arXiv subjects

E. Tosatti

Publications and source records attributed to E. Tosatti.

At least 37 records · Page 2Linked to original sources

arXiv2012

Kondo effect of magnetic impurities on nanotubes

The effect of magnetic impurities on the ballistic conductance of nanocontacts is, as suggested in recent work, amenable to ab initio study \cite{naturemat}. Our method proceeds via a conventional density functional calculation of spin and symmetry dependent electron scattering phase shifts, followed by the subsequent numerical renormalization group solution of Anderson models -- whose ingredients and parameters are chosen so as to reproduce these phase shifts. We apply this method to investigate the Kondo zero bias anomalies that would be caused in the ballistic conductance of perfect metallic (4,4) and (8,8) single wall carbon nanotubes, ideally connected to leads at the two ends, by externally adsorbed Co and Fe adatoms. The different spin and electronic structure of these impurities are predicted to lead to a variety of Kondo temperatures, generally well below 10 K, and to interference between channels leading to Fano-like conductance minima at zero bias.

cond-mat.str-el↗

arXiv2012

Kondo impurities in nanotubes: the importance of being "in"

Transition metal impurities will yield zero bias anomalies in the conductance of well contacted metallic carbon nanotubes, but Kondo temperatures and geometry dependences have not been anticipated so far. Applying the density functional plus numerical renormalization group approach of Lucignano \textit{et al.} to Co and Fe impurities in (4,4) and (8,8) nanotubes, we discover a huge difference of behaviour between outside versus inside adsorption of the impurity. The predicted Kondo temperatures and zero bias anomalies, tiny outside the nanotube, turn large and strongly radius dependent inside, owing to a change of symmetry of the magnetic orbital. Observation of this Kondo effect should open the way to a host of future experiments.

cond-mat.str-el↗

arXiv2011

Irreducible finite-size effects in surface free energies from crystal-nucleation data

In this Letter we report a simulation study in which we compare the solid-liquid interfacial free energy of NaCl at coexistence, with the value that follows from the height of the homogeneous nucleation barrier. We find that the two estimates differ by more than 100%. Similar, although smaller discrepancies are found for crystals of hard-sphere colloids and of Lennard-Jones ("argon") particles. We consider a variety of possible causes for this discrepancy and are forced to conclude that it is due to a finite-size effect that cannot be corrected for by any simple thermodynamic procedure. Importantly, we find that the surface free energies that follow from real nucleation experiments should be subject to large finite size effects. Taking this in to account, we obtain quantitative agreement between the simulation data and the surface free energy of NaCl that follows from nucleation experiments. Our finding suggests that most published solid-liquid surface free energies derived from nucleation experiments will have to be revised.

cond-mat.mtrl-sci↗

arXiv2009

Connection Between Magnetism and Structure in Fe Double Chains on the Ir(100) Surface

The magnetic ground state of nanosized systems such as Fe double chains, recently shown to form in the early stages of Fe deposition on Ir(100), is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir(100) for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is generally antiferromagnetic (AFM), a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely a magnetocrystalline anisotropy, and a Dzyaloshinskii-Moriya (DM) spin-spin interaction, the former stabilizing the collinear AFM state, the second favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir(100) -- a state which we find to be substantially lower in energy than any reconstructed structure -- so that in this case the Fe double chain should remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed in a metastable state onto the (5x1) fully reconstructed Ir(100) surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure.

cond-mat.mtrl-sci↗

arXiv2009

Dynamical chiral symmetry breaking in sliding nanotubes

We discovered in simulations of sliding coaxial nanotubes an unanticipated example of dynamical symmetry breaking taking place at the nanoscale. While both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero angular momentum of phonon origin appears spontaneously at a series of critical sliding velocities, in correspondence with large peaks of the sliding friction. The non-linear equations governing this phenomenon resemble the rotational instability of a forced string. However, several new elements, exquisitely "nano" appear here, with the crucial involvement of Umklapp and of sliding nanofriction.

cond-mat.mtrl-sci↗

arXiv2008

Interaction of a CO molecule with a Pt monatomic wire: electronic structure and ballistic conductance

We carry out a first-principles density functional study of the interaction between a monatomic Pt wire and a CO molecule, comparing the energy of different adsorption configurations (bridge, on top, substitutional, and tilted bridge) and discussing the effects of spin-orbit (SO) coupling on the electronic structure and on the ballistic conductance of two of these systems (bridge and substitutional). We find that, when the wire is unstrained, the bridge configuration is energetically favored, while the substitutional geometry becomes possible only after the breaking of the Pt-Pt bond next to CO. The interaction can be described by a donation/back-donation process similar to that occurring when CO adsorbs on transition-metal surfaces, a picture which remains valid also in presence of SO coupling. The ballistic conductance of the (tipless) nanowire is not much reduced by the adsorption of the molecule on the bridge and on-top sites, but shows a significant drop in the substitutional case. The differences in the electronic structure due to the SO coupling influence the transmission only at energies far away from the Fermi level so that fully- and scalar-relativistic conductances do not differ significantly.

