Showing papers in "Advances in Physics in 2006"

Statistical models of fracture

Abstract: Disorder and long-range interactions are two of the key components that make material failure an interesting playfield for the application of statistical mechanics. The cornerstone in this respect has been lattice models of the fracture in which a network of elastic beams, bonds, or electrical fuses with random failure thresholds are subject to an increasing external load. These models describe on a qualitative level the failure processes of real, brittle, or quasi-brittle materials. This has been particularly important in solving the classical engineering problems of material strength: the size dependence of maximum stress and its sample-to-sample statistical fluctuations. At the same time, lattice models pose many new fundamental questions in statistical physics, such as the relation between fracture and phase transitions. Experimental results point out to the existence of an intriguing crackling noise in the acoustic emission and of self-affine fractals in the crack surface morphology. Recent advances ...

Wet granular materials

Abstract: Most studies on granular physics have focused on dry granular media, with no liquids between the grains. However, in geology and many real world applications (e.g. food processing, pharmaceuticals, ceramics, civil engineering, construction, and many industrial applications), liquid is present between the grains. This produces inter-grain cohesion and drastically modifies the mechanical properties of the granular media (e.g. the surface angle can be larger than 90 degrees). Here we present a review of the mechanical properties of wet granular media, with particular emphasis on the effect of cohesion. We also list several open problems that might motivate future studies in this exciting but mostly unexplored field.

Solid friction from stick–slip down to pinning and aging

Abstract: We review the present state of understanding of solid friction at low velocities and for systems with negligibly small wear effects. We first analyze in detail the behavior of friction at interfaces between macroscopic hard rough solids, whose main dynamical features are well described by the Rice–Ruina rate and state-dependent constitutive law. We show that it results from two combined effects: (i) the threshold rheology of nanometer-thick junctions jammed under confinement into a soft glassy structure and (ii) the geometric aging, i.e. slow growth of the real area of contact via asperity creep interrupted by sliding. Closer analysis leads to identifying a second aging-rejuvenation process, at work within the junctions themselves. We compare the effects of structural aging at such multicontact, very highly confined, interfaces with those met under different confinement levels, namely boundary lubricated contacts and extended adhesive interfaces involving soft materials (hydrogels, elastomers). This leads...

Quantum graphs: Applications to quantum chaos and universal spectral statistics

Abstract: During the last few years quantum graphs have become a paradigm of quantum chaos with applications from spectral statistics to chaotic scattering and wavefunction statistics. In the first part of this review we give a detailed introduction to the spectral theory of quantum graphs and discuss exact trace formulae for the spectrum and the quantum-to-classical correspondence. The second part of this review is devoted to the spectral statistics of quantum graphs as an application to quantum chaos. In particular, we summarize recent developments on the spectral statistics of generic large quantum graphs based on two approaches: the periodic-orbit approach and the supersymmetry approach. The latter provides a condition and a proof for universal spectral statistics as predicted by random-matrix theory.

Scale invariance in plastic flow of crystalline solids

Abstract: From the traditional viewpoint of continuum plasticity, plastic deformation of crystalline solids is, at least in the absence of so-called plastic instabilities, envisaged as a smooth and quasi-lam...

New trends in density matrix renormalization

Abstract: The density matrix renormalization group (DMRG) has become a powerful numerical method that can be applied to low-dimensional strongly correlated fermionic and bosonic systems. It allows for a very precise calculation of static, dynamic and thermodynamic properties. Its field of applicability has now extended beyond condensed matter, and is successfully used in quantum chemistry, statistical mechanics, quantum information theory, and nuclear and high-energy physics as well. In this article, we briefly review the main aspects of the method and present some of the most relevant applications so as to give an overview of the scope and possibilities of DMRG. We focus on the most important extensions of the method such as the calculation of dynamical properties, the application to classical systems, finite-temperature simulations, phonons and disorder, field theory, time-dependent properties and the ab initio calculation of electronic states in molecules. The recent quantum information interpretation, the development of highly accurate time-dependent algorithms and the possibility of using the DMRG as the impurity-solver of the dynamical mean field method (DMFT) give new insights into its present and potential uses. We review the numerous very recent applications of these techniques where the DMRG has shown to be one of the most reliable and versatile methods in modern computational physics.

