SPIN TRANSPORT MEASUREMENTS IN GaAs QUANTUM DOTS

This thesis explores the spin dynamics of lateral quantum dots in GaAs/AlGaAs heterostructures, including the first demonstration of a geometry capable of measuring mesoscopic spin currents. The term “quantum dot” as used in this thesis refers to partiallyconfined regions connected to two leads by quantum point contacts. Because the quantum dots investigated in these experiments are gate-tunable, they are used to probe the range from open systems (dot conductance g > e2/h) to confined electron systems (dot conductance g < e2/h). In open dots, there is at least one fully transmitting channel of conductance in both the entrance and exit point contacts, and transport is characterized by conductance fluctuations. These fluctuations are analogous to the universal conductance fluctuations (UCF) observed in disordered systems, and reflect the quantum interference of transport paths through the device. In Chapter 2 we explore the degree to which the spin physics of such systems is reflected in their conductance, and shows non-trivial effects such as spin-orbit interaction or broken spin degeneracy. Even a simple picture of the spin physics in open dots, however, would predict that at large in-plane fields the transport through such a device would consist of spin-polarized electrons. This is explored in Chapter 3: here we present, for the first time, a direct measurement of the spin currents emitted from open quantum dots (and other mesoscopic structures) at high field. We find that the transport behavior of open dots is indeed consistent with a simple picture of the two spin species, separated by a Zeeman energy, undergoing uncorrelated spin-resolved conductance fluctuations and therefore leading to fluctuations in the spin polarization of emitted current. Finally, in Chapter 4, the measurement of quantum dot spin states is extended into the Coulomb blockade regime. Energy spectroscopy measurements of quantum dot ground and excited states show clear signatures of ground state spin transitions as consecutive electrons