The superconductor–insulator–superconductor (SIS) tunnel junction based on Nb material has been investigated theoretically at frequencies of 200–1400 GHz by modeling it as a segment of a superconducting microstrip transmission line and using the Mattis–Bardeen theory of anomalous skin effect. The modeling was used to predict the performance of a flux-flow oscillator (FFO) and a SIS frequency down-converter (mixer). It is shown that in a FFO the internal loss (quasiparticle and surface) limits flux flow to the frequency band of 260–770 GHz. A SIS junction of micron size employed as a mixer for frequency down-conversion demonstrates distributed behavior at frequencies above 550 GHz. We suggest that electrically long SIS junctions of resonant size can be used as self-compensating mixing elements with attractive performance in Nb as well as NbN mixers.
The superconductor–insulator–superconductor (SIS) tunnel junction based on Nb material has been investigated theoretically at frequencies of 200–1400 GHz by modeling it as a segment of a superconducting microstrip transmission line and using the Mattis–Bardeen theory of anomalous skin effect. The modeling was used to predict the performance of a flux-flow oscillator (FFO) and a SIS frequency down-converter (mixer). It is shown that in a FFO the internal loss (quasiparticle and surface) limits flux flow to the frequency band of 260–770 GHz. A SIS junction of micron size employed as a mixer for frequency down-conversion demonstrates distributed behavior at frequencies above 550 GHz. We suggest that electrically long SIS junctions of resonant size can be used as self-compensating mixing elements with attractive performance in Nb as well as NbN mixers.
A circuit that produces broadband impedance matching with low-current-density junctions at submillimeter wavelengths for tuning superconductor–insulator–superconductor (SIS) mixers is described; it consists of half-wavelength distributed SIS junctions connected alternately by half-wavelength microstrip lines. It is made of Nb and was designed for two practical source impedances using numerical calculation based on the Mattis–Bardeen theory of the anomalous skin effect at 660 GHz, assuming SIS junctions with a current density of 5 kA/cm 2 . An SIS mixer tuned using this circuit had a large fractional bandwidth of more than 18%, below -10 dB matching, even though the assumed ω C J R N product was about 16. Investigation of the mixer performance using Tucker's quantum theory of mixing showed that it has excellent broadband and low-noise characteristics compared with conventional mixers consisting of half- or full-wavelength distributed elements.
A 600-700GHz SIS mixer with the twin-junction tuning circuit has been designed and fabricated [1] The surface impedance of thin-film superconducting microstrip lines, based on Mattis-Bardeen theory [2], is included in the optimization of RF impedance matching. The receiver noise temperature measured in the frequency range of 630-660GHz is below 200K and the lowest receiver noise temperature of 181K is achieved at 656GHz. The FTS response of the SIS mixer shows a good RF coupling from 600GHz to 700GHz. Both the noise performance and the FTS response can be quantitatively described by numerical results using the quantum theory of mixing. Some detail considerations on the mixer model calculation, such as spreading inductance around the junction tuning structure and mixer's embedding impedance, are discussed. Introduction Nb-based superconductor-insulator-superconductor (SIS) mixers have been used very successfully in sub-mm wavelength detection. Their receiver noise temperature has reached to three times the quantum limit, —3 hfrkB, below the junction's gap frequency. However, at high frequency bands, Nb SIS mixers need to be designed carefully because the photon energy is close to or higher than the gap energy of Nb, A-1.4meV. When the frequency band covers the gap frequency of Nb, —670GHz, the RF loss of Nb superconducting transmission lines increases dramatically and the surface impedance has strong frequency dependence. Usually, the RF properties of a superconducting transmission line can be described by the Mattis-Bardeen theory [2] in good accuracy. From results reported so far [4-9], the best receiver noise temperature is close to 5 hi7kB in the band of 600-700GHz. Despite of the input loss contributed by the experimental setup, some other properties of SIS mixers should be taken into account. In this paper, we characterize the performance of a 600-700GHz mixer by comparing the experimental and simulated results.
