Spectroscopy of Andreev States in Atomic Contacts & Graphene: The Fermionic Side of Josephson Effect

Superconductors are characterized by zero resistivity. Since the work of Josephson 55 years ago, it is known that a dissipationless supercurrent can even flow through tunnel junctions between superconductors. This Josephson effect also occurs through any type of "weak links" - such as normal metals, nanowires, or molecules - between superconductors. A unified understanding of the Josephson effect has emerged from a mesoscopic description of weak links. It predicts that the supercurrent is carried by entangled electron-hole states localized at the weak link, called Andreev states. These fermionic states come in doublets with energies symmetric about the Fermi energy, and carry supercurrent in opposite directions. By exciting the Andreev states, one can in principle reverse the sign of the Josephson supercurrent. To illustrate these concepts, I will present experiments performed on two very different systems: a single-atom contact and a 2D graphene flake. In the first experiment, we perform a photon absorption spectroscopy of an atomic contact and observe discrete energy-resolved Andreev states [1]. We further detect changes in the supercurrent flowing through the atomic contact associated with this excitation, thus demonstrating that Andreev states indeed carry the Josephson supercurrent [2]. In the second experiment, we perform a tunneling spectroscopy of graphene proximitized by superconducting electrodes and we observe a continuum of Andreev states, which depends on the graphene Fermi energy [3]. We further derive the supercurrent spectral density and determine the supercurrent-phase relation, thus experimentally establishing the connection between Andreev physics at finite energy and the Josephson effect. These results demonstrate the accessibility of spin-like internal degrees of freedom in Josephson junctions.

[1] L. Bretheau, Ç.Ö. Girit, H. Pothier, D. Esteve & C. Urbina, Exciting Andreev Pairs in a Superconducting Atomic Contact , Nature (London) 499 , 312 (2013)

[2] L. Bretheau, Ç.Ö. Girit, C. Urbina, D. Esteve & H. Pothier, Supercurrent Spectroscopy of Andreev States , PRX 3 , 041034 (2013)

[3] L. Bretheau, J. I-J. Wang, R. Pisoni, K. Watanabe, T. Taniguchi and P. Jarillo-Herrero, Tunnelling Spectroscopy of Andreev States in Graphene, submitted to Nature Physics (2017)