The understanding of friction at nano-scales, ruled by the regular arrangement of atoms, is surprisingly incomplete. Here we provide a unified understanding by studying the interlocking potential ...energy of two infinite contacting surfaces with arbitrary lattice symmetries, and extending it to finite contacts. We categorize, based purely on geometrical features, all possible contacts into three different types: a structurally lubric contact where the monolayer can move isotropically without friction, a corrugated and strongly interlocked contact, and a newly discovered directionally structurally lubric contact where the layer can move frictionlessly along one specific direction and retains finite friction along all other directions. This novel category is energetically stable against rotational perturbations and provides extreme friction anisotropy. The finite-size analysis shows that our categorization applies to a wide range of technologically relevant materials in contact, from adsorbates on crystal surfaces to layered two-dimensional materials and colloidal monolayers.
A novel kind of geometry of nanoscale contacts realizes stable directional locking, with one structurally lubric sliding direction - the frictionless nanohighway, and large friction perpendicular to it.
Fullerides in a Squeeze Tosatti, Erio
Science (American Association for the Advancement of Science),
03/2009, Letnik:
323, Številka:
5921
Journal Article
John Ziman with his old-fashioned ways, was a real British gentleman of the colonies. Born and raised in New Zealand, Ziman belonged to that large group of men and women that went back to their ...fathers’ land in the last century from the Commonwealth countries. In many cases, they were individuals with an outstanding intellect and, therefore, a real tresure trove for Great Britain, which drew from those remote places not only gems, tea, perfumes and raw materials, but also enlightened minds and reliable personalities.
The Gutzwiller projector technique has long been known as a method to include correlations in electronic structure calculations. We describe a model implementation for a Gutzwiller + LDA calculation ...in a localized-orbital restricted basis framework, emphasizing the protocol step by step and illustrating our specific procedure for this and future applications. We demonstrate the method with a classic problem, the ferromagnetism of bulk bcc Fe, whose nature is attracting fresh interest. In the conventional Stoner-Wohlfarth model, and in spin-polarized LDA calculations, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion, at the cost of some increase of electron kinetic energy. This balance may, however, be altered by correlations, which are strong for localized d orbitals. The present localized basis Gutzwiller + LDA calculation demonstrates how the ferromagnetic ordering of Fe may, in fact, entrain a decrease of kinetic energy at the cost of some increase of potential energy. This happens because, as foreshadowed long ago by Goodenough and others and more recently supported by LDA-DMFT calculations, correlations cause e sub(g) and t sub(2g) d orbitals to behave differently, with the weakly propagating e sub(g) states fully spin polarized and almost localized, and only t sub(2g) states forming a broad partly filled itinerant band. Owing to an intra-atomic Hund's rule exchange that aligns e sub(g) and t sub(2g) spins, the propagation of itinerant t sub(2g) holes is favored when different atomic spins are ferromagnetically aligned. This suggests a strong analogy with double exchange in iron ferromagnetism.
We analyse in some detail the recently discovered velocity quantization phenomena in the classical motion of an idealized one-dimensional solid lubricant, consisting of a harmonic chain interposed ...between two periodic sliders. The ratio w = vcm/vext of the chain centre-of-mass velocity to the externally imposed relative velocity of the sliders is pinned to exact 'plateau' values for wide ranges of parameters, such as sliders corrugation amplitudes, external velocity, chain stiffness and dissipation, and is strictly determined by the commensurability ratios alone. The phenomenon is caused by one slider rigidly dragging the density solitons (kinks/antikinks) that the chain forms with the other slider. Possible consequences of these results for some real systems are discussed.
Physics. Fullerides in a squeeze Tosatti, Erio
Science (American Association for the Advancement of Science),
2009-Mar-20, 20090320, Letnik:
323, Številka:
5921
Journal Article
We extend the study of velocity quantization phenomena recently found in the classical motion of an idealized 1D model solid lubricant – consisting of a harmonic chain interposed between two periodic ...sliding potentials A. Vanossi, M. Manini, G. Divitini, G.E. Santoro, E. Tosatti, Phys. Rev. Lett. 97 (2006) 056101. This quantization is due to one slider rigidly dragging the
commensurate lattice of kinks that the chain forms with the other slider. In this follow-up work we consider finite-size chains rather than infinite chains. The finite-size (i) permits the development of robust velocity plateaus as a function of the lubricant stiffness, and (ii) allows an overall chain length re-adjustment which spontaneously promotes single-particle
periodic oscillations. These periodic oscillations replace the quasi-periodic motion produced by general incommensurate periods of the sliders and the lubricant in the infinite-size model. Possible consequences of these results for some real systems are discussed.
Solid-liquid interfaces play a fundamental role in surface electrochemistry, catalysis, wetting, self-assembly and biomolecular functions. The interfacial energy determines many of the properties of ...such interfaces, including the arrangement of the liquid molecules at the surface of the solid. Diffraction techniques are often used to investigate the structure of solid-liquid interfaces, but measurements of irregular or inhomogeneous interfaces remain challenging. Here, we report atomic- and molecular-resolution images of various organic and inorganic samples in liquids, obtained with a commercial atomic force microscope operated dynamically with small-amplitude modulation. This approach uses the structured liquid layers close to the solid to enhance lateral resolution. We propose a model to explain the mechanism dominating the image formation, and show that the energy dissipated during this process is related to the interfacial energy through a readily achievable calibration curve. Our topographic images and interfacial energy maps could provide insights into important interfaces.
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