Charge Transport in Semiconductor Nanowire Quantum Devices: From Single Quantum Dots to Topological Superconductors
Abstract: This thesis focuses on charge transport in semiconductor InSb nanowire quantum devices, including the electron transport, the hole transport, and the Cooper pair transport. Devices in which InSb semiconductor nanowire quantum dots are coupled with normal metals, superconductors or the proximity effect induced topological superconductors are fabricated and measured. Firstly, we have fabricated and measured normal metal contacted InSb nanowire devices. In each of these devices, a quantum dot is formed in the InSb nanowire between the contacts. We report on the magnetotransport measurements performed to these quantum dot devices, and reveal several novel transport features. First, we demonstrate the ambipolar quantum dot devices in which the quantum dots can be tuned from the n-type regime to the p-type regime. The transport measurements in both of the n-type regime and the p-type regime are performed. We also show that two methods can be used to estimate the effective g-factor of the quantum dot, but they can give very different estimation in the presence of a Kondo effect. In the p-type regime of an ambipolar quantum dot, we observe conductance peaks in the stability diagram which can be attributed to the quasi-1D lead states. Secondly, we have fabricated and characterized the superconductor coupled InSb nanowire quantum dots. We probe the density of states of the quasi-particles in the superconductor contacts, via a weakly coupled InSb quantum dot. In the strongly coupled InSb nanowire-superconductor junctions, dissipationless Josephson currents are observed. A SQUIDS device is also fabricated and measured, in which an anomalous modulation of the Josephson current in the magnetic field is observed. In the medium coupling regime, we observe the signatures of the multiple Andreev reflections, the sub-gab bound states, and the Josephson current, interplaying with the Kondo effect. By adjusting the gate voltages, we can control the dot-lead coupling strength and asymmetry. Here, we report the quantum phase transition induced by tuning the dot-lead coupling and the quantum phase transition induced by a magnetic field. We have also found the coupling asymmetry is very important for the observation of the Josephson current. In the magnetic field, the evolution of the Kondo effect enhanced Josephson current is found to be strongly dependent on the energy ratio of Kondo energy and superconducting gap. Finally, an anomalous low-field suppression of the zero-bias conductance peak in the Kondo regime is observed. In the last part of the thesis, we report on our efforts to search for Majorana fermions in solid state systems. Nb-InSb nanowire quantum dot-Nb hybrid devices were fabricated and the transport measurements were performed at low temperatures for these devices. We have observed anomalous zero-bias conductance peaks emerging in finite magnetic fields in the Nb-InSb nanowire quantum dot-Nb hybrid devices as a signature of the Majorana bound states in such hybrid devices. We have also found that the zero-bias conductance peak are independent of the even-odd parity of the quasi-particle number in the quantum dots and are associated with interesting fine structures. As a validation, a Au-InSb nanowire quantum dot-Nb device is fabricated and measured. Here, signatures of Majorana bound states, i.e., the zero-bias conductance peaks in finite magnetic fields are also observed. In addition, we analyze several other mechanisms that can lead to the emergence of zero-bias conductance peaks in finite magnetic fields, and discuss the results in comparison with the signatures of Majorana bound states.
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