The influence of short range forces on melting along grain boundaries

We investigate a model which couples diffusional melting and nanoscale structural forces via a combined nano-mesoscale description. Specifically, we obtain analytic and numerical solutions for melting processes at grain boundaries influenced by structural disjoining forces in the experimentally relevant regime of small deviations from the melting temperature. Though spatially limited to the close vicinity of the tip of the propagating melt finger, the influence of the disjoining forces is remarkable and leads to a strong modification of the penetration velocity. The problem is represented in terms of a sharp interface model to capture the wide range of relevant length scales, predicting the growth velocity and the length scale describing the pattern, depending on temperature, grain boundary energy, strength and length scale of the exponential decay of the disjoining potential. Close to equilibrium the short-range effects near the triple junctions can be expressed through a contact angle renormalisation in a mesoscale formulation. For higher driving forces strong deviations are found, leading to a significantly higher melting velocity than predicted from a purely mesoscopic description.Comment: 10 page

Kinetic cross coupling between non-conserved and conserved fields in phase field models

We present a phase field model for isothermal transformations of two component alloys that includes Onsager kinetic cross coupling between the non-conserved phase field and the conserved concentration field. We also provide the reduction of the phase field model to the corresponding macroscopic description of the free boundary problem. The reduction is given in a general form. Additionally we use an explicit example of a phase field model and check that the reduced macroscopic description, in the range of its applicability, is in excellent agreement with direct phase field simulations. The relevance of the newly introduced terms to solute trapping is also discussed

Successful invasions of hymenopteran insects into NW Patagonia

We describe the successful invasion of hymenopteran insects into NW Patagonia. We analyse the importance of the invading species and the characteristics of the invaded community, as well as the role of disturbance on the invasion process, by presenting the most conspicuous of the best documented case studies: the wasps Vespula germanica and Sirex noctilio, the bumblebee Bombus ruderatus, and the leaf cutting ant Acromyrmex lobicornis. In their native habitats, these insects are common and have a wide geographical range. In turn, ecological plasticity appears to be the most important demographic trait related to invasion success shared by these species. We believe that climatic matching between the community invaded and the invader’s native range together with the absence of natural enemies are the community characteristics better related to invasion success. The role played by biotic resistance remains unclear. The successful establishment of the studied cases is related to some extent to resource liberation due to exogenous disturbance, or competitive displacement of a native species. This might suggest that the native hymenoptera community of NW Patagonia is species saturated, which in turn, could imply that species interactions are important in the community structure in environments where physical variables have been regarded as key factors

Anomalous Dynamic Scaling in Locally-Conserved Coarsening of Fractal Clusters

We report two-dimensional phase-field simulations of locally-conserved coarsening dynamics of random fractal clusters with fractal dimension D=1.7 and 1.5. The correlation function, cluster perimeter and solute mass are measured as functions of time. Analyzing the correlation function dynamics, we identify two different time-dependent length scales that exhibit power laws in time. The exponents of these power laws are independent of D, one of them is apparently the classic exponent 1/3. The solute mass versus time exhibits dynamic scaling with a D-dependent exponent, in agreement with a simple scaling theory.Comment: 5 pages, 4 figure

Probing terahertz surface plasmon waves in graphene structures

Epitaxial graphene mesas and ribbons are investigated using terahertz (THz) nearfield microscopy to probe surface plasmon excitation and THz transmission properties on the sub-wavelength scale. The THz near-field images show variation of graphene properties on a scale smaller than the wavelength, and excitation of THz surface waves occurring at graphene edges, similar to that observed at metallic edges. The Fresnel reflection at the substrate SiC/air interface is also found to be altered by the presence of graphene ribbon arrays, leading to either reduced or enhanced transmission of the THz wave depending on the wave polarization and the ribbon width.Comment: accepted for publication in Applied Physics Lette

Phase Field Modeling of Fast Crack Propagation

We present a continuum theory which predicts the steady state propagation of cracks. The theory overcomes the usual problem of a finite time cusp singularity of the Grinfeld instability by the inclusion of elastodynamic effects which restore selection of the steady state tip radius and velocity. We developed a phase field model for elastically induced phase transitions; in the limit of small or vanishing elastic coefficients in the new phase, fracture can be studied. The simulations confirm analytical predictions for fast crack propagation.Comment: 5 pages, 11 figure

Gold substrate-induced single-mode lasing of GaN nanowires

We demonstrate a method for mode-selection by coupling a GaN nanowire laser to an underlying gold substrate. Multimode lasing of GaN nanowires is converted to single-mode behavior following placement onto a gold film. A mode-dependent loss is generated by the absorbing substrate to suppress multiple transverse-mode operation with a concomitant increase in lasing threshold of only ∼13%. This method provides greater flexibility in realizing practical single-mode nanowire lasers and offers insight into the design of metal-contacted nanoscale optoelectronics