Richard Gass

Carrier Dynamics and Quantum Confinement in type II ZB-WZ InP Nanowire Homostructures

We use time-resolved photoluminescence from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to measure the carrier dynamics of quantum confined excitons in a type-II homostructure. The observed recombination lifetime increases by nearly 2 orders of magnitude from 170 ps for excitons above the conduction and valence band barriers to more than 8400 ps for electrons and holes that are strongly confined in quantum wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. A simple computational model, guided by detailed high-resolution transmission electron microscopy measurements from a single nanowire, demonstrates that the dynamics are consistent with the calculated distribution of confined states for the electrons and holes.

Maximum entropy method of obtaining thermodynamic properties from quantum Monte Carlo simulations

We describe a method of obtaining thermodynamic properties of quantum systems using Bayesian inference maximum entropy techniques. The method is applicable to energy values sampled at a discrete set of temperatures from quantum Monte Carlo simulations. The internal energy and the specific heat of the system are easily obtained as are errorbars on these quantities. The entropy and the free energy are also obtainable. No assumptions as to the specific functional form of the energy are made. The use of a priori information, such as a sum rule on the entropy, is built into the method. As a nontrivial example of the method, we obtain the specific heat of the three-dimensional periodic Anderson model.

Domain Wall Spacetimes and Particle Motion

We present a mathematical framework for generating thick domain wall solutions to the coupled Einstein-scalar field equations which are (locally) plane symmetric. This approach leads naturally to two broad classes of wall-like solutions. The two classes include all previously known thick domain walls. Although one of these classes is static and the other dynamic, the corresponding Einstein-scalar equations share the same mathematical structure independent of the assumption of any reflection symmetry. We also exhibit a class of thick static domain wall spacetimes with different asymptotic vacua. Our analyses of particle motion in such spacetimes raises the interesting possibility that static domain walls will possess a unique experimental signature.

The time evolution of interdiffusion in multiple quantum wells: a new model of impurity-induced compositional intermixing

A new model for impurity-induced compositional interdiffusion which depends explicitly on the time evolution of the impurity-induced vacancy spatial profile is explored. The inclusion of a new phenomenological term depending on the time derivative of the vacancy spatial profile provides a time scale, as well as a depth profile of the resulting interdiffusion. Calculations are presented as a function of time for a variety of vacancy concentrations and contrasted to the calculations using the model of Kahen, Rajeswaren, and Lee. Our model generates good agreement with a range of experiments including Si-focused ion beam implantation experiments in AlGaAs multiple quantum wells.