Quantization of the superconducting energy gap in an intense microwave field
Number of Authors: 2
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 17, 174506Article in journal (Refereed) Published
We study experimentally photon-assisted tunneling in Nb/AlOx/Nb Josephson junctions. We perform a quantitative calibration of the microwave field inside the junction. This allows direct verification of the quantum efficiency of microwave photon detection, which corresponds to tunneling of one electron per one absorbed microwave photon. We observe that voltages of photon-assisted tunneling steps vary both with the microwave power and the tunneling current. However, this variation is not monotonous but staircaselike. The phenomenon is caused by mutual locking of positive and negative step series. A similar locking is observed with Shapiro steps. As a result, the superconducting gap assumes quantized values equal to multiples of the quarter of the photon energy. The quantization is a manifestation of nonequilibrium tuning (suppression or enhancement) of superconductivity by the microwave field.
Place, publisher, year, edition, pages
2015. Vol. 92, no 17, 174506
IdentifiersURN: urn:nbn:se:su:diva-124184DOI: 10.1103/PhysRevB.92.174506ISI: 000364013400003OAI: oai:DiVA.org:su-124184DiVA: diva2:883221