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Non-equilibrium molecular dynamics simulations, of crucial importance in sliding friction, are hampered by arbitrariness and uncertainties in the removal of the frictionally generated Joule heat. Building upon general pre-existing formulation, we implement a fully microscopic dissipation approach which, based on a parameter-free, non-Markovian, stochastic dynamics, absorbs Joule heat equivalently to a semi-infinite solid, and harmonic substrate. As a test case, we investigate the stick-slip friction of a slider over a two-dimensional Lennard-Jones solid, comparing our virtually exact frictional results with approximate ones from commonly adopted dissipation schemes. Remarkably, the exact results can be closely reproduced by a standard Langevin dissipation scheme, once its parameters are determined according to a general and self-standing variational procedure. [ABSTRACT FROM AUTHOR]

We present first-principles calculations of the effects of magnetism on the ballistic conductance of a model Pd nanocontact, made of a short Pd monatomic stretched chain placed between two Pd leads, simulated by semi-infinite (100) slabs. The stretching makes the suspended Pd chain generally ferromagnetic. The spin-resolved ballistic conductance, calculated according to the Landauer-Büttiker formula is found to be 0.85G0 for the spin-up and 1.15G0 for the spin-down electrons (G0 = 2e2/h is the conductance quantum). The total conductance ~2G0 is lower, but still relatively close to that of the nonmagnetic Pd nanocontact with the same geometry, calculated to be 2.3G0. To illustrate how magnetism and conductance depend on structural details, we change the three atom chain docking from the top to a hollow surface site, where at the same stress the Pd contact is nonmagnetic and the conductance decreases to 1.8G0. Overall we find these calculated ballistic conductance values of very similar magnitude to the first histogram peak in the experimental data obtained for Pd at low temperature in mechanically controllable break junctions. We conclude that the 15% conductance changes caused by the onset or the demise of local magnetism, similar in magnitude to geometry-related conductance changes, are probably too small to be used as a diagnostic for the presence or absence of nanocontact magnetism. [ABSTRACT FROM AUTHOR]

Whenever a nanosystem such as an adatom, a cluster or a nanowire spontaneously magnetizes, a crucial parameter is its magnetic anisotropy, the intrinsic preference of magnetization to lie along an easy axis. Anisotropy is important in nanosystems because it helps reduce the magnitude of thermal (superparamagnetic) fluctuations, it can modify the flow of current, and it can induce new phenomena, such as the quantum tunnelling of magnetization. We discuss here, on the basis of density functional calculations, the novel and unconventional feature of colossal magnetic anisotropy—the strict impossibility of magnetization to rotate from the parallel to the orthogonal direction—which, owing to a quantum mechanical selection rule, the recently predicted Pt nanowire magnetism should exhibit. Model calculations suggest that the colossal magnetic anisotropy of a Pt chain should persist after weak adsorption on an inert substrate or surface step. [ABSTRACT FROM AUTHOR]

The physics of sliding nanofriction at high temperature near the substrate melting point, TM, is so far unexplored. We conducted simulations of hard tips sliding on a prototype non-melting surface, NaCl(100), revealing two distinct and opposite phenomena for ploughing and for grazing friction in this regime. We found a frictional drop close to TM 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, and the latter to linear response properties of the free substrate surface. We argue that both phenomena occur more generally on surfaces other than NaCl and should be pursued experimentally. Most metals, in particular those possessing one or more close-packed non-melting surfaces, such as Pb, Al or Au(111), are likely to behave similarly. [ABSTRACT FROM AUTHOR]

This is a concise review of some facts and theory work on the equilibrium behavior of solid surfaces and nanosystems close to the bulk melting point. We focus in particular on computer simulation results obtained for metals and ionic insulators. [ABSTRACT FROM AUTHOR]

Loss of braking power and rubber skidding on a wet road is still an open physics problem, as neither the hydrodynamic effects nor the loss of surface adhesion that are sometimes blamed really manage to explain the 20–30% observed loss of low-speed tyre–road friction. Here we report a novel mechanism based on sealing of water-filled substrate pools by the rubber. The sealed-in water effectively smoothens the substrate, thus reducing the viscoelastic dissipation in bulk rubber induced by surface asperities—well established as a major friction contribution. Starting with the measured spectrum of asperities one can calculate the water-smoothened spectrum and from that the predicted friction reduction, which is of the correct magnitude. The theory is directly supported by fresh tyre–asphalt friction data. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the occurrence of Hund's rule magnetism in C60 molecular ions, by computing the ground-state spin for all charge states n from -3 to +5. Review of the model Hamiltonian; Zero-point quantum corrections; Discussion and conclusions.

