Quantum transport in hexagonal boron nitride-carbon nanotube heterostructures

Abulizi, Gulibusitan. Quantum transport in hexagonal boron nitride-carbon nanotube heterostructures. 2017, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_12901

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Carbon nanotubes (CNTs) are a versatile fundamental building block for classical small scale electronics and quantum electronics, and for the investigation of novel quantum states. However, the ideal properties of CNTs are usually masked by electrical potential fluctuations induced by the substrate. In this context, realization of disorder-free and clean CNT devices with outstanding quality is crucial for the fundamental studies of various interesting phenomena, such as Fabry-Perot interference, Klein tunneling, tunable quantum dots (QDs), spin-orbit interactions, valley spin-blockade, and the formation of Luttinger liquids, Wigner crystals, and Wigner molecules in one-dimensional (1D) systems.
In this PhD project, we introduce a new production scheme, where we combine the two-dimensional (2D) hexagonal boron nitride (hBN) with quasi-1D CNTs. This new approach aims to improve the device quality significantly, and eventually allow us to explore the electronic transport properties of CNTs in extended 1D geometries. In particular, we investigate hBN as clean substrates for CNT QDs, insulators for top finger gates, tunnel barriers to CNTs, and to fully encapsulate the CNTs. Our results are very promising first steps in the fabrication of substrate-bound very clean CNT devices. This allows us to explore many advantageous properties of CNTs in more versatile structures than possible in two-terminal devices with “ultra-clean” suspended CNTs.
This thesis is structured as follows. In Chapter 2, we introduce the theoretical background of the studied material systems, namely, the CNTs and hBN, with a focus on the basics of the CNT QDs. We discuss the superconductivity phenomena that may occur when a CNT is brought into contact with superconductors. Chapter 3 describes the fabrication details of hBN-CNT heterostructures and demonstrates the low-temperature measurement set-up. The main results of this thesis are presented in Chapters 4-7. We investigate the scanning electron microscopy (SEM) imaging contrast for locating CNTs on hBN flakes in Chapter 4. We discuss the low-temperature characteristics of CNT QDs fabricated on hBN flakes and of the dual-gated CNT QD devices with hBN top-gate dielectrics. We demonstrate that very good electrical device quality and stability can be achieved simply by introducing hBN flakes into the system. In Chapter 5, we focus on the CNT devices with atomically thin hBN tunnel barriers. We first characterize a CNT parallel double-QD, where we study the avoided crossings observed in its finite bias spectroscopy. In the second part of Chapter 5, we turn to the discussion on challenges of integrating atomically thin hBN into a CNT device. In Chapter 6, hBN encapsulated CNTs with zero-dimensional (0D) normal metal side contacts are investigated, while devices with 0D superconducting side contacts are characterized in Chapter 7. We demonstrate that low contact resistance with high-yield can be realized with 0D side contacts. This system allows us to study induced superconductivity in hBN encapsulated CNTs, where different transport regimes are identified. In an intermediate coupling regime, we observe Coulomb blockade, quasiparticle transport, resonant Andreev tunneling, and Andreev bound states, while in a strong coupling regime, multiple Andreev reflections and the magnetic field dependence of the critical current are discussed. Chapter 8 summarizes the experimental results and provides an outlook.
Advisors:Schönenberger, Christian and Grove-Rasmussen, Kasper and Meyer, Carola
Faculties and Departments:05 Faculty of Science > Departement Physik > Physik > Experimentalphysik Nanoelektronik (Schönenberger)
UniBasel Contributors:Abulizi, Gulibusitan and Schönenberger, Christian
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:12901
Thesis status:Complete
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (iv, 110 Seiten)
Identification Number:
Last Modified:28 Dec 2018 05:30
Deposited On:27 Dec 2018 14:19

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