The first object of this work is to theoretically study the effects of tunnel junction parameters on flux-flow Josephson oscillators (FFOs) by numerical simulation and to experimentally study the rf properties of FFOs by integrating them together with an SIS (superconductor-insulator-superconductor) mixer/detector on a chip. By using a CAD modeled integrated sub-mm wave receiver, the composite linewidth of two FFOs was measured to be less than 2.1 MHz in the band 280-330 GHz. The receiver consists of two identical FFOs and an SIS mixer. The linewidth was determined by mixing the two signals in the SIS mixer and analyzing the intermediate frequency. The output power coupled to a 10 .OMEGA. microstripline was about 0.5 µW, and could be adjusted by changing the current bias of the oscillator. The FFOs are connected to the SIS mixer via interdigital capacitors. The FFOs and the SIS element are made of Nb/NbOx/PbBi junctions by the "reactive ion beam oxidation" technique with the critical current density jc controlled by the oxidation time. Lift-off stencils for defining the junction sizes and the top-electrodes were made by using electron-beam writing on PMMA/Copolymer double layer resist. The jc of a long Josephson junction (LJJ) determines the fluxon motion. For LJJs which had jc > 1 kA/cm2, velocity-matching (VM, where the phase velocity of propagating fluxons equals that of the electromagnetic wave in the LJJ) steps could be tuned linearly from about 0.2 to 1.4 mV by an applied magnetic field. Coupled radiation at around 100 GHz (about 3 nW), 261 GHz (4.6 nW), and 330 GHz (0.43 µW), from flux-flow oscillations, was observed. From the step's steepness, the junction quality factor QJ is determined. QJ is between 4 to 7 in the voltage range of 0.4 to 1 mV. Coupled radiation (> 3 nW) at around 100 GHz corresponding to resonant soliton oscillation in low jc samples (250 to 1000 A/cm2) was detected. Simulations show that, for a uniform bias feed, the height of the VM step .gammam increases almost linearly with the external magnetic field .beta.e and the quasiparticle loss .alpha., but is less dependent on the surface loss .beta.. An approximate expression .gammam = a + .alpha(1+b).beta.e agrees with simulations. The maximum ac output voltage decreases with increasing dampings .alpha. and .beta.. It is important to include the .beta.-term in the mathematical model. The second part of this work is devoted to rf properties of long YBa2Cu3O7-.delta. (YBCO) grain boundary junctions made on 0- 32 °. [001]-tilt YSZ bicrystals. Josephson flux-flow resonances and Fiske resonances were observed. The first resonances could be tuned from about 0.5 to 2.0 mV in a 30 µm long junction at 16 K, and we determined the ratio between barrier thickness and relative dielectric constant td/.epsilon.r=2.9 Å and the loss .alpha.= 1.3 at 16 K. Simulations support the experimental observation that Josephson vortices can propagate unidirectionally in an overdamped, homogeneous junction up to .alpha.Ã? 2. From the Fiske resonances at 256-638 GHz, their Q-values, and the Steward-McCumber parameter, we find td/.epsilon.r=4 Å, the London penetration depth .gamma.L=145±5 nm and the surface resistance RS ~20 m.OMEGA. at 280 GHz. We conclude that the boundary is dielectric and the YBCO grains are well superconducting within 5-20 nm from the boundary. The dielectric between the grains could be part of a tunnel barrier and/or a matrix for filamentary conduction. Josephson flux-flow transitors based on long bicrystal junctions were made with an Au loop used as the control line. The line modulated the critical current Ic. Current gain larger than unity was obtained below 70 K. The gain increased with the junction length and varied roughly like the square root Ic when adjusting the temperature. A transresistance larger than 5 .OMEGA. was obtained at 15 K. Better performance is expected from junctions with high current densities.
The frequency response of three superconductive niobium resonating circuits, formed by a Nb microstrip and a Nb tunnel junction, is measured and analyzed at frequencies above the superconducting gap frequency. The circuits are placed in a waveguide system and the frequency response is determined with a Fourier transform spectrometer. The calculated and measured resonance frequencies and bandwidths are in good agreement with the extreme anomalous limit of the Mattis–Bardeen theory on the anomalous skin effect in superconductors [D.C. Mattis and J. Bardeen, Phys. Rev. 111, 412 (1958)]. The observed loss is higher than predicted by this theory, in agreement with previous observations on Nb films. The use of other materials for striplines as tuning circuits for heterodyne superconducting tunnel junction mixers is analyzed.
