Scanning Microscopy Probes of Majorana States

Majorana bound states are difficult to distinguish from trivial near-zero energy states. Single-electron transistors (SETs) have been proposed as on-chip probes to solve this problem. Combining these devices with scanning probe methods allows us to extend the reach of these probes to test many of the systems that theoretically support Majorana bound states (vortices in a 2D topologogical superconductor, for example.) In this talk, we present progress on (i) scanning tunneling microscopy (STM) probes of Majorana states in Josephson junctions and (ii) development and testing of the Scanning Majorana Microscope (SMM). We present a platform that implements a capacitance-based device constructed from a high-electron-mobility transistor that can measure the counting statistics of electrons tunneling into a quantum dot. Theoretically, the cumulants of the counting statistics change as the quantum dot hybridizes with a Majorana bound state. This device is integrated into a probe tip which fabricated by thermally evaporating two Al leads (one tunneling lead and one capacitance lead) onto a sharply-pointed optical fiber. Using a focused ion beam, we create a flat end of the pointed optical fiber where we deposit our quantum dot. For this talk, we will focus on key milestones that have been accomplished. The first, is a demonstration of the probes counting statistics in vacuum. Second, is confirmation that the probe can adequately perform the basic functions of an STM tip: approaching safely, and measuring the topography of the sample's surface. Additionally, we present progress on developing a device featuring planar Josephson junctions on the surface of a topological insulator. This device is fabricated with conventional electron-beam lithography and reactive ion etching. Such a device should theoretically support Majorana bound states in the presence of an external magnetic field. Our near-term goal is to probe this device with STM and the SMM.