Elastic contact between self-affine surfaces: Comparison of numerical stress and contact correlation functions with analytic predictions

Contact between an elastic manifold and a rigid substrate with a self-affine fractal surface is reinvestigated with Green's function molecular dynamics. Stress and contact autocorrelation functions (ACFs) are found to decrease algebraically. A rationale is provided for the observed similarity in the exponents for stress and contact ACFs. Both exponents differ substantially from analytic predictions over the range of Hurst roughness exponents studied. The effect of increasing the range of interactions from a hard sphere repulsion to exponential decay is analyzed. Results for exponential interactions are accurately described by recent systematic corrections to Persson's contact mechanics theory. The relation between the area of simply connected contact patches and the normal force is also studied. Below a threshold size the contact area and force are consistent with Hertzian contact mechanics, while area and force are linearly related in larger contact patches.Comment: 12 pages, 9 figure

Deformed Harmonic Oscillators for Metal Clusters: Analytic Properties and Supershells

The analytic properties of Nilsson's Modified Oscillator (MO), which was first introduced in nuclear structure, and of the recently introduced, based on quantum algebraic techniques, 3-dimensional q-deformed harmonic oscillator (3-dim q-HO) with Uq(3) > SOq(3) symmetry, which is known to reproduce correctly in terms of only one parameter the magic numbers of alkali clusters up to 1500 (the expected limit of validity for theories based on the filling of electronic shells), are considered. Exact expressions for the total energy of closed shells are determined and compared among them. Furthermore, the systematics of the appearance of supershells in the spectra of the two oscillators is considered, showing that the 3-dim q-HO correctly predicts the first supershell closure in alkali clusters without use of any extra parameter.Comment: 25 pages LaTeX plus 21 postscript figure

Infrared spectra of one- and two-dimensional fullerene polymer structures: RbC60 and rhombohedral C-60

We compare the infrared spectra of two types of fullerene polymers: the linear-chain RbC60 and the two-dimensional pressure-polymerized rhombohedral C-60. Both the splitting of the F-1u modes and the structure of newly activated Lines are in agreement with fully ordered structures of molecular symmetry D-2h and D-3d, respectively

Quantum Transport Length Scales in Silicon-based Semiconducting Nanowires: Surface Roughness Effects

We report on a theoretical study of quantum charge transport in atomistic models of silicon nanowires with surface roughness-based disorder. Depending on the nanowires features (length, roughness profile) various conduction regimes are explored numerically by using efficient real space order N computational approaches of both Kubo-Greenwood and Landauer-Buttiker transport frameworks. Quantitative estimations of the elastic mean free paths, charge mobilities and localization lengths are performed as a function of the correlation length of the surface roughness disorder. The obtained values for charge mobilities well compare with the experimental estimates of the most performant undoped nanowires. Further the limitations of the Thouless relationship between the mean free path and the localization length are outlined.Comment: 13 pages, to appear in PR

Thermal effects on atomic friction

We model friction acting on the tip of an atomic force microscope as it is dragged across a surface at non-zero temperatures. We find that stick-slip motion occurs and that the average frictional force follows $|\ln v|^{2/3}$, where $v$ is the tip velocity. This compares well to recent experimental work (Gnecco et al, PRL 84, 1172), permitting the quantitative extraction of all microscopic parameters. We calculate the scaled form of the average frictional force's dependence on both temperature and tip speed as well as the form of the friction-force distribution function.Comment: Accepted for publication, Physical Review Letter

Light emission from a scanning tunneling microscope: Fully retarded calculation

The light emission rate from a scanning tunneling microscope (STM) scanning a noble metal surface is calculated taking retardation effects into account. As in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev. B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric properties of tip and sample are described by experimentally measured dielectric functions. The calculations are based on exact diffraction theory through the vector equivalent of the Kirchoff integral. The present results are qualitatively similar to those of the non-retarded calculations. The light emission spectra have pronounced resonance peaks due to the formation of a tip-induced plasmon mode localized to the cavity between the tip and the sample. At a quantitative level, the effects of retardation are rather small as long as the sample material is Au or Cu, and the tip consists of W or Ir. However, for Ag samples, in which the resistive losses are smaller, the inclusion of retardation effects in the calculation leads to larger changes: the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These changes improve the agreement with experiment. For a Ag sample and an Ir tip, the quantum efficiency is $\approx$ 10$^{-4}$ emitted photons in the visible frequency range per tunneling electron. A study of the energy dissipation into the tip and sample shows that in total about 1 % of the electrons undergo inelastic processes while tunneling.Comment: 16 pages, 10 figures (1 ps, 9 tex, automatically included); To appear in Phys. Rev. B (15 October 1998

The Distance to SN 1999em from the Expanding Photosphere Method

We present optical and IR spectroscopy of the first two months of evolution of the Type II SN 1999em. We combine these data with high-quality optical/IR photometry beginning only three days after shock breakout, in order to study the performance of the ``Expanding Photosphere Method'' (EPM) in the determination of distances. With this purpose we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra. The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11% in velocity from an individual spectrum. Our analysis shows that EPM is quite robust to the effects of dust. In particular, the distances derived from the VI filters change by only 7% when the adopted visual extinction in the host galaxy is varied by 0.45 mag. The superb time sampling of the BVIZJHK light-curves of SN 1999em permits us to study the internal consistency of EPM and test the dilution factors computed from atmosphere models for Type II plateau supernovae. We find that, in the first week since explosion, the EPM distances are up to 50% lower than the average, possibly due the presence of circumstellar material. Over the following 65 days, on the other hand, our tests lend strong credence to the atmosphere models, and confirm previous claims that EPM can produce consistent distances without having to craft specific models to each supernova. This is particularly true for the VI filters which yield distances with an internal consistency of 4%. From the whole set of BVIZJHK photometry, we obtain an average distance of 7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of the internal precision of the method obtained from the analysis of the first 70 days of the supernova evolution.Comment: 68 pages, 15 tables, 22 figures, to appear in Ap

Simulations of the Static Friction Due to Adsorbed Molecules

The static friction between crystalline surfaces separated by a molecularly thin layer of adsorbed molecules is calculated using molecular dynamics simulations. These molecules naturally lead to a finite static friction that is consistent with macroscopic friction laws. Crystalline alignment, sliding direction, and the number of adsorbed molecules are not controlled in most experiments and are shown to have little effect on the friction. Temperature, molecular geometry and interaction potentials can have larger effects on friction. The observed trends in friction can be understood in terms of a simple hard sphere model.Comment: 13 pages, 13 figure