Andreev reflection spectroscopy in strongly paired superconductors

Abstract

Motivated by recent experiments on low-carrier-density superconductors, including twisted multilayer graphene, we study signatures of the BCS to BEC evolution in Andreev reflection spectroscopy. We establish that in a standard quantum point contact geometry, Andreev reflection in a BEC superconductor is unable to mediate a zero-bias conductance beyond e 2 /h per lead channel. This bound is shown to result from a duality that links the sub-gap conductance of BCS and BEC superconductors. We then demonstrate that sharp signatures of BEC superconductivity, including perfect Andreev reflection, can be recovered by tunneling through a suitably designed potential well. We propose various tunneling spectroscopy setups to experimentally probe this recovery.

Local probes and superconductivity in magic angle twisted bilayer and trilayer graphene

“…[14] suggests that the phase with an U shaped gap could be a strongly coupled superconductor, best described as a Bose-Einstein condensate. This point has been related to the observed subgap Andreev scattering using models presented in [15].…”

Evolution from Bardeen–Cooper–Schrieffer to Bose–Einstein Condensation in Two Dimensions: Crossovers and Topological Quantum Phase Transitions

“…The theoretically expected topological quantum phase transitions in such systems have not yet been observed experimentally. However, in materials with particle-and hole-like bands, such as magic-angle twisted trilayer graphene (3)(4)(5), there is experimental evidence that the BCS-BEC evolution is occurring via a topological quantum phase transition from gapless to fully gapped d-wave superconductivity (3,92,93). The generic discussion encompassing Equations 7 through 9, covering two bands with particle-like dispersions, also applies to two bands with particle-and hole-like kinetic energies.…”

Majorana fermions in Kitaev chains side-coupled to normal metals

“…The limit of µ < −2t corresponds to Bose-Einstein condensate (BEC) superconductors hosting strongly bound pairs when s-wave superconductivity is switched on in spin-half tight binding chain [13][14][15]. Andreev reflection spectroscopy in such BEC superconductors is expected to show finite subgap conductance when the barrier separating the NM and superconductor is turned into a well [16]. Physically this can be understood as due to the formation of Andreev bound state due to potential well at the interface.…”