This thesis describes the development, modeling, and optimization of Nb superconducting microstrip tuning circuits for Superconductor-Insulator-Superconductor mixers. The ultimate goal is to extend the quantum limited noise performance of Nb/AlOx/Nb SIS mixers to above the Nb gap frequency (680 GHz). The mixers are intended to be used in the Atacama Large Millimeter Array (ALMA) band 9 frequency range (600-720 GHz). The behavior of Nb microstrip tuning circuits especially above the gap frequency is modeled using the complex conductivity given by the Mattis-Bardeen theory of the anomalous skin effect [72]. A computer code utilizing two-port network theory is utilized to predict and optimize the RF coupling efficiency and bandwidth of the tuning circuits and embedding impedances. The effect of material and geometrical variations on the coupling has been evaluated. The installment of the devices in the waveguide has been found to be a crucial factor explaining the observed scattering in measurement results for nominally identical devices. Despite parameter variations, modeling of the RF coupling efficiency proved to be successful within the limitations set by scattering of the measured results. Using this model, the tuning circuit geometry has been optimized for bandwidth and maximum coupling efficiency. During the work several SIS devices have been fabricated and Fourier transform spectroscopy measurements have been made for RF coupling evaluation. Finally, using simulations, it has been found that application of Nb/AlN/Nb junctions would increase the bandwidth to an extent which should dramatically decrease the sensitivity to parameter variations.
Three Nb-A10x-Nb SIS mixers tuned to operate in the 400-550, 550-700 and 600-750 GHz frequency bands have been investigated by modeling and experimentally by employing Fourier Transform Spectrometer (FTS) technique and mixer measurements. The mixers were of quasioptical type with spiral antennas and twin junction tuning circuits integrated onto the apex area of the antenna arms. Modeling was employed for optimizing the tuning circuits and evaluating its performance. The comprehensive model for the SIS mixer bases on Tucker-Feldman theory of quasiparticle SIS mixer and Mattis-Bardeen theory of anomalous skin-effect. Very good agreement between modeling, FTS measurements and mixer experiment results was achieved. Accuracy in prediction of the tuning band and the coupling efficiency for the tuning circuits is better or about 15% within 400-1000 GHz band. Introduction The SIS mixer optimum performance can be reached when the intrinsic capacitance of the SIS junction is tuned out. Different integrated on chip tuning circuits have been developed for waveguide and open structure (quasioptical) Nb-A10x-Nb SIS mixers for 100-820 GHz band [16] providing broadband low-noise operation of the receivers [7-11]. These tuning circuits mainly utilize different microstrip lines, made from Nb with SiO or Si02 insulator, to resonate out SIS junction capacitance. The focus of this work is theoretical (by modeling) and experimental investigation of twin junction tuning circuit [4-6] for frequency band about 400-1000 GHz. Mattis-Bardeen theory was employed for the tuning circuit modeling. Fourier Transformer Spectrometer technique as well as mixer measurements of developed SIS mixers were used to determine tuning circuits performance, to compare experimental data and results of modeling. At the first step of the tuning circuit development simplified free-of-loss model has been employed for evaluating of the tuning circuit behavior and its optimization [11,191 The coupling efficiency of the spiral antenna output and the tuning circuit input was used as a quality parameter for loss-free circuitry (Fig. 1). However Fourier Transform Spectrometer (FTS) measurements of the tuning circuits [19] have shown noticeable discrepancy between tuning circuit coupling efficiency at above the gap frequency and model predicted performance. This associated with loss in Nb microstrip tuning circuitry. Near the gap frequency of Nb material (-690 GHz for film) the Nb based microstrip lines become dispersing [12, 13] and above this frequency having increasing losses [14, 15]. Mattis– Bardeen theory [16] of skin anomalous effect describes behavior of a superconductor at these frequencies in teillis of complex conductivity a: cr= G–ja2 , (1) where real a l and imaginary 32 components translate directly into the normal-electron and cooperpare currents in a superconductor. Following fundamental paper of R.L. Kautz [14] these components above can be presented in integral form: zo (7) Page 88 Sixth International Symposium on Space Terahertz Technology 2 E2±2 + E j = de[f (s)— f (e + hco)} O 7: ha) 2 2 1/2 2 2 A h + 1 ' er dep. 2f(hco hw i, 21 — A2 — A2} 112 (s2 _2 ) / (11(0 _ e) ha)