Publications

Earlier computations on the work of separation of boundaries with adsorbed solute atmospheres are reconsidered in terms of reversible work cycles. Special attention is given to two limiting cases. These are the separation of a material interface under fully equilibrated conditions, for which the chemical potential of the adsorbed solute remains constant, and separation under constrained conditions for which the surface excess solute concentration remains constant (i.e., the same on the two newly created free surfaces as present initially on the unstressed interface). The results are consistent with the limiting cases treated before and include the extension to more general cases of solute interactions, including multi-component systems. The work terms are conveniently represented on diagrams of chemical potential versus surface excess solute concentration. A general separation process is then represented as a path in this diagram which begins on the adsorption isotherm for the unstressed interface and ends on the adsorption isotherm for the pair of newly created surfaces.

Abstract Die Reaktion derPeroxymetallverbindungen(I),(IV),(V),(VI)und (VII)mit n‐Butyllithium (II) wird untersucht.

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We are developing an expert system environment for solving elliptic partial differential equations (PDEs) defined on two and three dimensional domains on MIMD type parallel machines. According to its design objectives, it will provide a uniform programming environment for implementing parallel MIMD PDE solution solvers, an automatic partitioning and allocation of the PDE computation, a very high level problem specification language, an interactive high level environment for grid selection, a domain partitioning and mapping facility, a uniform environment for obtaining software engineering measurements and a graphical display of solution output. The ‖ELLPACK is implemented on a hardware facility consisting of a graphics workstation supporting X11 window system and connected to NCUBE and SEQUENT machines through a wide bandwidth local network. The software infrastructure includes (i) a PDE problem oriented language processor, (ii) a geometry processing tool which is capable of generating fixed meshes and domain decompositions automatically and interactively, (iii) an algorithm mapper facility for partitioning and mapping the underlying PDE computation and (iv) an expert front end that selects the discretization mesh, the parallel algorithm⧸machine pair and its configuration. In order to support the solution of elliptic PDEs on fixed meshes, we are building a library of finite difference and element discretization methods and direct⧸iterative solvers for the NCUBE and SEQUENT machines.

We report on numerical simulations of slip evolution along a two dimensional (slip varies only with depth) vertical strike‐slip fault in an elastic half‐space, using a framework incorporating full inertial elastodynamics. The model is a follow‐up on earlier quasi‐static and quasi‐dynamic simulations of deformations along smooth fault systems in elastic continua. The fault is driven below a crustal depth of 24 km by a constant plate velocity of 35‐mm/yr. Deformation at each fault location in the crustal zone is the sum of slip rate contributions from rate‐ and state‐dependent friction and power law creep, where both processes have temperature‐dependent (and hence depth‐varying) coefficients and both take place locally under the same stress. The simulations employ two versions of rate‐ and state‐dependent friction: a “slip” law, which requires nonzero velocity for state evolution, and an ”ageing/slowness“ law, which incorporates state evolution and restrengthening in stationary contact. The assumed constitutive laws and distribution of frictional parameters are compatible with laboratory experiments. The elastodynamic calculations are based on spectral representations of variables and a new algorithm providing a unified computational framework for calculations of long deformational histories containing short periods of rapid instabilities. The simulations show dynamic rupture propagation and wave phenomena not accounted for in the previous quasi‐static and quasi‐dynamic works. However, the results are qualitatively similar to those obtained by corresponding quasi‐static and quasi‐dynamic calculations. Slip histories along a smooth fault, simulated here with full elastodynamics for various constitutive laws and model parameters, consist mostly of quasi‐periodic large events. This finding indicates that inertial dynamics does not provide a generic mechanism for generating spatio‐temporal complexities of slip. On the other hand, calculations done for cases representing, approximately, strongly disordered systems do show rich slip histories with a range of event sizes. This result is compatible with our previous conclusions that the origin of observed broad distributions of earthquake sizes is strong fault zone heterogeneities. The fully dynamic calculations illustrate the evolution of nucleation phases of instabilities associated with accelerating and expanding creep. Final slip values of model earthquakes in full elastodynamic calculations are larger than those of corresponding quasi‐static and quasi‐dynamic events. The dynamic overshoot in simulations with the slip version of friction is larger than in those employing the ageing/slowness law.

Semiconductor nanowires can function as both gain medium and optical cavity, and thus represent a unique class of miniaturized laser sources for the assembly of nanoscale photonic systems. In this talk we will review the rational design and synthesis of nanowires and nanowire heterostructures as laser sources, describe how their structure design interplays with optical properties, and discuss exciting device applications. Group III-nitride nanowire structures will be used as a model system to illustrate our approach towards nanowire lasers. First, the general synthetic strategy for rational growth of semiconductor nanowires and the underlying physical mechanism of lasing in these materials will be reviewed. Second, structural characterization and photoluminescence studies of homogeneous GaN nanowires will be discussed. These studies will illuminate how basic structural characteristics affect threshold for ultraviolet room-temperature lasing in these homogeneous structures. Third, multicolor nanowire lasers based on InGaN multi-quantum well (MQW) radial nanowire heterostructures will be described. Cross-sectional transmission electron microscopy studies allow direct visualization of well-defined internal interfaces and demonstrate our ability to control quantum well growth down to atomic level. These functional nanowire structures were optically pumped individually to lasing from 380 to 494 nm at room temperature, depending on the alloy composition of MQWs. Key factors contributing to the lasing threshold were evaluated by three-dimensional finite-difference time-domain calculations. Last, current injection schemes for electrically-driven nanowire light-emitting diodes/lasers with the emphasis on n-GaN/InGaN MQW/p-AlGaN/p-GaN radial nanowire heterostructures will be discussed.

Field observations of mature crustal faults suggest that slip in individual events occurs primarily within a thin shear zone, <1–5 mm, within a finely granulated, ultracataclastic fault core. Relevant weakening processes in large crustal events are therefore suggested to be thermal, and to involve the following: (1) thermal pressurization of pore fluid within and adjacent to the deforming fault core, which reduces the effective normal stress and hence also the shear strength for a given friction coefficient and (2) flash heating at highly stressed frictional microcontacts during rapid slip, which reduces the friction coefficient. (Macroscopic melting, or possibly gel formation in silica‐rich lithologies, may become important too at large enough slip.) Theoretical modeling of mechanisms 1 and 2 is constrained with lab‐determined hydrologic and poroelastic properties of fault core materials and lab friction studies at high slip rates. Predictions are that strength drop should often be nearly complete at large slip and that the onset of melting should be precluded over much (and, for small enough slip, all) of the seismogenic zone. A testable prediction is of the shear fracture energies that would be implied if actual earthquake ruptures were controlled by those thermal mechanisms. Seismic data have been compiled on the fracture energy of crustal events, including its variation with slip in an event. It is plausibly described by theoretical predictions based on the above mechanisms, within a considerable range of uncertainty of parameter choices, thus allowing the possibility that such thermal weakening prevails in the Earth.

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