arXiv subjects

Erio Tosatti

Publications and source records attributed to Erio Tosatti.

159 records · Page 9Linked to original sources

arXiv1998

Non-Crystalline Structures of Ultra-Thin 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.

cond-mat.mtrl-sci↗

arXiv1997

Realistic grand canonical Monte Carlo surface simulation: application to Ar(111)

Most realistic, off-lattice surface simulations are done canonically--- conserving particles. For some applications, however, such as studying the thermal behavior of rare gas solid surfaces, these constitute bad working conditions. Surface layer occupancies are believed to change with temperature, particularly at preroughening, and naturally call for a grand canonical approach, where particle number is controlled by a chemical potential. We report preliminary results of novel realistic grand canonical Monte Carlo simulations of the Lennard-Jones (LJ) fcc(111) surface, believed to represent a quantitative model of e.g. Ar(111). The results are successful and highly informative for temperatures up to roughly 0.8 T_m, where clear precursor signals of preroughening are found. At higher temperatures, convergence to equilibrium is hampered by large fluctuations.

cond-mat.mtrl-sci↗

arXiv1997

Can liquid metal surfaces have hexatic order?

We propose that extended orientational correlations can appear at the surface of supercooled heavy noble liquid metals, due to the same compressive forces that cause reconstruction of their crystal surfaces. Simulations for liquid Au show a packed surface layer structurally akin to a defected 2D triangular solid. Upon supercooling, the density of unbound disclinations decreases as expected in Nelson-Halperin's theory. It extrapolates to a hexatic transition about 350 K below melting, where a sharp growth of orientational correlation length and time is also found. True hexatic order is preempted by surface-initiated recrystallisation.

cond-mat.mtrl-sci↗

arXiv1997

Solid molecular hydrogen: The Broken Symmetry Phase

By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure, of $Pca2_1$ symmetry, for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150 GPa. We present results for the equation of state, lattice parameters and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H2 and D2. We assign the observed modes to specific symmetry representations.

cond-mat.mtrl-sci↗

arXiv1997

Non magnetic molecular Jahn-Teller Mott insulators

Narrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a non-magnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of non-magnetic insulator in a very simple $E\otimes e$ Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K_{4}C_{60} and Rb_{4}C_{60}.

cond-mat.str-el↗

arXiv1996

Surprises in the Orbital Magnetic Moment and g-Factor of the Dynamic Jahn-Teller Ion C_{60}^-

We calculate the magnetic susceptibility and g-factor of the isolated C_{60}^- ion at zero temperature, with a proper treatment of the dynamical Jahn-Teller effect, and of the associated orbital angular momentum, Ham-reduced gyromagnetic ratio, and molecular spin-orbit coupling. A number of surprises emerge. First, the predicted molecular spin-orbit splitting is two orders of magnitude smaller than in the bare carbon atom, due to the large radius of curvature of the molecule. Second, this reduced spin-orbit splitting is comparable to Zeeman energies, for instance, in X-band EPR at 3.39KGauss, and a field dependence of the g-factor is predicted. Third, the orbital gyromagnetic factor is strongly reduced by vibron coupling, and so therefore are the effective weak-field g-factors of all low-lying states. In particular, the ground-state doublet of C_{60}^- is predicted to show a negative g-factor of \sim -0.1.

cond-mat↗

arXiv1995

Role of Layering Oscillations at Liquid Metal Surfaces in Bulk Recrystallization and Surface Melting

The contrasting melting behavior of different surface orientations in metals can be explained in terms of a repulsive or attractive effective interaction between the solid-liquid and the liquid-vapor interface. We show how a crucial part of this interaction originates from the layering effects near the liquid metal surface. Its sign depends on the relative tuning of layering oscillations to the crystal interplanar spacing, thus explaining the orientational dependence. Molecular dynamics recrystallization simulations of Au surfaces provide direct and quantitative evidence of this phenomenon.

cond-mat↗

arXiv1995

Phase Diagram of a Model of Correlated Hopping of Electrons in a Lattice of Berry Molecules

The $1D$ phase diagram of a model for correlated hopping of electrons in a lattice of Berry phase molecules is presented. Electrons hop in presence of an extra orbital degree of freedom at each site. This is mimicked as a spin-1 variable whose allowed states depend on the electron occupancy so as to take into account the orbital degeneracies of different molecular occupancies. In the $1D$ case we find that at low electron densities $n<<1$ there is a region with dominant superconducting correlations surviving an additional repulsive on-site interaction $U$ as strong as the bandwidth, $W=4t$. The critical value $U_c$ of $U$ below which superconductivity is found to be dominant decreases with increasing density $n$. For $n=1/2$ we find $U_c/t\approx 1$, whereas at $n=1$ (half-filling) our (less accurate) results are compatible with $U_c/t\approx 0$. For $U>U_c(n)$ and away from half-filling ($n\neq 1$) the system is metallic with dominant $2k_F$ charge density wave (CDW) correlations. At half-filling a charge gap opens for $U>U_c$ and the system becomes an insulator. A spin-gap characterizes the phase-diagram for all densities and for all values of $U$, even in the metallic regime $U>U_c$.

cond-mat↗

arXiv1995

Enhanced electron-phonon coupling at the Mo and W(110) surfaces induced by adsorbed hydrogen

