Condensed Matter Theory

Abstract:

We experimentally determine the density dependence of the transport lifetime (τ t) obtained from low-field Hall measurements and the quantum lifetime (τ q) derived from analysis of the amplitude of Shubnikov-de Haas oscillations in a tunable high mobility two-dimensional electron gas (2DEG) in a Al 0.06Ga 0.94N/GaN heterostructure. Using an insulated gate structure, we are able to tune the 2DEG density from 2 × 10 11 to 2 × 10 12 cm -2, and thus, monitor the evolution of the scattering times in a single sample at T=0.3 K in a previously unexplored density regime. The transport lifetime τ t is a strong function of electron density, increasing from ∼2.7 ps at n e=2 × 10 11 cm -2 to ∼11 ps at n e= 1.75 × 10 12cm -2. Conversely, we find that the quantum scattering time τ q is relatively insensitive to changes in electron density over this range. The data suggest that dislocation scattering accounts for the density dependence of τ q as well as τ t in our low-density sample. © 2004 American Institute of Physics.

Abstract:

We present an experimental study of the coiling instability of a liquid "rope" falling on a solid surface. Coiling can occur in three different regimes--viscous, gravitational, or inertial--depending on the fluid viscosity and density, the fall height, and the flow rate. The competition among the different forces causes the coiling frequency first to decrease and subsequently to increase with increasing height. We also observe an oscillation between two coiling states in the gravitational-to-inertial transitional range, reflecting the multivaluedness of the dependence of coiling frequency on fall height. The data can be rescaled in a universal way, and agree very well with numerically predicted coiling frequencies.

Abstract:

We use the stochastic rotation dynamics algorithm to investigate the packaging of flexible and semiflexible polymers into a capsid that is permeable to solvent molecules. The model takes into account hydrodynamic interactions arising due to local flow. The flexible chain maintains a random configuration as it is being fed into the capsid, in contrast to the semiflexible chain, whose configuration is initially spool-like, becoming more random at high packing. We measure the packing rate, which is found to decrease with the percentage of the chain packed and highlight the difference between the flexible and semiflexible chains. Reflecting experiments, we find pauses in the packing process for individual chains as the motor loses grip of the fluctuating beads. We also find that hydrodynamics is important, in that the packaging rate is faster when flow is included.