Zero-energy states in Majorana nanowire devices

In the voyage towards solving increasingly challenging computations of physical systems, quantum computation has arisen as a contender for conventional computational approaches. To address the issue of keeping the required quantum mechanical states sufficiently stable against environmental disturbances, novel proposals suggested to employ topological quantum states, where information can be stored nonlocally, essentially by sharing the information over physically different locations. Because suitable topological states are elusive in existing materials, an approach of great interest is to engineer the required topological Majorana modes by combining a spin-orbit coupled semiconductor nanowire exposed to a magnetic field with a superconducting material: a Majorana nanowire. After the first experimental signs of Majorana modes were observed in 2012, it also became clear that the experiments showed deviations from the theoretical expectations and alternative interpretations were suggested. This dissertation explores the intricate physics that emerges in Majorana nanowires, with the aim to find improved Majorana signatures in transport experiments. By addressing disorder at the interface between the nanowire and the superconductor, we find Majorana signatures through the electrical transport through a ballistic tunnel junction, which allows us to exclude certain alternative explanations based on disorder. We also look into two key elements required to obtain Majorana modes: spin-orbit interaction and induced superconductivity. First, through measurements of the effect of a magnetic field and its direction on the size of the induced superconducting gap, we show that spin-orbit interaction counteracts the closing of the superconducting gap. This protection of the superconducting gap is ultimately responsible for the possibility of a topological nontrivial phase in nanowires. Second, we investigate the influence of an electric field in the nanowire on the coupling between electronic states in the nanowire and the superconductor and find that the electric field modifies the strength of the effective nanowire parameters essential to Majorana physics. Returning to the study of transport signatures of Majorana modes, we explore plateaus in the zero-bias conductance near the quantization value predicted for topological Majorana modes. Instabilities of the observed quantized plateaus on tunnel-barrier details indicate instead the presence of topologically trivial zero-energy states, which can be described as local Majorana modes and may offer an alternative route towards the demonstration of non-Abelian exchange statistics. Finally, we address the nonlocal distribution of Majorana nanowire zero-energy states through the modulation of the energy splitting due to a remote electrostatic gate decoupled from the tunneling barrier region. We identify states consistent with overlapping Majorana modes in a short nanowire. The dissertation is concluded by discussing interesting future avenues to solidify the understanding of Majorana nanowires and we indicating a possible alternative approach to demonstrate non-Abelian properties by deliberately stabilizing local Majorana modes.