Subharmonic Shapiro steps and assisted tunneling in superconducting point contacts

•Juan Carlos Cuevas1, Jan Heurich1, Alvaro Martin-Rodero2, Alfredo Levy-Yeyati2 und Gerd Schön1
2Departamento de Fisica Teorica de la Materia Condensada C-V, Universidad Autonoma de Madrid, E-28049 Madrid, Spain
1Institut für Theoretische Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany

We present a theoretical analysis of the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. We show that the interplay between the ac Josephson current and the microwave signal gives rise to appearance of Shapiro steps at voltages V = (m/n)(h/2p)wr/2e , where n,m are integer numbers and wr is the radiation frequency. The subharmonic steps (n ¹ 1 ) are a consequence of the occurrence of multiple Andreev reflections (MAR) and provide an unambiguous signature of the peculiar ac Josephson effect at high transmission. Moreover, the background current, in which the Shapiro steps are superimposed, exhibits a rich subgap structure, which can be understood in terms of photon-assisted MARs. Our predictions are amenable to a quantitative experimental test using superconducting atomic-size contacts.