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Anomalous Pumped and Unpumped I-V Characteristics of Nb SIS Terahertz Mixers with NbTiN Striplines

Authors:
B. Leone, B. D. Jackson, J. R. Gao, T. M. Klapwijk, W. M. Laauwen, G. de Lange
Abstract:
To achieve nearly quantum limited noise temperatures around 1 THz in Nb SIS mixers NbTiN is used as stripline material because its high DC conductivity and gap frequency above 1 THz should minimize RF losses. However, one expects problems at the Nb/NbTiN interface due to an energy gap discontinuity. Indeed, the measured I-V curves show features that are absent in all Nb SIS mixers or Nb junctions with normal metal striplines. These include the gap voltage backbending and a severe gap voltage reduction in the pumped case. In an analysis of the unpumped case, it W as previously shown that the backbending feature is caused by the DC heat trapped at the Nb/NbTiN interface and that the heat flow is limited by the electron-phonon interaction time. In the present paper, the analysis is extended to the pumped case by introducing an additional power contribution to explain the severe gap depression. This additional power results from the energy spectrum of the photon assisted tunneling electrons as given in the Tien-Gordon theory. Theoretical fits are obtained that faithfully reproduce the main features of the anomalous pumped I-V characteristics.
Categories:
SIS
Year:
2000
Session:
5
Full-text:
Download a PDF of this paper.
Page Number(s):
228-237