Physics & Astronomy ETDs

Publication Date

2-9-2010

Abstract

This dissertation describes a series of three experiments focused on low electronic temperature transport and Coulomb drag in GaAs electron-hole bilayers. Electron-hole bilayers are of immense interest for exciton condensation studies since the exciton, predicted to form here, has a comparably light mass. This should lead to condensation at temperatures relatively easily obtained in a 3He-fridge, while the bilayer's device geometry allows for unambiguous detection of condensation effects via Coulomb drag measurements. General transport measurements of each layer are also of interest since an additional source of correlation, via the attractive Coulomb interaction from the nearby layer, is present. These interlayer effects are expected to become more visible as the layer separation is reduced and depend on the densities, relative and total, in each well, as well as the application of an external perpendicular or parallel magnetic field. Exploring these questions, measuring and discussing the results of experiments which probe them, was the main point of this dissertation. The first experiment examined the layer interdependence of transport in an undoped electron-hole bilayer (uEHBL) device with a relatively large 30 nm Al$_{0.9}$Ga$_{0.1}$As barrier between the two quantum wells. The results here were consistent with mobility of each layer being only indirectly dependent on the adjacent layer density and dominated by background impurity scattering. A decreasing interlayer separation, estimated via Coulomb drag measurements, was also observed with increased interlayer electric-field. The other two experiments were centered on the possibility of detecting electron-hole pairing and condensation in the bilayer using Coulomb drag measurements. Hints of condensation were observed in previous bilayer drag experiments and the follow-on experiments, described in this dissertation, sought to further elucidate the nature of these initial effects. The main result previously determined, which the experiments here were intended to elaborate upon, was an upturn in the Coulomb drag signal measured at low temperatures in the hole layer of bilayer devices with relatively small-barrier widths between the wells. The initial upturn results, which are fully reviewed and expanded upon here, ideally indicated a dramatic change in the coupling between the layers and, at least initially, appeared consistent with formation of a superfluid condensate, as predicted over a decade ago.\cite{Vignale1996, Hu2000} However, several issues with the upturn results have arisen since its initial discovery, the main one being the lack of drag symmetry under layer reversal. These issues are discussed in detail, along with some different mechanisms that might account for the phenomena. The other two experiments, related to the upturn phenomena, were studies on the effects of a density imbalance and an external perpendicular or parallel magnetic field. The former showed the drag upturn to be inconsistent with the predictions of electron-hole pairing fluctuations, for which a set of numerical calculations were also done, and have a stronger density dependence than transport theory predicts. The perpendicular magnetic field study showed drag in the upturn regime was barely enhanced by a small perpendicular magnetic field up to the point where oscillations began occurring. A small phase offset between sets of oscillations in the layer resistivity and drag was also observed. Following this, a parallel magnetic field was found to diminish the upturn magnitude. At higher temperatures, above the upturn regime, the drag was enhanced, as expected, in concert with the rise in drive layer magnetoresistivity. All the magnetic field effects were weaker than expected. Summarizing the conclusions from these experiments, the results indicated that in wide-barrier devices the transport was only indirectly affected by the appearance of a second layer, while in the narrow-barrier device there was enhanced drag and a strong departure from Fermi-liquid physics at low temperatures. Experiments on the latter here have shown that it may be due to a phase transition, but that it is not entirely consistent with formation of a superfluid condensate. In the future, newly designed dual-gate devices with separate gates for the central Hall bar and contact regions of the uEHBLs promise more robust fabrication and expanded experimental options, such as lower density and operation at lower temperatures.

Degree Name

Physics

Level of Degree

Doctoral

Department Name

Physics & Astronomy

First Advisor

Dunlap, David

First Committee Member (Chair)

Dunlap, David

Second Committee Member

Lilly, Mike

Third Committee Member

Malloy, Kevin

Fourth Committee Member

Krishna, Sanjay

Fifth Committee Member

Cardimona, David

Sixth Committee Member

Huang, Danhong

Project Sponsors

I acknowledge that this work has been supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.

Language

English

Keywords

Exciton theory, Superfluidity, Charge density waves.

Document Type

Dissertation

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