Thesis presented June 09, 2017

Abstract:
In my thesis, I studied low-temperature electronic transport in semiconductor nanowires coupled to superconductors, with the goal of understanding the requirements to observe Majorana bound states. Unexpectedly, I found dramatic examples of the pervasiveness of spatial localization of electrons even in seemingly ballistic one-dimensional (1D) nanowires. Localization could replicate signatures of one-dimensionality, helicity and Majorana bound states, casting a shadow of doubt on their interpretation.
1D nanowires are expected to show plateaus of quantized conductance. Curiously, transport through an InAs nanowire hosting a single-level quantum dot showed that it could mimic the first two spin-resolved plateaus. A measurement of the Josephson supercurrent under magnetic field revealed the ground-state transitions of an electron occupying this level, confirming its localized nature.
In the helical regime, a conductance dip is predicted to appear in each of the conductance plateaus. Surprisingly, InSb nanowire devices hosting a quantum dot conducting in parallel with a 1D channel reproduced this signature.
The presence of Majorana bound states, in turn, should be revealed by a zero-bias peak (ZBP) in tunnel spectroscopy. In one of the two-path devices mentioned above, when the 1D path was closed, a zero-bias peak emerged inside the superconducting gap under a magnetic field parallel to the nanowire. This ZBP was related to trivial Andreev bound states from the quantum dot in parallel to the 1D channel. In a different experiment done in an InAs nanowire Josephson junction device hosting a quantum dot, a ZBP related to a Josephson supercurrent appeared inside of the superconducting gap as a result of a transition of the ground-state of the dot from a singlet to a doublet.
In spite of localization, it was possible to extract some meaningful information about the 1D regime. The role of the gates was major in determining the degeneracy of the subbands in an InSb nanowire with two 1D conduction paths in parallel under magnetic field. Through a direct influence on their threshold voltages, orbital effects, and g-factors, the gate voltage could shift the energies of the subbands and lock them together. Via this mechanism, it was possible to observe a 2e²/h plateau lasting until very large field without the appearance of a 1e²/h plateau. The possible existence of two quantum wires in a single nanowire opens the door for novel helical and Majorana bound states of fractional nature.
Altogether, these results point to the need of a better understanding of the physics of simpler few-gates short-channel InAs and InSb nanowire superconducting and normal-state devices, before committing to the utterly complex devices that should be fabricated to study and manipulate Majorana bound states, in which signatures of localization could be better hidden. These original results will be published in the coming months in four different articles.

Keywords:
Mesoscopic physics, Low temperature, Superconductivity, Nanowire, Solid state physics, Nanosciences

On-line thesis.