Erio Tosatti

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion

Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the atomic force microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.

Simulation of gold in the glue model

Abstract Many well known difficulties associated with the use of two-body forces for the description of metallic systems may be overcome by using an expression for the total energy of the form V = ½Σ ij φ(r ij ) + Σ i U(ni), where n i = Σiρ(r ij ) is a generalized atomic coordination. The three functions φ(r), U(n) and ρ(r) are constructed empirically, by fitting several physical quantities including thermal and surface properties. This simple many-body force scheme can be used in molecular-dynamics simulations with few overheads compared with pair-wise systems. We present our realization for gold and summarize the results of recent structural and dynamical studies of Au surfaces.

Theory of Quantum Annealing of an Ising Spin Glass

Probing the lowest energy configuration of a complex system by quantum annealing was recently found to be more effective than its classical, thermal counterpart. By comparing classical and quantum Monte Carlo annealing protocols on the two-dimensional random Ising model (a prototype spin glass), we confirm the superiority of quantum annealing relative to classical annealing. We also propose a theory of quantum annealing based on a cascade of Landau-Zener tunneling events. For both classical and quantum annealing, the residual energy after annealing is inversely proportional to a power of the logarithm of the annealing time, but the quantum case has a larger power that makes it faster.

Noncrystalline Structures of Ultrathin Unsupported Nanowires

Computer simulations suggest that ultrathin metal wires should develop exotic, non-crystalline stable atomic structures, once their diameter decreases below a critical size of the order of a few atomic spacings. The new structures, whose details depend upon the material and the wire thickness, may be dominated by icosahedral packings. Helical, spiral-structured wires with multi-atom pitches are also predicted. The phenomenon, analogous to the appearance of icosahedral and other non-crystalline shapes in small clusters, can be rationalized in terms of surface energy anisotropy and optimal packing.

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

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 Benfords law. We analyze in detail this property for different data sets and give a general explanation for the origin of the Benfords 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 Benfords law and the popular Zipfs law which characterize the rank order statistics and has been extensively applied to many problems ranging from city population to linguistics.

Magnetic Tunnel Junctions with Ferroelectric Barriers: Prediction of Four Resistance States from First Principles

Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization alignment of the electrodes (tunneling magnetoresistance) and the polarization orientation in the ferroelectric barrier (tunneling electroresistance), these multiferroic tunnel junctions (MFTJs) may serve as four-state resistance devices. On the basis of first-principles calculations, we demonstrate four resistance states in SrRuO(3)/BaTiO(3)/SrRuO(3) MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal the exciting prospects of MFTJs for application as multifunctional spintronic devices.

Colloquium : Modeling friction: from nanoscale to mesoscale

The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling. This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems.

Indication for a novel phase in the quantum paraelectric regime of SrTiO3

Electron paramagnetic resonance of Fe3+ gives evidence for a phase-transition-like feature in SrTiO3 belowTq=37±1 K, both in the tetragonal and the pressure-induced trigonal phase. In the latter,Tq shifts to lower temperatures as a function of uniaxial stressp111. The new phenomenon is very tentatively discussed in terms of a possible transition to a novel coherent quantum state.

Premelting of thin wires

Recent work has raised considerable interest on the nature of thin metallic wires. We have investigated the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential. We find that---in analogy with cluster melting---the melting temperature

Melting and nonmelting of solid surfaces and nanosystems

T_m (R)