Steve Simon

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:

The dramatic appearance of luminescence rings with radius of several hundred microns in quantum well structures can be understood through a fairly simple nonlinear model of the diffusion and recombination of electrons and holes in a driven nonequilibrium system. The ring corresponds to the boundary between a positive hole gas and a negative electron gas in steady state. While this basic effect is now well understood, we discuss several other experimental results which cannot be explained by this simple model.

Abstract:

Using a character expansion method, we calculate exactly the eigenvalue density of random matrices of the form M dagger M where M is a complex matrix drawn from a normalized distribution P(M) approximately exp(-Tr [AMB M dagger]) with A and B positive definite (square) matrices of arbitrary dimensions. Such so-called correlated Wishart matrices occur in many fields ranging from information theory to multivariate analysis.

Abstract:

We report on new experiments and theory that unambiguously resolve the recent puzzling observation of large diameter exciton emission halos around a laser excitation spot in two dimensional systems. We find a novel separation of plasmas of opposite charge with emission from the sharp circular boundary between these two regions. This charge separation allows for cooling of initially hot optically generated carriers as they dwell in the charge reservoirs for very long times.