Crystal growth from a supersaturated melt: relaxation of the solid-liquid dynamic stiffness

We discuss the growth process of a crystalline phase out of a metastable over-compressed liquid that is brought into contact with a crystalline substrate. The process is modeled by means of molecular dynamics. The particles interact via the Lennard-Jones potential and their motion is locally thermalized by Langevin dynamics. We characterize the relaxation process of the solid-liquid interface, showing that the growth speed is maximal for liquid densities above the solid coexistence density, and that the structural properties of the interface rapidly converge to equilibrium-like properties. In particular, we show that the off-equilibrium dynamic stiffness can be extracted using capillary wave theory arguments, even if the growth front moves fast compared to the typical diffusion time of the compressed liquid, and that the dynamic stiffness converges to the equilibrium stiffness in times much shorter than the diffusion time

Self-Diffusion in Random-Tiling Quasicrystals

The first explicit realization of the conjecture that phason dynamics leads to self-diffusion in quasicrystals is presented for the icosahedral Ammann tilings. On short time scales, the transport is found to be subdiffusive with the exponent $\beta\approx0.57(1)$, while on long time scales it is consistent with normal diffusion that is up to an order of magnitude larger than in the typical room temperature vacancy-assisted self-diffusion. No simple finite-size scaling is found, suggesting anomalous corrections to normal diffusion, or existence of at least two independent length scales.Comment: 11 pages + 2 figures, COMPRESSED postscript figures available by anonymous ftp to black_hole.physics.ubc.ca directory outgoing/diffuse (use bi for binary mode to transfer), REVTeX 3.0, CTP-TAMU 21/9

Magnetic Diffusion in Star Formation

Magnetic diffusion plays a vital role in star formation. We trace its influence from interstellar cloud scales down to star-disk scales. On both scales, we find that magnetic diffusion can be significantly enhanced by the buildup of strong gradients in magnetic field structure. Large scale nonlinear flows can create compressed cloud layers within which ambipolar diffusion occurs rapidly. However, in the flux-freezing limit that may be applicable to photoionized molecular cloud envelopes, supersonic motions can persist for long times if driven by an externally generated magnetic field that corresponds to a subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid magnetic diffusion (through Ohmic dissipation with additional support from ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By doing so, it also allows the formation of a centrifugal disk, thereby avoiding the magnetic braking catastrophe.Comment: 5 pages, 4 figures. Conference proceedings of IAU Symposium 270, Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble

From Labyrinthine Patterns to Spiral Turbulence

A new mechanism for spiral vortex nucleation in nongradient reaction diffusion systems is proposed. It involves two key ingredients: An Ising-Bloch type front bifurcation and an instability of a planar front to transverse perturbations. Vortex nucleation by this mechanism plays an important role in inducing a transition from labyrinthine patterns to spiral turbulence. PACS numbers: 05.45.+b, 82.20.MjComment: 4 pages uuencoded compressed postscrip