Charge Transport in Semiconductor Nanowire Quantum Devices: From Single Quantum Dots to Topological Superconductors
(2013) 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... (More)  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 ntype regime to the ptype regime. The transport measurements in both of the ntype regime and the ptype regime are performed. We also show that two methods can be used to estimate the effective gfactor of the quantum dot, but they can give very different estimation in the presence of a Kondo effect. In the ptype regime of an ambipolar quantum dot, we observe conductance peaks in the stability diagram which can be attributed to the quasi1D lead states.
Secondly, we have fabricated and characterized the superconductor coupled InSb nanowire quantum dots. We probe the density of states of the quasiparticles in the superconductor contacts, via a weakly coupled InSb quantum dot. In the strongly coupled InSb nanowiresuperconductor 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 subgab bound states, and the Josephson current, interplaying with the Kondo effect. By adjusting the gate voltages, we can control the dotlead coupling strength and asymmetry. Here, we report the quantum phase transition induced by tuning the dotlead 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 lowfield suppression of the zerobias 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. NbInSb nanowire quantum dotNb hybrid devices were fabricated and the transport measurements were performed at low temperatures for these devices. We have observed anomalous zerobias conductance peaks emerging in finite magnetic fields in the NbInSb nanowire quantum dotNb hybrid devices as a signature of the Majorana bound states in such hybrid devices. We have also found that the zerobias conductance peak are independent of the evenodd parity of the quasiparticle number in the quantum dots and are associated with interesting fine structures. As a validation, a AuInSb nanowire quantum dotNb device is fabricated and measured. Here, signatures of Majorana bound states, i.e., the zerobias conductance peaks in finite magnetic fields are also observed. In addition, we analyze several other mechanisms that can lead to the emergence of zerobias conductance peaks in finite magnetic fields, and discuss the results in comparison with the signatures of Majorana bound states. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/record/4024868
 author
 Deng, Mingtang ^{LU}
 supervisor

 Hongqi Xu ^{LU}
 opponent

 Professor Hirakawa, Kazuhiko, Institute of Industrial Science, University of Tokyo, Japan
 organization
 publishing date
 2013
 type
 Thesis
 publication status
 published
 subject
 keywords
 Multiple Andreev reflection, SQUID, Proximity effect, the Josephson effect, Cooper pair, the Kondo effect, the Zeeman effect, Quantum dot, InSb, Nanowires, Transport, Fysicumarkivet A:2013:Deng, Majorana bound state, YuShibaRusinov state, Andreev bound state
 defense location
 Lecture hall Rydbergsalen, Department of Physics, Professorsgatan 1, Lund University, Faculty of Engineering
 defense date
 20131011 09:15:00
 ISBN
 9789174736908 (pdf)
 9789174736908 (print)
 language
 English
 LU publication?
 yes
 id
 8d86e82eb8ce4a469bb330090ddff4d1 (old id 4024868)
 date added to LUP
 20160404 14:02:49
 date last changed
 20181122 17:08:25
@phdthesis{8d86e82eb8ce4a469bb330090ddff4d1, 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.<br/><br> <br/><br> 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 ntype regime to the ptype regime. The transport measurements in both of the ntype regime and the ptype regime are performed. We also show that two methods can be used to estimate the effective gfactor of the quantum dot, but they can give very different estimation in the presence of a Kondo effect. In the ptype regime of an ambipolar quantum dot, we observe conductance peaks in the stability diagram which can be attributed to the quasi1D lead states. <br/><br> <br/><br> Secondly, we have fabricated and characterized the superconductor coupled InSb nanowire quantum dots. We probe the density of states of the quasiparticles in the superconductor contacts, via a weakly coupled InSb quantum dot. In the strongly coupled InSb nanowiresuperconductor 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 subgab bound states, and the Josephson current, interplaying with the Kondo effect. By adjusting the gate voltages, we can control the dotlead coupling strength and asymmetry. Here, we report the quantum phase transition induced by tuning the dotlead 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 lowfield suppression of the zerobias conductance peak in the Kondo regime is observed.<br/><br> <br/><br> In the last part of the thesis, we report on our efforts to search for Majorana fermions in solid state systems. NbInSb nanowire quantum dotNb hybrid devices were fabricated and the transport measurements were performed at low temperatures for these devices. We have observed anomalous zerobias conductance peaks emerging in finite magnetic fields in the NbInSb nanowire quantum dotNb hybrid devices as a signature of the Majorana bound states in such hybrid devices. We have also found that the zerobias conductance peak are independent of the evenodd parity of the quasiparticle number in the quantum dots and are associated with interesting fine structures. As a validation, a AuInSb nanowire quantum dotNb device is fabricated and measured. Here, signatures of Majorana bound states, i.e., the zerobias conductance peaks in finite magnetic fields are also observed. In addition, we analyze several other mechanisms that can lead to the emergence of zerobias conductance peaks in finite magnetic fields, and discuss the results in comparison with the signatures of Majorana bound states.}, author = {Deng, Mingtang}, isbn = {9789174736908 (pdf)}, language = {eng}, school = {Lund University}, title = {Charge Transport in Semiconductor Nanowire Quantum Devices: From Single Quantum Dots to Topological Superconductors}, url = {https://lup.lub.lu.se/search/ws/files/6267623/4024941.pdf}, year = {2013}, }