Quantum-coherent transport in low-dimensional mesoscopic structures and thin films
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This thesis experimentally studies quantum interference phenomena and quantum coherence in mesoscopic systems, and quantum transport as well as magnetotransport in various materials system. One overarching aim is exploring the different mechanisms that give rise to quantum phase decoherence in lithographically patterned mesoscopic structures, of importance in the field of quantum technologies and spintronics. Various mesoscopic structures, namely quantum stadia, quantum wires, and side-gated rings, were fabricated to function as quantum interference devices and platforms to study quantum coherence on two-dimensional electron systems in InGaAs/InAlAs heterostructures. The mesoscopic structures were fabricated by photolithography and electron-beam lithography. The dependence of quantum coherence on geometry or temperature is investigated for each of the quantum interference devices. In the case of quantum stadia, phase coherence lengths were extracted by universal conductance fluctuations, and the extracted phase coherence lengths show a dependence on both temperature and geometry. Phase coherence lengths decreased with increasing temperature, as expected. Moreover, phase coherence lengths also varied with the width-to-length ratio and length of the side wires connected to the stadia, where competition between Nyquist decoherence and environmental coupling decoherence mechanisms coexists. For the quantum wires studied, the phase coherence lengths were extracted from antilocalization signals. Antilocalization measurements provide a sensitive mean of probing the quantum mechanical correction to electronic transport. The phase coherence lengths increased as the wire length increased, due to reduction of the environmental coupling that induces decoherence at the ends of a wire; longer wires tend to have longer phase coherence lengths. In related work, the thesis shows that the spin coherence length, as limited by spin-orbit interaction, increases as the wire width decreases. Decoherence in side-gated rings was measured from the amplitudes of the quantum-mechanical Aharonov-Bohm oscillations. The side gates allow for an in-plane controllable electric field. Asymmetrically biased side-gate voltages allow for the breaking of the two-dimensional parity symmetry of the ring device, effectively resulting in reduced amplitude of the Aharonov-Bohm oscillations. The mechanism that contributes to decoherence in these rings appears to be related to the breaking of the spatial symmetry. Measurements of antilocalization and weak-localization as well as magnetotransport were used to probe interesting or unique quantum mechanical phenomena in the following two, quite different, materials system: bismuth iridate thin films, and Ge/AlAs heterostructures on GaAs or Si substrates. Both materials are of interest for future quantum technologies and devices. Measurements in bismuth iridate thin films reveal interesting transport characteristics such as logarithmic temperature dependence of the resistivity, multiple charge carriers, and antilocalization due to spin-orbit interaction in the system. Weak localization measurements in the Ge/AlAs heterostructure on GaAs or Si substrates show that single carrier transport is essentially located in the Ge layer only. Further, the weak localization results indicate the near-absence of spin-orbit interaction for carriers in the electronically active Ge layer, suggesting the potential use of this materials system as a promising candidate for future electronic device applications. In short, quantum transport and interference measurements probe the quantum-mechanical behavior of materials system for future quantum, spin and electronic technologies. Mesoscopic patterned geometries in InGaAs/InAlAs heterostructures offer a wide range of interesting and unique platforms to study quantum-mechanical phenomena, specifically quantum decoherence, in the solid state. The decoherence phenomena observed and the investigations to the underlying mechanisms studied and modeled in this thesis may be transferred to similar materials system, enriching the knowledge in the field of quantum technologies. Magnetotransport and quantum transport were also applied to Ge/AlAs heterostructures and bismuth iridate thin films, to study the properties of their carrier systems.
- Doctoral Dissertations