cond-mat.mtrl-sci↗

arXiv2008

Modeling the Unconventional Superconducting Properties of Expanded A$_3$C$_{60}$ Fullerides

The trivalent alkali fullerides A$_3$C$_{60}$, where C$_{60}$ are a well established family of molecular superconductors. The electron pairing has s-wave symmetry and is due to standard electron-phonon coupling, in particular by Jahn-Teller intramolecular C$_{60}$ vibrations. A source of renewed interest in these systems are indications of strong electron-electron repulsion, which emerges especially in compounds where the C$_{60}$-C$_{60}$ distance is expanded. In several compounds after an initial increases T$_c$, further expansions leads to a decline of superconductivity and its eventual disappearance in favor of a Mott insulating state We theoretically study a three-orbital Hubbard model including the phonon-mediated interaction using Dynamical Mean-Field Theory, which is particularly suitable due to the local nature of all the interactions. We studied the system as a function of the ratio of intra-molecular repulsion $U$ over the electron bandwidth $W$, the increase of $U/W$ representing the main effect of lattice expansion. The phase diagram is close to that of actual materials, with a dome-shaped superconducting region preceding the Mott transition. Unconventional properties predicted by this model include: (i) a pseudogap in the normal phase; (ii) a gain of kinetic energy and of d.c. conductivity at the onset of superconductivity; (iii) regular spin susceptibility and specific heat despite strong correlations; (iv) the emergence of more than one energy scale governing the renormalized single particle dispersion. These predictions, if confirmed, would establish fullerides, especially the expanded ones, as members of the wider family of strongly correlated superconductors.

cond-mat.supr-con↗

arXiv2008

Interaction of a CO molecule with a Pt monoatomic chain: the top geometry

Recent experiments showed that the conductance of Pt nanocontacts and nanowires is measurably reduced by adsorption of CO. We present DFT calculations of the electronic structure and ballistic conductance of a Pt monoatomic chain and a CO molecule adsorbed in an on-top position. We find that the main electronic molecule-chain interaction occurs via the $5σ$ and $2π^{\star}$ orbitals of the molecule, involved in a donation/back-donation process similar to that of CO on transition-metal surfaces. The ideal ballistic conductance of the monoatomic chain undergoes a moderate reduction by about 1.0 G_0 (from 4 G_0 to 3.1 G_0) upon adsorption of CO. By repeating all calculations with and without spin-orbit coupling, no substantial spin-orbit induced change emerges either in the chain-molecule interaction mechanism or in the conductance.

cond-mat.mtrl-sci↗

arXiv2008

Two-level Physics in a Model Metallic Break Junction

We consider a model inspired by a metal break-junction hypothetically caught at its breaking point, where the non-adiabatic center-of-mass motion of the bridging atom can be treated as a two-level system. By means of Numerical Renormalization Group (NRG) we calculate the influence of the two level system on the ballistic conductance across the bridge atom. The results are shown to be fully consistent with a conformal field theory treatment. We find that the conductance, calculated by coupling Fermi liquid theory to our NRG is always finite and fractional at zero temperature, but drops quite fast as the temperature increases.

cond-mat.str-el↗

arXiv2008

Nonlinear hysteretic behavior of a confined sliding layer

A nonlinear model representing the tribological problem of a thin solid lubricant layer between two sliding periodic surfaces is used to analyze the phenomenon of hysteresis at pinning/depinning around a moving state rather than around a statically pinned state. The cycling of an external driving force F_ext is used as a simple means to destroy and then to recover the dynamically pinned state previously discovered for the lubricant center-of-mass velocity. De-pinning to a quasi-freely sliding state occurs either directly, with a single jump, or through a sequence of discontinuous transitions. The intermediate sliding steps are reminiscent of phase-locked states and stick-slip motion in static friction, and can be interpreted in terms of the appearance of travelling density defects in an otherwise regular arrangement of kinks. Re-pinning occurs more smoothly, through the successive disappearance of different travelling defects. The resulting bistability and multistability regions may also be explored by varying mechanical parameters other than F_ext, e.g. the sliding velocity or the corrugation amplitude of the sliders.

cond-mat.mtrl-sci↗

arXiv2007

Static friction on the fly: velocity depinning transitions of lubricants in motion

The dragging velocity of a model solid lubricant confined between sliding periodic substrates exhibits a phase transition between two regimes, respectively with quantized and with continuous lubricant center-of-mass velocity. The transition, occurring for increasing external driving force F_ext acting on the lubricant, displays a large hysteresis, and has the features of depinning transitions in static friction, only taking place on the fly. Although different in nature, this phenomenon appears isomorphic to a static Aubry depinning transition in a Frenkel-Kontorova model, the role of particles now taken by the moving kinks of the lubricant-substrate interface. We suggest a possible realization in 2D optical lattice experiments.

