Comment on "Perfect Andreev reflection due to the Klein paradox in a topological superconducting state (Nature \textbf{570}, 344 (2019))"

Abstract

In a recent publication, Lee $et$ $al.$ discussed experimental observation of Klein tunneling into a proximity-induced topological superconducting state of SmB$_{6}$ through the measurement of Andreev reflection with low-bias conductance doubling across point-contact junctions between sharp Pt-Ir tips and SmB$_{6}$/YB$_{6}$ heterostructures. However, the interpretation of the presented point-contact data is rather ambiguous because observing a low-bias conductance enhancement by a factor of 2 is not special for Klein tunneling into topological superconductors. Such an enhanced conductance can be observed in point contacts between all types of superconductors and normal metals when the contacts are neither in the ballistic nor in the diffusive regimes of mesoscopic transport, but in a dissipative thermal regime. The thermal regime is expected for the point contacts presented in Lee $et$ $al.$ primarily because of the anticipated very short electron mean free path of SmB$_6$ -- we will discuss this in more detail here.