Condensed Matter Physics Seminar

Topological superconductors have been predicted to host exotic bound states commonly referred to as Majorana fermions, which possess non-Abelian exchange statistics.  One can engineer a topological superconductor by coupling a conventional s-wave superconductor (e.g. niobium) with the helical surface states present in a topological insulator (e.g. bismuth selenide).  By placing two closely-spaced niobium leads on the surface of bismuth selenide, a Josephson junction can be realized with supercurrent flowing between the leads mediated by Andreev bound states within the bismuth selenide.  By adding a normal metal lead, one can perform spectroscopic studies of the induced superconductivity.  In this talk, I will describe two types of phase transitions that we observe in such hybrid devices.  First, by tuning the chemical potential with an electrostatic top gate, we can drive the system through a topological phase transition associated with dramatic changes in the temperature dependence of the Josephson supercurrent as well as its phase-sensitive properties.  This transition is caused by the depletion of trivial states and a subsequent change in the transport properties of the surface states.  We find evidence for the emergence of low-energy Andreev bound states, which could include Majorana bound states.  Second, in the presence of a large magnetic field perpendicular to the bismuth selenide surface, we observe multiple abrupt changes in the subgap transport at specific field values.  These signatures are consistent with vortices nucleated within the bismuth selenide, which can localize Majorana bound states.  Our results characterize hybrid superconductor/topological insulator devices and explore the feasibility of realizing Majorana bound states in them.