Mechanical properties of carbynes investigated by ab initio total-energy calculations

As sp carbon chains (carbynes) are relatively rigid molecular objects, can we exploit them as construction elements in nanomechanics? To answer this question, we investigate their remarkable mechanical properties by ab-initio total-energy simulations. In particular, we evaluate their linear response to small longitudinal and bending deformations and their failure limits for longitudinal compression and elongation.Comment: 6 pages, 4 figures, 1 tabl

AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers

In the atomic force microscope, the nanoscale force topography of even complex surface superstructures is extracted by the changing vibration frequency of a scanning tip. An alternative dissipation topography with similar or even better contrast has been demonstrated recently by mapping the (x,y)-dependent tip damping but the detailed damping mechanism is still unknown. Here we identify two different tip dissipation mechanisms: local mechanical softness and hysteresis. Motivated by recent data, we describe both of them in a onedimensional model of Moire' superstructures of incommensurate overlayers. Local softness at "soliton" defects yields a dissipation contrast that can be much larger than the corresponding density or corrugation contrast. At realistically low vibration frequencies, however, a much stronger and more effective dissipation is caused by the tip-induced nonlinear jumping of the soliton, naturally developing bistability and hysteresis. Signatures of this mechanism are proposed for experimental identification.Comment: 5 pages, 5 figures, Phys Rev B 81, 045417 (2010

Solitons and exact velocity quantization of incommensurate sliders

We analyze 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 = v_cm/v_ext of the chain center-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.Comment: 12 pages 6 figures. Small fixup after Referee's comments. In print in J. Phys.: Condens. Matte

Hysteresis from dynamically pinned sliding states

We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F_ext. A critical threshold value F_c of the adiabatically applied force F_ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of ``dynamic'' stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F_ext. On the contrary no hysteretic behavior is observed as a function of the external driving velocity.Comment: 12 pages, 5 figures, ECOSS 200

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.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let

Axial collective excitations of a degenerate Fermi gas in the BEC to unitarity crossover

We show that, with reasonable hypotheses leading to a simple modeling,a link can be obtained from experiments on the axial low frequency collective modes between the molecular scattering length $a_M$ and the energy parameter $\xi \equiv 1 + \beta$ of the gas at the unitarity limit. We also point out that, in order to reach the range where the features of the Bose limit can be clearly seen, experiments have to go to more dilute situations than have been achieved presently.Comment: 5 pages, revtex, 2 figure

Exactly quantized dynamics of classical incommensurate sliders

We report peculiar velocity quantization phenomena in the classical motion of an idealized 1D solid lubricant, consisting of a harmonic chain interposed between two periodic sliders. The ratio v_cm/v_ext of the chain center-of-mass velocity to the externally imposed relative velocity of the sliders stays 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 explained by one slider rigidly dragging the kinks that the chain forms with the other slider. Possible consequences of these results for some real systems are discussed.Comment: 5 pags 4 fig

The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules

A model for correlated electrons in a lattice with local additional spin--1 degrees of freedom inducing constrained hopping, is studied both in the low density limit and at quarter filling. We show that in both 1D and 2D two particles form a bound state even in presence of a repulsive U<U_c. A picture of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D (quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow for a determination of the phase diagram.Comment: 7 pages + 2 ps figures, published versio

Large Spatial Scale Variability in Bathyal Macrobenthos Abundance, Biomass, a- and b-Diversity along the Mediterranean Continental Margin

The large-scale deep-sea biodiversity distribution of the benthic fauna was explored in the Mediterranean Sea, which can beseen as a miniature model of the oceans of the world. Within the framework of the BIOFUN project (‘‘Biodiversity andEcosystem Functioning in Contrasting Southern European Deep-sea Environments: from viruses to megafauna’’), weinvestigated the large spatial scale variability (over .1,000 km) of the bathyal macrofauna communities that inhabit theMediterranean basin, and their relationships with the environmental variables. The macrofauna abundance, biomass,community structure and functional diversity were analysed and the a-diversity and b-diversity were estimated across sixselected slope areas at different longitudes and along three main depths. The macrobenthic standing stock and a-diversitywere lower in the deep-sea sediments of the eastern Mediterranean basin, compared to the western and central basins. Themacrofaunal standing stock and diversity decreased significantly from the upper bathyal to the lower bathyal slope stations.The major changes in the community composition of the higher taxa and in the trophic (functional) structure occurred atdifferent longitudes, rather than at increasing water depth. For the b-diversity, very high dissimilarities emerged at all levels:(i) between basins; (ii) between slopes within the same basin; and (iii) between stations at different depths; this thereforedemonstrates the high macrofaunal diversity of the Mediterranean basins at large spatial scales. Overall, the food sources(i.e., quantity and quality) that characterised the west, central and eastern Mediterranean basins, as well as sediment grainsize, appear to influence the macrobenthic standing stock and the biodiversity along the different slope areas