Energy resolved STM mapping of C60_{60} on metal surfaces: A theoretical study

We present a detailed theoretical study of scanning tunneling imaging and spectroscopy of \Csixty on silver and gold surfaces, motivated by the recent experiments and discussion by X. Lu et al. [PRL \textbf{90}, 096802 (2003) and PRB \textbf{70}, 115418 (2004)]. The surface/sample/tip system is described within a self--consistent DFT based tight--binding model. The topographic and conductance images are computed at constant current from a full self--consistent transport theory based on nonequilibrium Green's functions and compared with those simulated from the local density of states. The molecular orbitals of \Csixty are clearly identified in the energy resolved maps, in close correspondence with the experimental results. We show how the tip structure and orientation can affect the images. In particular, we consider the effects of truncated tips on the energy resolved maps.Comment: 9 pages, 8 figure

Turbulent self-organized criticality

In the prototype sandpile model of self-organized criticality time series obtained by decomposing avalanches into waves of toppling show intermittent fluctuations. The q-th moments of wave size differences possess local multiscaling and global simple scaling regimes analogous to those holding for velocity structure functions in fluid turbulence. The correspondence involves identity of a basic scaling relation and of the form of relevant probability distributions. The sandpile provides a qualitative analog of many features of turbulent phenomena.Comment: Revised version. 5 RevTex pages and 4 postscript figure

Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters

Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.Comment: 5 pages, 3 figure

Modeling of droplet breakup in a microfluidic T--shaped junction with a phase--field model

A phase--field method is applied to the modeling of flow and breakup of droplets in a T--shaped junction in the hydrodynamic regime where capillary and viscous stresses dominate over inertial forces, which is characteristic of microfluidic devices. The transport equations are solved numerically in the three--dimensional geometry, and the dependence of the droplet breakup on the flow rates, surface tension and viscosities of the two components is investigated in detail. The model reproduces quite accurately the phase diagram observed in experiments performed with immiscible fluids. The critical capillary number for droplet breakup depends on the viscosity contrast, with a trend which is analogous to that observed for free isolated droplets in hyperbolic flow

From waves to avalanches: two different mechanisms of sandpile dynamics

Time series resulting from wave decomposition show the existence of different correlation patterns for avalanche dynamics. For the d=2 Bak-Tang-Wiesenfeld model, long range correlations determine a modification of the wave size distribution under coarse graining in time, and multifractal scaling for avalanches. In the Manna model, the distribution of avalanches coincides with that of waves, which are uncorrelated and obey finite size scaling, a result expected also for the d=3 Bak et al. model.Comment: 5 pages, 4 figure

Multifractal scaling in the Bak-Tang-Wiesenfeld Sandpile and edge events

An analysis of moments and spectra shows that, while the distribution of avalanche areas obeys finite size scaling, that of toppling numbers is universally characterized by a full, nonlinear multifractal spectrum. Rare, large avalanches dissipating at the border influence the statistics very sensibly. Only once they are excluded from the sample, the conditional toppling distribution for given area simplifies enough to show also a well defined, multifractal scaling. The resulting picture brings to light unsuspected, novel physics in the model.Comment: 5 pages, 4 figure