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Physica C Superconductivity

Giant shot-like voltage noise in high current density Nb–AlOx–Nb superconducting tunnel junctions

Bonaventura Savo, Patrizia Romano, A. Nigro, R. Di Leo

Published May 1, 2002• 0 citations
Publisher
DOI
10.1016/s0921-4534(01)00954-6

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IEICE Transactions on Electronics•Jan 1, 2012•2 citations
In this paper we focus on the numerical simulation of the mixing behaviors of distributed superconducting junction arrays at 1.2 THz. A novel type of superconducting tunnel junctions, i.e., NbN/AlN/Nb, which have a relatively high gap voltage (4.3 mV) and can reach a critical current density as high as several tens of kA/cm 2 , are proposed for this characterization along with conventional Nb/AlOx/Nb junctions. The former is incorporated with a NbN/SiO 2 /Al tuning circuit, and the latter with a Nb/SiO 2 /Al and a NbTiN/SiO 2 /Al tuning circuits. The noise performance, local-oscillator power requirement, IF bandwidth, and optimum embedding impedance are thoroughly characterized for the two types of distributed superconducting junction arrays.
In this paper we focus on the numerical simulation of the mixing behaviors of distributed superconducting junction arrays at 1.2 THz. A novel type of superconducting tunnel junctions, i.e., NbN/AlN/Nb, which have a relatively high gap voltage (4.3 mV) and can reach a critical current density as high as several tens of kA/cm(2), are proposed for this characterization along with conventional Nb/AlOx/Nb junctions. The former is incorporated with a NbN/SiO2/Al tuning circuit, and the latter with a Nb/SiO2/Al and a NbTiN/SiO2/Al tuning circuits. The noise performance, local-oscillator power requirement, IF bandwidth, and optimum embedding impedance are thoroughly characterized for the two types of distributed superconducting junction arrays.
We have developed a process for the fabrication of high-quality Nb/AlOx/Nb tunnel junctions with small area and high current densities for the heterodyne mixers at millimeter and submillimeter wavelengths. Their dc I-V curves are numerically studied, including the broadening of quasiparticle density of states resulting from the existence of an imaginary part of the gap energy of Nb. We have found both experimentally and numerically that the subgap current is strongly dependent on bias voltage at temperatures below 4.2K unlike the prediction of the BCS tunneling theory. It is shown that calculated dc I-V curves taking into account the complex number of the gap energy agree well with those of Nb/AlOx/Nb junctions measured at temperatures from 0.4 to 4.2K. We have successfully built receivers at millimeter and submillimeter wavelengths with the noise temperature as low as 4 times the quantum photon noise, employing those high-quality Nb/AlOx/Nb junctions. Those low-noise receivers are to be installed in the ALMA (Atacama Large Millimeter/Submillimeter Array) telescope and they are going into series production now.