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Andreev bound states in superconductor-quantum dot chains

Su, Zhaoen (2018) Andreev bound states in superconductor-quantum dot chains. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Andreev bound states in superconductor-quantum dot chains can provide a platform for quantum simulation and topologically protected quantum computation. This thesis focuses on quantum transport in superconductor-semiconductor nanowire hybrid structures. With InSb nanowires, we study Andreev bound states in single, double and triple dot chains. We first implement highly tunable single quantum dots in nanowires coupled to superconductors facilitated by local gates and transparent contacts. We explore the tunneling resonance of Andreev bound states in a wide parameter regime: from co-tunneling regime to spinfull singlet Andreev bound states, and find simultaneous transitions of superconducting and normal transports as the dot is tuned to be strongly coupled to the superconductor. In the open dot regime we investigate the zero bias feature that is strongly relevant to Majorana zero modes based on continuous nanowire sections. With two copies of this hybrid structure, we study the hybridization of Andreev bound states in a double dot. We observe tunneling spectra of the hybridized Andreev bound states and resolve their spin structure. Finally we implement a chain made of three superconductors and three quantum dots in series. Each dot is strongly coupled to a superconductor and has a single electron near the superconductor chemical potential. Spectroscopy measurement shows hybridization of Andreev bound states in the triple dot. A zero-bias peak is observed when a magnetic field is applied and it sustains in magnetic fields for a wide range, which can provide a signature of Majorana zero modes in this chain structure. We also evaluate the potential of Ge/Si core/shell nanowires for the realization of Majorana zero modes. To that end we establish three of the necessary ingredients for realizing Majorana zero modes based in nanowires: we achieve induced superconductivity from NbTiN, we estimate spin-orbit coupling ($l_{SO}\approx 100-500$ nm) based on spin blockade, and we measure g-factors (up to 8) in Ge/Si double dots.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Su, Zhaoenzhs19@pitt.eduzhs19
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairSergey,
Committee MemberDutt, M. V.
Committee MemberLiu, W.
Committee MemberSavinov,
Committee MemberXiao,
Date: 31 January 2018
Date Type: Publication
Defense Date: 15 September 2017
Approval Date: 31 January 2018
Submission Date: 9 August 2017
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 185
Institution: University of Pittsburgh
Schools and Programs: Dietrich School of Arts and Sciences > Physics
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Andreev bound states, quantum dots, nanowires, chain, Majorana zero modes
Date Deposited: 31 Jan 2018 18:55
Last Modified: 31 Jan 2018 18:55


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