We consider dewetting at soft interfaces, e.g., a rubber ball squeezed in a fluid against a flat hard substrate. We show that the applied squeezing pressure, which was not taken into account in earlier studies of the dewetting transition, may have a big effect on the squeeze-out dynamics. [ABSTRACT FROM AUTHOR]

We study the quantum phase transition between a band (“ionic”) insulator and a Mott-Hubbard insulator, realized at a critical value in a bipartite Hubbard model with two inequivalent sites, whose on-site energies differ by an offset . The study is carried out both in D=1 and D=2 (square and honeycomb lattices), using exact Lanczos diagonalization, finite-size scaling, and Berry's phase calculations of the polarization. The Born effective charge jump from positive infinity to negative infinity previously discovered in D=1 by Resta and Sorella is confirmed to be directly connected with the transition from the band insulator to the Mott insulating state, in agreement with recent work of Ortiz et al. In addition, symmetry is analysed, and the transition is found to be associated with a reversal of inversion symmetry in the ground state, of magnetic origin. We also study the D=1 excitation spectrum by Lanczos diagonalization and finite-size scaling. Not only the spin gap closes at the transition, consistent with the magnetic nature of the Mott state, but also the charge gap closes, so that the intermediate state between the two insulators appears to be metallic. This finding, rationalized within Hartree-Fock as due to a sign change of the effective on-site energy offset for the minority spin electrons, underlines the profound difference between the two insulators. The band-to-Mott insulator transition is also studied and found in the same model in D=2. There too we find an associated, although weaker, polarization anomaly, with some differences between square and honeycomb lattices. The honeycomb lattice, which does not possess an inversion symmetry, is used to demonstrate the possibility of an inverted piezoelectric effect in this kind of ionic Mott insulator. [ABSTRACT FROM AUTHOR]

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ground state of two-dimensional electrons of density N/L in a strong transverse magnetic field B is discussed in terms of localized magnetic functions. For all « commensurate » fractional fillings of the n = 0 Landau level, occurring at B = (s + t + st) 2πhcN/eL, with s, t integers, it is found that the ground state is a triangular lattice. This lattice has unusual properties, because it is tied to the magnetic functions. In particular, it has a finite Hall conductivity σ = e/2πh(s + t + st) and it also exhibits perfect diagmagnetism relative to B. It does, however, display no proper Meissner effect, because the London depth is macroscopically large. The excess field B - B gives rise instead to defects in the lattice, where the extra electrons (holes) become « interstitials » (« vacancies »). If the defects are free to move, the Hall conductivity will not stay quantized. On the other hand, if all defects are pinned by inhomogeneities, Hall plateaux are expected around each B. This picture, while providing a natural explanation for the quantized Hall effect at both integer and fractional filling, leads to a simple understanding of the plateau width vs. temperature and simple quality, and can also explain, at finite temperatures, the behaviour of the longitudinal conductivity σ and its observed asymmetry for integer filling. [ABSTRACT FROM AUTHOR]

Two main reconstructions have been reported for the α-Sn (111) surfaces (epitaxially grown on InSb(111)), namely 2 x 2 and a metastable 3 x 3. We discuss the possible nature of these reconstructions, based on first-principles electronic structure calculations carried out in our group. The lowest surface energy is attained by an 'adatom-restatom'(AR) reconstruction, whose basic building block can be either c(4 x 2) or 2 x 2, analogous to those of Si(111)7 x 7, and of Ge(111)c(2 x 8). We thus propose that Sn(111)2 x 2 could be explained by the AR model, and we compute surface core level shifts, as a prediction for future experiments. For the 3 x 3, the scenario appears less obvious, and we consider two different models: a dimer-adatom-stacking fault (DAS) model and a distorted $$\sqrt 3 \times \sqrt 3 $$ , inspired by existing Charge-Density-Wave (CDW) distorted surfaces. Both models yield a higher surface energy when compared to the 2 x 2 AR, the DAS only slightly worse than the other. Thus the nature of the metastable 3 x 3, possibly a result of growth kinetics, remains an open issue. [ABSTRACT FROM AUTHOR]

Surface melting is strongly influenced by the interaction between the solid-liquid interface and the liquid-vapour interface. Unretarded electromagnetic dispersion forces yield a potential of the form H/ l , where l is the distance between the interfaces (thickness of the quasi-liquid film) and H is the appropriate Hamaker constant. We have adopted the well-known treatment of dispersion forces of Lifshitz et al., where the frequency-dependent conductivities of solid and liquid are the primary quantities for the calculation of this Hamaker constant. Using the dielectric functions obtained from experimental data, numerical values for H are obtained for Al, Pb, Au, Bi and Ge. The Hamaker constant of the metals is found to be positive and is discussed in connection with recent experimental data on surface melting, particularly of Pb(110) and Al(110). The Hamaker constants of Bi (semimetal) and Ge (semiconductor) are negative, allowing at most blocked surface melting. [ABSTRACT FROM AUTHOR]

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: D is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: B is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)