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IEEE Transactions on Applied Superconductivity

Subharmonic Photon-Assisted Tunneling Effect in Superconducting NbN Tunnel Junctions

Yoshinori Uzawa, Mahoto Takeda, Akira Kawakami, Z. Wang

Published Jun 1, 2005• 1 citation
Publisher

Abstract

We investigated subharmonic gap structures in the current-voltage (I-V) characteristics of NbN tunnel junctions by observing frequency response using a quasioptical mixer configuration. We chose a 10.5-/spl mu/m-long by 0.6-/spl mu/m-wide junction for the mixer so that the junction behaved as a distributed element at submillimeter-wave frequencies in wideband observations. In the presence of electromagnetic waves, subharmonic photon-assisted tunneling steps with a voltage width of hf/2e could clearly be observed below and above the half-gap voltage in the I-V curves of junctions at measured frequencies from 600 to 950 GHz, where h is Planck's constant, f is frequency, and e is electron charge. This evidence is consistent with a consequence of multiple Andreev reflection at low transmissions or multiparticle tunneling.

DOI
10.1109/tasc.2005.849760

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IEEE Transactions on Applied Superconductivity•Jun 1, 1995•10 citations
We investigated subharmonic gap structures in the current-voltage (I-V) characteristics of NbN tunnel junctions by observing frequency response using a quasioptical mixer configuration. We chose a 10.5-/spl mu/m-long by 0.6-/spl mu/m-wide junction for the mixer so that the junction behaved as a distributed element at submillimeter-wave frequencies in wideband observations. In the presence of electromagnetic waves, subharmonic photon-assisted tunneling steps with a voltage width of hf/2e could clearly be observed below and above the half-gap voltage in the I-V curves of junctions at measured frequencies from 600 to 950 GHz, where h is Planck's constant, f is frequency, and e is electron charge. This evidence is consistent with a consequence of multiple Andreev reflection at low transmissions or multiparticle tunneling.
We measured shot noise and submillimeter-wave response in a superconducting NbN tunnel junction that had a subharmonic gap structure on the current-voltage (I-V) curve. We found that the observed effective charge, defined from the noise-current ratio, tends to a steplike function of voltage. In the presence of submillimeter-wave radiation of frequency v, novel step structures spaced by hv/2e below and above the half-gap voltage clearly appeared on the I-V curve, overlapping the ordinary photon-assisted tunneling steps spaced by hv/e. Observation of these features provides clear evidence that coherent multiple Andreev reflection processes occur in the NbN tunnel junction with low barrier transparency.
The submillimeter wave frequency responses of NbN/AlN/NbN tunnel junctions have been investigated using quasioptical coupling techniques and an optically pumped far-infrared laser. A two junction array was integrated at the feeds of a bowtie antenna. Measurements on the junctions were made by monitoring photon-assisted tunneling steps induced in the current–voltage characteristics. These n=±1 photon steps were clearly observed with irradiation at 303, 594, 693, and 762 GHz. The results suggest it will be possible to realize submillimeter wave mixers by using these junctions.
We report on the fabrication and properties of high current density NbN/AlN/NbN tunnel junctions for the application of superconducting SIS mixers in the submillimeter wave regions. Junctions having current densities as high as 50 kA/cm/sup 2/ exhibited a good Josephson tunneling behaviour, excellent terahertz response, and sensitive heterodyne mixing properties. Accurate values of junction capacitance were estimated by measuring dc-SQUID resonant steps for the design of tuning circuits. The measured specific capacitance gives values in the range of 20-200 fF//spl mu/m/sup 2/ for the current density range of 1-100 kA/cm/sup 2/. The NbN/AlN/NbN tunnel junctions were integrated with a NbN thin film antenna to investigate the terahertz responses and mixing tests in a quasi-optical testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1.02 THz, and a low-noise heterodyne mixing was demonstrated in the 300-GHz band.
The subharmonic gap structure (SGS) in long-overlap Nb-AlOx-Nb Josephson tunnel junctions has been investigated. The experimental results show peaks in the differential conductance at both odd and even integer fractions of the gap voltage, VG. Furthermore, the conductance peaks at VG/2 has been observed to split into two peaks with different characteristics. At high magnetic fields, the I-V characteristics approach a single curve, while retaining the SGS conductance peaks. The gap structure and the SGS show the same temperature dependence. The SGS can be explained by a Josephson self-coupling effect or multiple Andreev reflections. The results are important for the development of the flux flow oscillator used a local oscillator in integrated all-superconducting 200-800 GHz receivers.
We have fabricated tunnel-type Josephson junctions with very small junction area by pressing the tip of a sharpened Nb wire onto a thinly oxidized Sn film, and studied their submillimeter-wave responses. It turns out that the junctions have a very high cutoff frequency, and are suitable for the investigation of the response to the submillimeter waves with the frequency higher than 1 THz. From the frequency and laser-power dependence of the Shapiro step, it is found that the tunnel-type junction shows much larger higher-order steps, and accordingly has much stronger nonlinearity than the bridge-type junction in the submillimeter-wave region. The large enhancement of the steps due to the Riedel singularity is also observed in the tunnel-type junction. In addition to the Shapiro steps, photon-assisted tunneling steps are observed clearly, reflecting the large and sharp gap structure in the dc IV curves of the tunnel-type junctions.
Short metallic weak links of Nb are used for millimeter-wave mixer experiments at 100-110 GHz. When the weak links are connected to a voltage bias circuit with zero output impedance, subharmonic gap structures are observed as a large dynamic resistance region or a negative differential resistance region at voltages V/sub u/=2/spl Delta//ne on the I-V curve, where n=1, 2, 3, ... . These strong structures are caused by a resonance effect between the energy gap and the multiple Andreev reflections that are present whenever there is a spatial inhomogeneity of the pair potential. Enhancement of IF signal peaks is observed at the subgap voltages. These experimental results suggest the appearance of the density of states predicted theoretically by R. Kummel et al. The subharmonic gap structure and details of the SNS mixer experiment will be described.>