cond-mat.mtrl-sci↗

arXiv2007

Alkali Halide Surfaces Near Melting: Wetting and Nanofriction Properties

Alkali halide (100) crystal surfaces are poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within the well tested BMHFT model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a non-melting surface, and explain its bad wetting in detail. The extreme stability of NaCl(100) is ideal for a study of the nanofriction in the high temperature regime, close to and even above the bulk melting temperature (T_M). Our simulations reveal in this regime two distinct and opposite phenomena for plowing and for grazing friction. We found a frictional drop close to T_M for deep ploughing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena we obtain a fresh microscopic understanding, relating the former to ``skating'' through a local liquid cloud, the latter to softening of the free substrate surface. It is argued that both phenomena, to be pursued experimentally, should be much more general than the specific NaCl surface case. Most metals in particular possessing one or more close packed non-melting surface, such as Pb, Al or Au(111), should behave quite similarly.

cond-mat.mtrl-sci↗

arXiv2007

Electron corrected Lorentz forces in solids and molecules in magnetic field

We describe the effective Lorentz forces on the ions of a generic insulating system in an magnetic field, in the context of Born-Oppenheimer ab-initio molecular dynamics. The force on each ion includes an important contribution of electronic origin, which depends explicitly on the velocity of all other ions. It is formulated in terms of a Berry curvature, in a form directly suitable for future first principles classical dynamics simulations based {\it e.g.,} on density functional methods. As a preliminary analytical demonstration we present the dynamics of an H$_2$ molecule in a field of intermediate strength, approximately describing the electrons through Slater's variational wavefunction.

cond-mat.mtrl-sci↗

arXiv2007

Peak Effect versus Skating in High Temperature Nanofriction

The physics of sliding nanofriction at high temperature near the substrate melting point $\Tmelt$ is so far unexplored. We conducted simulations of hard tips sliding on a prototype non-melting surface, NaCl(100), revealing in this regime two distinct and opposite phenomena for plowing and for grazing friction. We found a frictional drop close to $\Tmelt$ for deep plowing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena we obtain a fresh microscopic understanding, relating the former to ``skating'' through a local liquid cloud, the latter to linear response properties of the free substrate surface. It is argued that both phenomena should be pursued experimentally, and much more general than the specific NaCl surface case. Most metals in particular possessing one or more close packed non-melting surface, such as Pb, Al or Au(111), that should behave quite similarly.

cond-mat.mtrl-sci↗

arXiv2007

Triangular Mott-Hubbard Insulator Phases of Sn/Si(111) and Sn/Ge(111) Surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface $α$-phases is reexamined theoretically, based on $ab-initio$ calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density + Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed $Γ$-point photoemission intensity, as well asfor the partial metallization observed above above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, $e.g.$ alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.

cond-mat.str-el↗

arXiv2006

Physics and Nanofriction of Alkali Halide Solid Surfaces at the Melting Point

Alkali halide (100) surfaces are anomalously poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within a simple (BMHFT) model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a nonmelting surface, and that the incomplete wetting can be traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface, reducing in particular its entropy much below that of solid NaCl(100). Presently, we are making use of the nonmelting properties of this surface to conduct case study simulations of hard tips sliding on a hot stable crystal surface. Preliminary results reveal novel phenomena whose applicability is likely of greater generality.

cond-mat.mtrl-sci↗

arXiv2006

Electron-Stimulated Emission of Na Atoms from NaCl Nanocube Corners

We performed first principles density functional calculations and simulations of magic-size neutral NaCl nanocubes, and computed the the extraction of a Na neutral corner atom after donating an electron. The atomic structure of the resulting Na corner vacancy is presented.

cond-mat.mtrl-sci↗

arXiv2006

Ballistic Conductance and Magnetism in Short Tip-Suspended Ni Nanowires

Electronic and transport properties of a short Ni nanowire suspended between two semi-infinite ferromagnetic Ni leads are explored in the framework of density-functional theory. The spin-dependent ballistic conductance of the nanowire is calculated using a scattering-based approach and the Landauer-Buttiker formula. The total calculated conductance in units of $G_0 = 2e^2/h$ is around 1.6, in fairly good agreement with the broad peak observed around 1.5 for the last conductance step in break junctions. Separating contributions from different spins, we find nearly 0.5 $G_0$ from the majority spin $s$-like channel, whereas the remaining minority spin conductance of 1.1 $G_0$ contains significant contributions from several $d$ states, but much less than 0.5 $G_0$ from $s$ states. The influence of the structural relaxation on the magnetic properties and the ballistic conductance of the nanowire is also studied.

cond-mat.mtrl-sci↗