Quasi-Majorana bound states in semiconductor-superconductor heterostructures

Date: 
Wednesday, February 20, 2019 - 4:00pm
Event Type: 

Originally proposed in high energy physics as particles that are their own anti-particles, Majorana fermions have yet to be observed experimentally. However, possible signatures of their condensed matter analog, zero energy, charge neutral, quasiparticle excitations, known as Majorana zero modes (MZMs), are beginning to emerge in experimental data. Recent tunneling transport experiments involving quantum dot-semiconductor nanowire systems with proximity-induced superconductivity, strong Rashba spin-orbit coupling, and an applied magnetic field, were capable for the first time of measuring quantized zero bias conductance peaks (ZBCPs), which remain quantized over a range of experimental control parameters. These robust quantized conductance plateaus have long been believed to be the smoking gun signature of the existence of MZMs. In this work, through numerical calculations on a tight-binding model, quantized conductance plateaus of height 2e2/ h are shown to identically arise in these systems as a result of low-energy bound states, generically induced by the quantum dot, in which the component Majorana bound states are partially separated in space without being topological MZMs. These results establish that the observation of quantized conductance plateaus of height 2e2/ h in local charge tunneling experiments does not represent sufficient evidence for the existence of topologically protected MZMs localized at the opposite ends of a wire. Finally, I will outline a two-terminal experiment involving charge tunneling at both ends of the wire capable of distinguishing between the generic quasi-Majorana bound states and the non-Abelian MZMs. So while claims of an experimental breakthrough may be extremely encouraging, they are somewhat premature.

Speaker: 
Dr. Christopher Moore, Clemson University
Host: 
Se Kwon Kim