The Glass Transition in Driven Granular Fluids: A Mode-Coupling Approach

We consider the stationary state of a fluid comprised of inelastic hard spheres or disks under the influence of a random, momentum-conserving external force. Starting from the microscopic description of the dynamics, we derive a nonlinear equation of motion for the coherent scattering function in two and three space dimensions. A glass transition is observed for all coefficients of restitution, epsilon, at a critical packing fraction, phi_c(epsilon), below random close packing. The divergence of timescales at the glass-transition implies a dependence on compression rate upon further increase of the density - similar to the cooling rate dependence of a thermal glass. The critical dynamics for coherent motion as well as tagged particle dynamics is analyzed and shown to be non-universal with exponents depending on space dimension and degree of dissipation.Comment: 16 pages, 9 figure

Partitioning of energy in highly polydisperse granular gases

A highly polydisperse granular gas is modeled by a continuous distribution of particle sizes, a, giving rise to a corresponding continuous temperature profile, T(a), which we compute approximately, generalizing previous results for binary or multicomponent mixtures. If the system is driven, it evolves towards a stationary temperature profile, which is discussed for several driving mechanisms in dependence on the variance of the size distribution. For a uniform distribution of sizes, the stationary temperature profile is nonuniform with either hot small particles (constant force driving) or hot large particles (constant velocity or constant energy driving). Polydispersity always gives rise to non-Gaussian velocity distributions. Depending on the driving mechanism the tails can be either overpopulated or underpopulated as compared to the molecular gas. The deviations are mainly due to small particles. In the case of free cooling the decay rate depends continuously on particle size, while all partial temperatures decay according to Haff's law. The analytical results are supported by event driven simulations for a large, but discrete number of species.Comment: 10 pages; 5 figure

Fracture of disordered solids in compression as a critical phenomenon: II. Model Hamiltonian for a population of interacting cracks

To obtain the probability distribution of 2D crack patterns in mesoscopic regions of a disordered solid, the formalism of Paper I requires that a functional form associating the crack patterns (or states) to their formation energy be developed. The crack states are here defined by an order parameter field representing both the presence and orientation of cracks at each site on a discrete square network. The associated Hamiltonian represents the total work required to lead an uncracked mesovolume into that state as averaged over the initial quenched disorder. The effect of cracks is to create mesovolumes having internal heterogeneity in their elastic moduli. To model the Hamiltonian, the effective elastic moduli corresponding to a given crack distribution are determined that includes crack-to-crack interactions. The interaction terms are entirely responsible for the localization transition analyzed in Paper III. The crack-opening energies are related to these effective moduli via Griffith's criterion as established in Paper I.Comment: 9 pages, 1 figur

Comparison of ICP-AlOx and ALD-Al2O3 layers for the rear surface passivation of c-Si solar cells

The deposition rate of the standard (i.e. sequential) atomic layer deposition (ALD) process is very low compared to the plasma-enhanced chemical vapour deposition (PECVD) process. Therefore, as a short- and medium-term perspective, PECVD aluminium oxide (AlOx) films might be better suited for the implementation into industrial-type solar cells than ALD-Al 2O3 films. In this paper, we report results achieved with a newly developed PECVD deposition process for AlOx using an inductively coupled plasma (ICP). We compare the results to high-quality ALDAl2O3 films. We examine a stack consisting of a thin AlOx passivation layer and a PECVD silicon nitride (SiNy) capping layer. Surface recombination velocities below 9 cm/s were measured on low-resistivity (1.4 Ωcm) p-type crystalline silicon wafers passivated either by ICP-PECVD-AlOx films or by ALD-Al2O3 films after annealing at 425°C. Both passivation schemes provide an excellent thermal stability during firing at 910°C with SRVs below 12 cm/s for both, Al2O3/SiNy stacks and single Al 2O3 layers. A fixed negative charge of -4×10 12 cm-2 is measured for ICP-AlOx and ALD-Al2O3, whereas the interface state density is higher for the ICP-AlOx layer with values of 11.0×1011 eV-1cm-2 compared to 1.3×1011 eV -1cm-2 for ALD-Al2O3. Implemented into large-area screen-printed PERC solar cells, an independently confirmed efficiency of 20.1% for ICP-AlOx and an efficiency of 19.6% for ALD-Al2O3 are achieved.BMU/0325296Solland Solar Cells BVSolarWorld Innovations GmbHSCHOTT Solar AGRENA GmbHSINGULUS TECHNOLOGIES A

Cost-effectiveness of Preventive Oral Health Care in Medical Offices for Young Medicaid Enrollees

Dental caries is the most common preventable chronic disease among preschool children. The pediatric primary care setting provides an alternative site to deliver preventive oral health. This study estimates the cost-effectiveness of a medical office-based preventive oral health program in North Carolina (“Into the Mouths of Babes,” IMB)

The Link between the Baryonic Mass Distribution and the Rotation Curve Shape

The observed rotation curves of disc galaxies, ranging from late-type dwarf galaxies to early-type spirals, can be fit remarkably well simply by scaling up the contributions of the stellar and HI discs. This `baryonic scaling model' can explain the full breadth of observed rotation curves with only two free parameters. For a small fraction of galaxies, in particular early-type spiral galaxies, HI scaling appears to fail in the outer parts, possibly due to observational effects or ionization of the HI. The overall success of the baryonic scaling model suggests that the well-known global coupling between the baryonic mass of a galaxy and its rotation velocity (known as the baryonic Tully-Fisher relation), applies at a more local level as well, and it seems to imply a link between the baryonic mass distribution and the distribution of total mass (including dark matter).Comment: 10 pages, accepted for publication in MNRA

Quantum point contact on graphite surface

The conductance through a quantum point contact created by a sharp and hard metal tip on the graphite surface has features which to our knowledge have not been encountered so far in metal contacts or in nanowires. In this paper we first investigate these features which emerge from the strongly directional bonding and electronic structure of graphite, and provide a theoretical understanding for the electronic conduction through quantum point contacts. Our study involves the molecular-dynamics simulations to reveal the variation of interlayer distances and atomic structure at the proximity of the contact that evolves by the tip pressing toward the surface. The effects of the elastic deformation on the electronic structure, state density at the Fermi level, and crystal potential are analyzed by performing self-consistent-field pseudopotential calculations within the local-density approximation. It is found that the metallicity of graphite increases under the uniaxial compressive strain perpendicular to the basal plane. The quantum point contact is modeled by a constriction with a realistic potential. The conductance is calculated by representing the current transporting states in Laue representation, and the variation of conductance with the evolution of contact is explained by taking the characteristic features of graphite into account. It is shown that the sequential puncturing of the layers characterizes the conductance.Comment: LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev. B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12