fact, this transition is slightly sharper than for the simple cubic lattice because of the initial twodimensional BKT behavior away from the transition temperature. We have also explored other plaquette sizes both with and without disorder and in no case do we see a rounded transition similar to the experimental signature. Our main conclusion disagrees with Ref. [1]. We find a sharp superfluid transition for a connected system of grain boundaries unlike the rounded transition predicted for the same system in Ref. [1]. The explanation of the strongly rounded NCRI signature seen in supersolid experiments remains a mystery. Support for this work was provided in part by NSF Grants No. DMR-0907235 and No. PHY-0653183.