arXiv subjects

E. Tosatti

Publications and source records attributed to E. Tosatti.

At least 73 records · Page 4Linked to original sources

arXiv2002

Strain effects at solid surfaces near the melting point

We investigate the effects of strain on a crystal surface close to the bulk melting temperature T_m, where surface melting usually sets in. Strain lowers the bulk melting point, so that at a fixed temperature below but close to T_m the thickness of the quasi-liquid film is expected to grow with strain, irrespective of sign. In addition, a strain-induced solid surface free energy increase/decrease takes place, favoring/disfavoring surface melting depending on the sign of strain relative to surface stress. In the latter case one can produce a strain-induced prewetting transition, where for increasing temperature the liquid film suddenly jumps from zero to a finite thickness. This phenomenology is illustrated by a realistic molecular dynamics simulation of strained Al(110).

cond-mat.mtrl-sci↗

arXiv2002

Phonons Softening in Tip-Stretched Monatomic Nanowires

It has been shown in recent experiments that electronic transport through a gold monatomic nanowire is dissipative above a threshold voltage due to excitation of phonons via the electron-phonon interaction. We address that data by computing, via density functional theory, the zone boundary longitudinal phonon frequency of a perfect monatomic nanowire during its mechanical elongation. The theoretical frequency that we find for an ideally strained nanowire is not compatible with experiment if a uniformly distributed stretch is assumed. With the help of a semi-empirical Au-Au potential, we model the realistic nanowire stretching as exerted by two tips. In this model we see that strain tends to concentrate in the junctions, so that the mean strain of the nanowire is roughly one half of the ideal value. With this reduced strain, the calculated phonon softening is in much better agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2002

Bending strain-driven modification of surface reconstructions: Au(111)

Strain can affect the morphology of a crystal surface, and cause modifications of its reconstruction even when weak, as in the case of mechanical bending. We carried out calculations of strain-dependent surface free energy and direct bending simulations demonstrating the change of incommensurate reconstruction in Au(111) under strain, in good agreement with recent data. Time-dependent strain should cause a sliding of the topmost layer over the second, suggesting an interesting case of nanofriction. Bending strain could also be used to fine tune the spacing of selectively absorbed nanoclusters.

cond-mat.mtrl-sci↗

arXiv2002

Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces

Distortions of the $\sqrt3\times\sqrt3$ Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, $Q$, related to the surface band occupancy. A novel understanding of the $(3\times3)$-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distorsions. Negative strain attracts pseudocharge from the valence band causing first a $(3\times3)$-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a $(\sqrt3\times\sqrt3)$-3U (``all up'') state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.

cond-mat.mtrl-sci↗

arXiv2002

Strongly Correlated Superconductivity

High temperature superconductivity in doped Mott insulators such as the cuprates contradicts the conventional wisdom that electron repulsion is detrimental to superconductivity. Because doped fullerene conductors are also strongly correlated, the recent discovery of high-critical-temperature, presumably s-wave, superconductivity in C$_{60}$ field effect devices is even more puzzling. We examine a dynamical-mean-field solution of a model for electron doped fullerenes which shows how strong correlations can indeed enhance superconductivity close to the Mott transition. We argue that the mechanism responsible for this enhancement could be common to a wider class of strongly correlated models, including those for cuprate superconductors.

cond-mat.str-el↗

arXiv2002

Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals

Based on an accurate first principles description of the energetics in H-bonded KDP, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Cluster tunneling involving also heavy ions is allowed, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples selfconsistently with the proton off-centering, thus explaining both the giant isotope effect, and its close connection with geometrical effects.

cond-mat.mtrl-sci↗

arXiv2001

Selective d-state Conduction Blocking in Nickel Nanocontacts

The lowest conductance step for a Ni nanocontact is anomalously small in comparison with the large expected number of conducting channels. We present electronic structure calculations for an extremely idealized Ni nanobridge consisting of just a monatomic nanowire. Our calculations show that no less than eight single spin bands cross the Fermi level in a nonmagnetic Ni monatomic wire, dropping marginally to seven in the more stable, fully ferromagnetic state. However, when we build in the wire a magnetization reversal, or domain wall, by forcing the net magnetization to be zero, we suddenly find that d electrons selectively cease to propagate across the wall. s electron propagation remains, and can account for the small observed conductance steps.

cond-mat.mtrl-sci↗

arXiv2001

Superconductivity Near Ferromagnetism in MgCNi3

An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs) lies very near the Fermi energy in MgCNi3, recently reported to superconduct at 8.5 K. This compound is strongly exchange enhanced and is unstable to ferromagnetism upon hole doping with 12% Mg --> Na or Li. The 1/4-depleted fcc (frustrated) Ni sublattice and lack of Fermi surface nesting argues against competing antiferromagnetic and charge density wave instabilities. We identify an essentially infinite mass along the M-Gamma line, leading to quasi-two-dimensionality of this vHs may promote unconventional p-wave pairing that could coexist with superconductivity.

cond-mat.supr-con↗

arXiv2001

Direct Transition Between a Singlet Mott Insulator and a Superconductor

We argue that a normal Fermi liquid and a singlet, spin gapped Mott insulator cannot be continuously connected, and that some intermediate phase must intrude between them. By explicitly working out a case study where the singlet insulator is stabilized by orbital degeneracy and an inverted Hund's rule coupling, mimicking a Jahn-Teller effect, we find that the intermediate phase is a superconductor.