The possible occurrence of either a charge-density-wave or a Kohn anomaly is governed by the presence of Fermi-surface nesting and the subtle interaction of electrons and phonons. Recent experimental and theoretical investigations suggest such an effect for the hydrogen covered Mo and W(110) surfaces. Using density-functional theory we examine the electronic structure and the electron-phonon coupling of these systems. Besides good agreement with the experimental phonon frequencies our study provides a characterization and quantitative analysis of an unusual scenario determining the electronic, vibrational, and structural properties of these surfaces.

mtrl-th↗

arXiv1995

Interplay of Orbital Degeneracy and Superconductivity in a Molecular Conductor

We study electron propagation in a molecular lattice model. Each molecular site involves doubly degenerate electronic states coupled to doubly degenerate molecular vibration, leading to a so--called E-e type of Jahn-Teller Hamiltonian. For weak electron-phonon coupling and in the anti-adiabatic limit we find that the orbital degeneracy induces an intersite pairing mechanism which is absent in the standard non-degenerate polaronic model. In this limit we analyse the model in the presence of an additional on-site repulsion and we determine, within BCS mean field theory, the region of stability of superconductivity. In one dimension, where powerful analytical techniques are available, we are able to calculate the phase diagram of the model both for weak and for strong electron-phonon coupling.

cond-mat↗

arXiv1995

Low lying excitations of the Dynamical Jahn-Teller ions C_60^- and C_60^2-

We compute the vibronic spectrum of gas phase fullerene negative ions C_{60}^{-} and C_{60}^{2-}. We treat accurately the linear dynamical Jahn-Teller of these ions in their ground state. In particular, coupling of the t_{1u} orbital to the eight H_g vibrational modes is handled by exact diagonalizations including up to six vibrons. The resulting spectrum is characterized by large splittings, which should be readily observable spectroscopically. The lowest excitation symmetry is argued to be related to the Berry phase present in this problem.

cond-mat↗

arXiv1994

Maximum Overheating and Partial Wetting of Nonmelting Solid Surfaces

Surfaces which do not exhibit surface melting below the melting point (nonmelting surfaces) have been recently observed to sustain a very large amount of overheating. We present a theory which identifies a maximum overheating temperature, and relates it to other thermodynamical properties of the surface, in particular to geometrical properties more readily accessible to experiment. These are the angle of partial wetting, and the nmelting-induced faceting angle. We also present molecular dynamics simulations of a liquid droplet deposited on Al(111), showing lack of spreading and partial wetting in good agreement with the theory. PACS numbers: 68.10.Cr, 68.45.Gd, 61.50.Jr

cond-mat↗

arXiv1994

Anomalous low-energy electron attachment in C_{60}

We propose that thermal electron attachment to C$_{60}$ should occur preferentially in the p-wave channel, following an analysis of the vibron excitation spectrum of C$_{60}^-$. A very simple model based on this idea is shown to account very well for recent attachment data. The unexplained activation energy of $\approx$ 0.26 eV found experimentally is attributed to the p-wave centrifugal barrier.

cond-mat↗

arXiv1994

Electron--Vibron Interactions and Berry Phases in Charged Buckminsterfullerene: Part I

A simple model for electron-vibron interactions on charged buckminsterfullerene C$_{60}^{n-}$, $n=1,\ldots 5$, is solved both at weak and strong couplings. We consider a single $H_g$ vibrational multiplet interacting with $t_{1u}$ electrons. At strong coupling the semiclassical dynamical Jahn-Teller theory is valid. The Jahn-Teller distortions are unimodal for $n$=1,2,4,5 electrons, and bimodal for 3 electrons. The distortions are quantized as rigid body pseudo--rotators which are subject to geometrical Berry phases. These impose ground state degeneracies and dramatically change zero point energies. Exact diagonalization shows that the semiclassical level degeneracies and ordering survive well into the weak coupling regime. At weak coupling, we discover an enhancement factor of $5/2$ for the pair binding energies over their classical values. This has potentially important implications for superconductivity in fullerides, and demonstrates the shortcoming of Migdal--Eliashberg theory for molecular crystals.

cond-mat↗

arXiv1994

Electron--Vibron Interactions in Charged Buckminsterfullerene: Pair Energies and Spectra (Part II)

The ground state energy shifts and excitation spectra of charged buckminsterfullerene C$_{60}^{n-}$, $n=1,\ldots 5$ are calculated. The electron--vibron Hamiltonian of Part I is extended to include all $A_g$ and $H_g$ modes with experimentally determined frequencies and theoretically estimated coupling constants. Complex splitting patterns of $H_g$ vibrational levels are found. Our results are relevant to EPR measurements of spin splittings in C$_{60}^{2-}$ and C$_{60}^{3-}$ in solution. Spectroscopic gas-phase experiments will be of interest for further testing of this theory. As found in Part I, degeneracies in the electron and vibron Hamiltonians give rise to a dynamical Jahn Teller effect, and to a considerable enhancement of the electronic pairing interaction. This helps to overcome repulsive Coulomb interactions and has important implications for superconductivity in K$_3$C$_{60}$ and the insulating state in K$_4$C$_{60}$.

cond-mat↗