One focus of our research is exploration of new quantum materials and their device applications. In this talk, I will describe our observation of perfect Andreev reflection in point contact spectroscopy – a signature of the Klein paradox in a topological superconducting state [1]. Klein tunneling is a manifestation of relativistic physics: shortly after Dirac proposed a new wave equation to describe relativistic electrons in 1928, Klein solved a simple potential step problem for the Dirac equation and stumbled upon an apparent paradox, where the potential becomes transparent when the height is larger than the electron energy. For massless particles, backscattering is completely forbidden in Klein tunneling, leading to perfect transmission through any potential barrier. Recent advent of condensed matter systems with Dirac-like excitations, such as graphene and topological insulators, has opened the possibility of directly observing the Klein tunneling experimentally. In the surface states of topological insulators, electrons are bound by spin-momentum locking, and are thus immune to backscattering due to time-reversal symmetry. We establish a proximity-induced topological superconducting state using a topological Kondo insulator SmB6 coupled to YB6. Klein tunneling through point contact Andreev reflection results in exact doubling of conductance within the superconducting gap, a very rarely observed phenomenon. I will discuss potential device applications of superconducting Klein tunneling. I will also discuss our methodology for high-throughput exploration of new superconductors. This work is funded by ONR and AFOSR.

[1] Lee et al., Nature 570, 344 (2019).