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Coherent electron and hole dynamics in semiconductor rings


Developments in materials synthesis and processing have allowed, for some time, the exploration of coherent electron dynamics in condensed matter systems. These studies, typically at low-temperatures and in highly pure and defect-free systems, reveal interesting properties in materials themselves. However, and perhaps more interestingly, these studies also probe the detailed quantum mechanical nature of the charge/current carriers in those structures, providing unique opportunities to study quantum coherence, quantum entanglement and the role of inter-particle interactions in the dynamical evolution of systems. The insights gained so far typically involve better understanding and the ability to manipulate the dynamical degrees of freedom of electrons in materials. And yet, the electronic spin has remained somewhat more elusive and difficult to study, since its coupling to external magnetic fields is through relatively slow probes which cannot provide spatially-localized couplings (and typically affect rather large regions). Consideration of the weak spin-orbit coupling on the electron dynamics, an intrinsically relativistic effect, has however allowed a new fast probe of spin dynamics in electronic systems by the application of fast gate voltages and electric fields. The combination of nanometer-scale structures designed on highly pure and well characterized materials (especially semiconductors), as well as the availability of strong electric fields that can access (via spin-orbit coupling) the spin degree of freedom of the electrons, promises a great deal of flexibility. The studies proposed here explore some of the richness of phenomena in nanoscale semiconductor systems, and the possibility of accessing the spins of the electrons in a deterministic fashion. The proposed visit of Prof. Sergio Ulloa, Ohio University, to UFSCar will deepen a long-standing collaboration between the two groups. More recent work has focused on the exploration and interplay of Berry phases and spin-orbit coupling (through the Rashba effect), and how they affect the electronic structure of semiconducting rings. That work also explored the observable consequences of rings and applied fields on the transport properties, accessible through controlled measurements of conductance through such structures. Building on our understanding of coherent effects for electrons in semiconductor rings, we plan to extend our work along the following three main lines: (i) developing appropriate models and theory to study the problem of Berry phases in hole systems using multiple band models with different angular momentum components and corresponding different Rashba effect amplitudes; (ii) study the role of electron-electron interactions, as the excitonic spectrum will be expected to also reflect the role of Berry and Rashba phases; (iii) in a more ambitious extension of the problem, we propose to explore the features of Majorana states that may appear near impurities embedded in such semiconductor ring structure, when the ring is placed in close proximity to a superconductor surface. (AU)

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