published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

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Nanoscale wires and molecules have remarkable electrical properties that make them well suited for new electronic devices. The projected device densities result in very small separation distances and therefore the possibility of device-device interactions. However, we do not know what impacts wire-wire interactions might have on the properties of closely spaced devices. If two quantum wires interact, what types of effects will there be on transport properties such as conductance? How would the coupling strength, length of wire, position of contact, or the energy of the electrons affect conductance? Understanding the effects of the interactions will assist the construction of efficient nanoscale devices.This thesis examined the effects of interaction on the low-field conductance using a simple classical model and two quantum models of coupled quantum wires fabricated electrostatically in the two-dimensional electron gas at the interface of the heterostructure A1GaAs/GaAs. We considered the effect of position and length of an interaction between two parallel quantum wires formed by hard wall boundaries and connected to electron reservoirs. Our second model consisted of two artificial molecular wires, i.e., parallel chains of quantum dots. We used a one-electron Schrodinger equation in the envelope approximation, a tight-binding Hamiltonian, and a recursive Green's function method to study the electron transport properties. Multi-parameter computations using a fortran-95 computer model provided data for an analysis of the relationships among conductance, the interaction strength, interaction location, and electron energy.In contrast to the monotonic changes predicted by the classical model, the lowfield conductance of interacting hard wall quantum wires varies in an oscillatory manner with the perturbing interaction strength and position. For electron energies below the first conductance plateau, Breit-Wigner resonances appear as a consequence of coupling. These conductance properties are explained with reference to quasi-bound states created by reflections at the end boundaries of the wires and the separating wall.At low electron energies, the conductance signature of a symmetric artificial molecule composed of serial quantum dots is a band of resonances. Coupled artificial molecular wires display a split-off molecular band with an energy separation that grows with the coupling strength and a bandwidth that narrows. The position of the Fermi energy relative to the molecular band states plays a dominant role in determining the lowfield conductance of interacting artificial molecules. The conductance variation with coupling ranges from oscillatory to monotonic, depending on the Fermi energy. Varying the atom-atom coupling position in the molecular wires causes a relatively small shift in the resonance band energies. / Department of Physics and Astronomy

by Chu Wing Fai. / Parallel title in Chinese characters. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1988. / Bibliography: leaves 89-92.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--Brigham Young University. Dept. of Chemistry and Biochemistry, 2006. / Includes bibliographical references (p. 191-200).

Much experimental research has been performed in the equilibrium regime on individual quantum dots and quantum point contacts (QPCs). The focus of the research presented here is electron transport in the nonequilibrium regime in coupled quantum dot and QPC systems fabricated on AlGaAs/GaAs material using the split gate technique. Near equilibrium magnetoconductance measurements were performed on a quantum dot and a QPC. Oscillations were seen in the conductance of the sensor which corresponded to Aharonov-Bohm oscillations in the quantum dot, to our knowledge the first such observation. Sudden jumps in the conductance of the QPC were observed under certain gate biases and under certain magnetic fields. When the gate biases and magnetic field were held constant and the conductance was observed over time, switching was observed with the form of a random telegraph signal (RTS). RTS switching is usually attributed to charging of a single impurity. However, in this case switching may have been due to tunneling via edge states in the dot. Nonequilibrium transport in single quantum dots was investigated. A knee or kink was observed in the current-voltage characteristics of two dots on different material. The bias conditions under which the knee occurred point to electron heating as the physical mechanism for the observed behavior. However, the data can not be fit accurately over all bias ranges with an energy balance hot electron model. Modifications to the model are needed to accurately represent the devices studied here. Finally, the effect of nonlinear transport through a one dimensional (1D) QPC on the equilibrium conductance of an adjacent OD quantum dot was explored. This was the first attempt to observe Coulomb drag between a OD and 1D system. It was observed that the equilibrium conductance peaks in the quantum dot were broadened as the current in the QPC increased. This apparent electron heating effect in the dot can be explained by a simple ballistic phonon model. However, reasonable phase coherence times can be estimated from peak fitting using a Breit- Wigner formula which points to a Coulomb interaction. More detailed numerical calculations should illuminate the dominant scattering processes. / Graduation date: 1999