This experimental research project utilizes double layer two- dimensional electron systems to study two exciting topics in quantum Hall research: composite Fermions (CF) and quantum Hall ferromagnetism (QHF). CF's, which are electrons with two magnetic flulx quanta attached, have provided a framework for understanding the diversity of fractional quantum Hall states. QHF, on the other hand, offers a beautiful and useful analogy between quantized Hall states in systems with discrete internal degrees of freedom (like spin or layer index) and ordinary magnetic materials. Remarkable new phenomena, like textural phase transitions and skyrmionic topological excitations, have been discovered in QHF's. In double layer 2D electron gases, simply changing the thickness of the separating barrier allows interpolation between a system best described as two weakly coupled composite fermion liquids to a single quantum Hall ferromagnet. In addition to conventional magneto-transport, novel experimental probes requiring separate electrical contact to the individual layers (e.g., Coulomb drag, tunneling spectroscopy, and counterflow transport) will be employed to uncover important new information in both the CF and QHF limits and, uniquely, on the transition between them. %%%% This experimental research project is devoted to basic properties of two conducting electron systems, each confined into a two- dimensional planar region, when the two systems are parallel and in close proximity separated only by a thin planar tunnel barrier. Such coupled parallel sets of two-dimensional electrons at low temperatures and with high magnetic fields exhibit unusual characteristics. In each layer the effective charge carriers are "composite Fermions", which are electrons with two magnetic flux quanta attached. As the spacing between the two layers is reduced, which increases the interaction, a crossover in behavior is expected from a case of weakly interacting composite Fermion systems to a state called a single quantum Hall ferromagnet. A ferromagnet is a system in which electron magnetic moments are aligned parallel, as in a bar magnet or compass needle. On the one hand the experiments will provide new basic information about electron systems when they are in interaction in confined layers at low temperatures. For example, one experiment will measure the degree to which motion (electrical current) in one system "drags" the second system. On the other hand the device structures which are used in these experiments are similar to those used in the most advanced microelectronic devices in the GaAs technology, and indeed fabricating these devices may lead to innovation in the GaAs technology. This research will involve students who will be well trained in the area of physics and will also gain familiarity with semiconductor device technology, and who will be excellently trained for employment in industry, government, or education. ***
