Condensed Matter Seminar: "Experiments with Majorana fermions and quantum point contacts."

Note the unusual weekday! -- Majorana modes in hybrid superconductor-semiconductor nanowire devices can be probed via tunneling spectroscopy which shows a zero bias peak (ZBP) in differential conductance. However, alternative mechanisms such as disorder or formation of quantum dots can also give rise to ZBPs, and obscure experimental studies of Majoranas. Further, a soft induced superconducting gap presents an outstanding challenge for the demonstration of their topological protection. In this talk we show that with device improvements, we reach low-disorder transport regime with clear quantized conductance plateaus and Andreev enhancement, approaching the theoretical limit. Tunneling spectroscopy shows a hard induced superconducting gap without any formation of quantum dots. Together with extremely stable ZBPs observed in large gate voltage and magnetic field ranges, we exclude various alternative theories besides the formation of localized Majorana modes. Majoranas are formed when the Zeeman energy E_Z and the chemical potential mu satisfy the condition E_Z>(Delta^2+mu^2)^(1/2), with Delta the superconducting gap. This Majorana condition outlines the topologically non-trivial phase and predicts a particular dependence of ZBPs on the gate voltage (chemical potential) and the external magnetic field (Zeeman). Our gate voltage and magnetic field dependence of ZBPs map out a ZBP phase diagram which is consistent with the Majorana topological phase diagram.