Coulomb Blockade and Few-Electron Energy Spectra of Quantum Rings

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This Condensed Matter Physics project deals with nanoscale quantum conductors that have a circular, i. e., "ring" geometry. Ideal quantum rings have a number of fascinating properties: Ground-state persistent currents, strong Aharonov-Bohm interference effects, and an oscillatory dependence of the wavefunctions and all thermodynamic quantities on the enclosed magnetic flux, with period given by the flux quantum. This research will focus on the properties of individual and multiply-connected quantum rings in the few-electron and Coulomb blockade regime. The PI has recently discovered that inhomogeneous strain relaxation in sufficiently small Si/SiGe quantum dots leads to ring-like confinement. Additionally, tunneling measurements have confirmed the expected flux quantum periodicity. Given the control over device geometry, elliptical rings and coupled ring structures will be fabricated and few-carrier energy spectra, spin interactions, and Coulomb blockade effects will be measured as these rings are filled by carriers from zero carrier occupancy. The work will be coupled to state-of-the-art finite-element solid mechanics simulations. Experiment data will provide feedback on the interplay between electronic and mechanical properties in nanostructures. Graduate students involved in this research will acquire a full complement of skills, from deep-submicron fabrication, to low-temperature, low-signal measurements, to numerical simulation, which will prepare them for employment in academia, the semiconductor industry, or government. A teaching plan focusing on improved undergraduate teaching of core courses in the Electrical Engineering program is an integral part of this project. Ideal quantum rings, in which carriers follow circular orbits without scattering, have fascinating properties: Currents that flow without resistance, phase-interference effects, and the oscillatory dependence of carrier energies and wavefunctions on the enclosed magnetic flux. Quantum rings have also been suggested as possible building blocks for quantum computation. This research will focus on the properties of individual and multiply-connected quantum rings in the few-electron and Coulomb blockade regime, where the one carrier circling the ring affects the probability of another tunneling into the ring. The rings in question arise from the strain relaxation at the sidewalls of etched structures, making it possible to control the geometry (circular or elliptical) and the topology (single-ring or multiply-connected ring) of the system. Tunneling current measurements will study the carrier and spin interactions as these rings are filled by carriers from zero; the results will be compared to state-of-the-art finite-element strain simulations and our data will provide experimental feedback on the interplay between electronic and mechanical properties of nanostructures. Graduate students involved in this research will acquire a full complement of skills, preparing them for research jobs in academia, the semiconductor industry, or government. A teaching plan focusing on improved undergraduate teaching of core courses in the Electrical Engineering program is an integral part of the project.