cond-mat.str-el↗

arXiv2001

The effect of surface roughness on the adhesion of elastic solids

We study the influence of surface roughness on the adhesion of elastic solids. Most real surfaces have roughness on many different length scales, and this fact is taken into account in our analysis. We consider in detail the case when the surface roughness can be described as a self affine fractal, and show that when the fractal dimension D_f >2.5, the adhesion force may vanish, or be at least strongly reduced. We consider the block-substrate pull-off force as a function of roughness, and find a partial detachment transition preceding a full detachment one. The theory is in good qualitative agreement with experimental data.

cond-mat.mtrl-sci↗

arXiv2001

Two-Stage Rotational Disordering of a Molecular Crystal Surface: C60

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C$_{60}$ fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low temperature ordered $(2 \times 2)$ state, and a high temperature $(1 \times 1)$ disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.

cond-mat↗

arXiv2000

Bent surface free energy differences from simulation

We present a calculation of the change of free energy of a solid surface upon bending of the solid. It is based on extracting the surface stress through a molecular dynamics simulation of a bent slab by using a generalized stress theorem formula, and subsequent integration of the stress with respect to strain as a function of bending curvature. The method is exemplified by obtaining and comparing free energy changes with curvature of various reconstructed Au(001) surfaces.

cond-mat.mtrl-sci↗

arXiv2000

Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem

Due to particle conservation, Canonical Molecular Dynamics (MD) simulations fail in the description of surface phase transitions involving coverage or lateral density changes. However, a step on the surface can act effectively as a source or a sink of atoms, in the simulation as well as in real life. A single surface step can be introduced by suitably modifying planar Periodic Boundary Conditions (PBC), to accommodate the generally inequivalent stacking of two adjacent layers. We discuss here how, through the introduction of two orthogonal surface steps, particle number conservation may no longer represent a fatal constraint for the study of these surface transitions. As an example, we apply the method for estimating temperature-induced lateral density increase of the reconstructed Au (001) surface; the resulting anisotropic cell change is consistent with experimental observations. Moreover, we implement this kind of scheme in conjunction with the variable curvature MD method, recently introduced by our group.

cond-mat.mtrl-sci↗

arXiv2000

Spectroscopic fingerprints of a surface Mott-Hubbard insulator: the case of SiC(0001)

We discuss the spectroscopic fingerprints that a surface Mott-Hubbard insulator should show at the intra-atomic level. The test case considered is that of the Si-terminated SiC(0001) sqrt{3}xsqrt{3} surface, which is known experimentally to be insulating. We argue that, due to the Mott-Hubbard phenomenon, spin unpaired electrons in the Si adatom dangling bonds are expected to give rise to a Si-2p core level spectrum with a characteristic three-peaked structure, as seen experimentally. This structure results from the joint effect of intra-atomic exchange, spatial anisotropy, and spin-orbit coupling. Auger intensities are also discussed.

cond-mat.str-el↗

arXiv2000

Electronic Properties of Ultra-Thin Aluminum Nanowires

We have carried out first principles electronic structure and total energy calculations for a series of ultrathin aluminum nanowires, based on structures obtained by relaxing the model wires of Gulseren et al. The number of conducting channels is followed as the wires radius is increased. The results suggest that pentagonal wires should be detectable, as the only ones who can yield a channel number between 8 and 10.

cond-mat↗

arXiv2000

(Meta-)stable reconstructions of the diamond(111) surface: interplay between diamond- and graphite-like bonding

Off-lattice Grand Canonical Monte Carlo simulations of the clean diamond (111) surface, based on the effective many-body Brenner potential, yield the $(2\times1)$ Pandey reconstruction in agreement with \emph{ab-initio} calculations and predict the existence of new meta-stable states, very near in energy, with all surface atoms in three-fold graphite-like bonding. We believe that the long-standing debate on the structural and electronic properties of this surface could be solved by considering this type of carbon-specific configurations.

cond-mat.mtrl-sci↗

arXiv2000

Realistic simulations of Au(100): Grand Canonical Monte Carlo and Molecular Dynamics

The large surface density changes associated with the (100) noble metals surface hex-reconstruction suggest the use of non-particle conserving simulation methods. We present an example of a surface Grand Canonical Monte Carlo applied to the transformation of a square non reconstructed surface to the hexagonally covered low temperature stable Au(100). On the other hand, classical Molecular Dynamics allows to investigate microscopic details of the reconstruction dynamics, and we show, as an example, retraction of a step and its interplay with the surface reconstruction/deconstruction mechanism.

cond-mat.mtrl-sci↗

arXiv2000

Islands, craters, and a moving surface step on a hexagonally reconstructed (100) noble metal surface

Deposition/removal of metal atoms on the hex reconstructed (100) surface of Au, Pt and Ir should present intriguing aspects, since a new island implies hex -> square deconstruction of the substrate, and a new crater the square -> hex reconstruction of the uncovered layer. To obtain a microscopic understanding of how islands/craters form in these conditions, we have conducted simulations of island and crater growth on Au(100), whose atomistic behavior, including the hex reconstruction on top of the square substrate, is well described by mean s of classical many-body forces. By increasing/decreasing the Au coverage on Au(100), we find that island/craters will not grow unless they exceed a critical size of about 8-10 atoms. This value is close to that which explains the nonlinear coverage dependence observed in molecular adsorption on the closely related surface Pt (100). This threshold size is rationalized in terms of a transverse step correlation length, measuring the spatial extent where reconstruction of a given plane is disturbed by the nearby step.

cond-mat.mtrl-sci↗