Observation of perfect Andreev reflection due to Klein paradox in a topological superconducting state

TL;DR

This paper reports the experimental observation of perfect Andreev reflection in a topological superconductor, demonstrating Klein tunneling and relativistic physics effects in condensed matter systems with Dirac-like surface states.

Contribution

It provides the first experimental evidence of Klein tunneling manifesting as perfect Andreev reflection in a topological Kondo insulator with proximity-induced superconductivity.

Findings

Observation of perfect Andreev reflection indicating Klein tunneling

Evidence of relativistic physics in topological superconducting states

Implications for quantum transport in topological materials

Abstract

In 1928, P. Dirac proposed a new wave equation to describe relativistic electrons. Shortly afterwards, O. Klein solved a simple potential step problem for the Dirac equation and stumbled upon an apparent paradox - 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 (TIs), has opened the possibility of observing the Klein tunneling experimentally. In the surface states of TIs, fermions are bound by spin-momentum locking, and are thus immune to backscattering due to time-reversal symmetry. Here we report the observation of perfect Andreev reflection in point contact spectroscopy - a clear signature of Klein tunneling and a manifestation of the underlying relativistic physics of a proximity-induced superconducting state in a topological Kondo insulator.