Rheology of giant micelles

Abstract: Giant micelles are elongated, polymer-like objects created by the self-assembly of amphiphilic molecules (such as detergents) in solution. Giant micelles are typically flexible, and can become highly entangled even at modest concentrations. The resulting viscoelastic solutions show fascinating flow behaviour (rheology) which we address theoretically in this article at two levels. First, we summarize advances in understanding linear viscoelastic spectra and steady-state nonlinear flows, based on microscopic constitutive models that combine the physics of polymer entanglement with the reversible kinetics of self-assembly. Such models were first introduced two decades ago, and since then have been shown to explain robustly several distinctive features of the rheology in the strongly entangled regime, including extreme shear thinning. We then turn to more complex rheological phenomena, particularly involving spatial heterogeneity, spontaneous oscillation, instability and chaos. Recent understanding of these complex flows is based largely on grossly simplified models which capture in outline just a few pertinent microscopic features, such as coupling between stresses and other order parameters such as concentration. The role of 'structural memory' (the dependence of structural parameters such as the micellar length distribution on the flow history) in explaining these highly nonlinear phenomena is addressed. Structural memory also plays an intriguing role in the little-understood shear thickening regime, which occurs in a concentration regime close to but below the onset of strong entanglement, and which is marked by a shear-induced transformation from an inviscid to a gelatinous state.

The effect of collective spin-1 excitations on electronic spectra in high- Tc superconductors

Abstract: We review recent experimental and theoretical results on the interaction between single-particle excitations and collective spin excitations in the superconducting state of high-Tc cuprates. We con...

Statistical models of brittle fragmentation

Abstract: Recent developments in statistical models for fragmentation of brittle material are reviewed. The generic objective of these models is understanding the origin of the fragment size distributions (FSDs) that result from fracturing brittle material. Brittle fragmentation can be divided into two categories: (1) Instantaneous fragmentation for which breakup generations are not distinguishable and (2) continuous fragmentation for which generations of chronological fragment breakups can be identified. This categorization becomes obvious in mining industry applications where instantaneous fragmentation refers to blasting of rock and continuous fragmentation to the consequent crushing and grinding of the blasted rock fragments. A model of unstable cracks and crack-branch merging contains both of the FSDs usually related to instantaneous fragmentation: the scale invariant FSD with the power exponent (2−1/D) and the double exponential FSD which relates to Poisson process fragmentation. The FSDs commonly related to ...

Superconductivity and magnetism and their interplay in quaternary borocarbides RNi2B2C

Abstract: Since 1986, most of the interest in superconductivity became focused on high-T c cuprates. The discovery of the superconducting quaternary borocarbide system Y–Ni–B–C with T c as high as ∼12 K inspired research into intermetallic superconductors (IMS) once again. Several reasons can be attributed to this revival of interest in IMS: (i) In the tetragonal quaternary magnetic superconductors RNi2B2C, superconductivity and magnetism occur with T c and T N ∼ 10 K, thereby allowing studies of exotic phenomena associated with, and arising from, the interplay of superconductivity and magnetism. (ii) High T N's and a variety of commensurate and incommensurate magnetic structures in RNi2B2C (Fermi surface nesting playing a central role) strongly suggest that R-spins are coupled via the RKKY-exchange interaction. Hence, unlike in most other magnetic superconductors known so far, conduction electrons take part in superconductivity and magnetism. (iii) Quaternary borocarbides open up new pathways to try and synthesize...

Misfit dislocations in nanocomposites with quantum dots, nanowires and their ensembles

Abstract: We review theoretical concepts and experimental results on the physics of misfit dislocations in nanocomposite solids with quantum dots (QDs) and nanowires (quantum wires). Special attention is paid to thermodynamic theoretical models of formation of misfit dislocations in QDs and nanowires, including composite core–shell nanowires. The effects of misfit dislocations on the film growth mode during heteroepitaxy and phase transitions in QD systems are analysed. Experimental results and theoretical models of the ordered spatial arrangement of QDs growing on composite substrates with misfit dislocation networks are discussed. The influence of subsurface dislocations in composite substrates on the nucleation of QDs and nanowires on the substrate surface is considered. Models of misfit strain relaxation and dislocation formation in nanofilms on compliant substrates are also reviewed.