Quantum Transport in Superconductor-Semiconductor Nanowire Hybrid Devices

Sammanfattning: In this thesis, we study quantum transport properties of superconductor-semiconductor nanowire hybrid devices. We mainly focus on the quantum transport in InSb nanowire Josephson quantum dot devices, i.e. quantum dots coupled to two pieces of superconductors. In such a structure, many-body interacting phenomena occur, such as Kondo correlation, multiple Andreev reflection, and the formation of Andreev bound states (ABSs) and Majorana bound states (MBSs).InSb nanowire is an ideal candidate for electronic and spintronic applications. Among all of the binary III-V semiconductors, InSb (bulk) has the narrowest band gap (Eg∼0.17 eV), the highest electron mobility (μe∼77,000 cm2V-1s-1), the smallest effective electron mass (m'∼0.014 me), the largest electron g'-factor (g'∼51), and the largest spin-orbit interaction (α up to 100 meV·nm). InSb nanowire is therefore predicted to be a promising harbouring system for MBSs --- a key ingredient for topological quantum computing --- when it is coupled to a s-wave superconductor. When an InSb nanowire is coupled to two s-wave superconductor leads (Al or Nb in this thesis), a quantum dot is naturally formed in the nanowire segment between the superconductors. Such a device can be configured to various regimes upon applying different strength of external electrical field and magnetic field. In particular, with a Zeeman field perpendicularly applied to the spin-orbit field direction, the device is predicted to host two pairs of MBSs mediated by the inter-quantum-dot, which will give rise to a zero-bias conductance peak (ZBCP) in the tunnelling spectrum as a signature of MBSs.However, except MBSs, other trivial mechanism can also cause emergent ZBCPs in a magnetic field, e.g. recovered Kondo resonances by a magnetic field, ABSs at quantum phase transition, etc. We hence take the emergent ZBCPs in magnetic field, both trivial and nontrivial, as the main thread of this thesis. We explore their physical origins and related physics with the main focuses on transport features in the Kondo-superconductor competing regime, lead state detection via a p-type quantum dot, anomalous negative magnetoresistance with Kondo correlations, and parity-independent ZBCPs. These measurements and discussion aim to be one step closer of the understandings of how a single magnetic impurity interacts with a superconductor, how to distinguish topologically trivial/non-trivial physics, and unveil more novel physics in the Josephson quantum dot structures.

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