Search

We have prepared an epitaxial niobium nitride (NbN) film and NbN/AlN/NbN tunnel junctions to investigate the energy gaps. By measuring the optical conductivity spectrum of a 41-nm-thick film with the critical temperature (TC) of about 14 K by terahertz time-domain spectroscopy, we obtained the gap frequency of about 1.2 THz. An ill-defined gap was suggested due to the broadening of the onset of absorption as the temperature increases. On the other hand, the current–voltage curve measurement of the tunnel junctions showed the current rise at the gap voltage of 5.6 mV (corresponding to 1.4 THz) with a smeared shape as the temperature increases. We found that both gap broadenings can be explained by introducing a temperature-dependent imaginary energy gap part into the superconducting energy gap, corresponding to a finite quasi-particle lifetime in the NbN films. [ABSTRACT FROM AUTHOR]

We have designed, fabricated, and tested terahertz waveguide superconductor-insulator-superconductor (SIS) mixers with Nb/A1Ox/Nb junctions and NbN/SiO2/AI microstriplines. The NbN ground plane used was an epitaxial film grown on a single-crystal MgO substrate. A two-junction tuning circuit, which was directly placed at the feed point of a bow-tie waveguide probe without impedance transformers, was used. A full-height waveguide, hammer-type choke filter, and zero-depth backshort were adopted for the design of the microstrip-waveguide transition structure. The Nb junctions in which the current density was 6.5 kA/cm2 showed good I V characteristics, yielding a subgap-to-normal-state resistance ratio greater than 20. The results obtained for the mixer performance showed a receiver noise temperature of 440 K (DSB) at 1 THz; the results were corrected for losses in the beam splitter and the vacuum window. A detailed analysis of the mixers suggested that the waveguide mixers composed of epitaxial NbN films and Nb junctions were effective in the terahertz band. [ABSTRACT FROM AUTHOR]

We report on the mixing properties of waveguide SIS mixers with epitaxial NbN/AIN/NbN junctions and NbN/SiO2 /NbTiN microstrips on MgO substrates. The superconducting transition temperature and 20-K resistivity of NbTiN film on the SiO2 dielectric were 14.7 K and 175 μΩcm, respectively. The junction tuning circuit was composed of an NbN/Si02 /NbTiN microstrip and two parallel-connected NbN/AIN/NbN junctions 0.9 μm in diameter. The critical current density and normal-state resistance of the fabricated NbN junction were 13 kA/cm2 and 22 Ω, respectively. The heterodyne response was measured using 295. and 77-K blackbody sources, a local oscillator (LO) source, and a 25-μm-thick Kapton film as a beam splitter. An intermediate frequency (IF) amplifier with a bandwidth of 4-12 GHz was used in the measurements. The receiver noise temperature corrected for losses in the beam splitter and in the vacuum window of the cryocooler was 370 K at 860 GHz and was comparable to that of all-NbN mixes with NbN/MgO/NbN microstrips. [ABSTRACT FROM AUTHOR]