Abstract
We studied the low-energy states of spin- quantum dots defined in nanowires and coupled to aluminum superconducting leads. By varying the superconducting gap with a magnetic field we investigated the transition from strong coupling to weak-coupling , where is the Kondo temperature. Below the critical field, we observe a persisting zero-bias Kondo resonance that vanishes only for low or higher temperatures, leaving the room to more robust subgap structures at bias voltages between and . For strong and approximately symmetric tunnel couplings, a Josephson supercurrent is observed in addition to the Kondo peak. We ascribe the coexistence of a Kondo resonance and a superconducting gap to a significant density of intragap quasiparticle states, and the finite-bias subgap structures to tunneling through Shiba states. Our results, supported by numerical calculations, own relevance also in relation to tunnel-spectroscopy experiments aiming at the observation of Majorana fermions in hybrid nanostructures.
- Received 5 July 2012
DOI:https://doi.org/10.1103/PhysRevLett.109.186802
© 2012 American Physical Society