Josephson Critical Current Density Research Articles

Anisotropy of the Josephson Critical Current betweenUPt3andNb

Josephson critical current densities ${J}_{\mathrm{c}}$ between a single crystal ${\mathrm{UPt}}_{3}$ and an $s$-wave superconductor have been investigated for ${\mathrm{UPt}}_{3}\ensuremath{-}\mathrm{Cu}\ensuremath{-}\mathrm{Nb}$ junctions and ${\mathrm{UPt}}_{3}\ensuremath{-}\mathrm{Nb}$ junctions. The anisotropic temperature dependence is observed at about the lower critical temperature ${T}_{\mathrm{c}}^{\ensuremath{-}}$ of ${\mathrm{UPt}}_{3}$; the increase in ${J}_{\mathrm{c}}$ with decreasing temperatures becomes fast below ${T}_{\mathrm{c}}^{\ensuremath{-}}$ for a current flow parallel to the crystallographic $c$ axis and it becomes rather slow below ${T}_{\mathrm{c}}^{\ensuremath{-}}$ for a current flow parallel to the $b$ axis. This anisotropy gives clear evidence that the Josephson effect reflects the anisotropic superconductivity of ${\mathrm{UPt}}_{3}$.

Proton damage on Nb-based Josephson junctions

The effects of proton irradiation on Nb-based Josephson junctions have been investigated. The fabrication technology employed is essentially the same of Josephson digital circuit designs, the main difference being a much lower Josephson critical current density. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunnelling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found. The profile of the energy lost by protons within the Nb-targets and the dose is calculated by means of Monte Carlo simulations. A schematic description of the vacuum scattering chamber used for proton irradiation is also reported.

Coherent response of two nearly identical stacked Josephson junctions to mm wave irradiation

Abstract Double-barrier Nb/Al-AlO x -Nb/Al-AlO x -Nb tunnel devices with nearly identical Josephson critical current densities of the single junctions have been investigated under influence of the electromagnetic irradiation in the frequency range from 78 to 119 GHz. Odd, even, and fractional Shapiro steps have been observed in the weak-signal regime. The switching of the device to the double gap-sum voltage occurs from even steps, whereas the switching to the single gap-sum voltage has usually been observed from odd steps. The even steps have a much larger amplitude than the odd steps. This behavior is explained as a manifestation of the coherent phase-locking of the two junctions to the applied mm wave signal. Fractional steps may be evidence of a nonlinear interaction between the junctions.

Noise and gain in frequency mixers with NbN SIS junctions

We study noise and conversion gain in the SIS mixers with Nb/MgO/NbN junctions at the millimeter and submillimeter wavelengths in order to estimate the possibility to extent the low noise operation towards the theoretical limit of 4/spl Delta//h=2.4 THz. We present as preliminary result the lowest achieved receiver noise with NbN SIS junctions of 65 K with conversion gain of -6 dB at 162 GHz and 230 K receiver noise with conversion gain of -10 dB at 306 GHz. The junction area is 2 /spl mu/m/sup 2/ and Josephson critical current density is 1.4 KA/cm/sup 2/. The optimum R/sub N//spl omega/C of NbN junction for mixers is estimated as 600 GHz/V. In preliminary tests the junctions R/sub N//spl omega/C is 10-20 time superior to the optimum and a significant improvement of NbN SIS mixers may be expected. The noise sources in the NbN SIS junctions are studied and a possible mixer sensitivity improvement is discussed. The thermal properties of the SIS NbN are studied. The frequency limit of the low loss integrated tuning structure in NbN is estimated as at least as 1.5 THz.

Improvement of junction properties of YBaCuO/PrBaCuO/YBaCuO trilayer Josephson junctions

We have demonstrated improvement of junction properties for YBaCuO/PrBaCuO/YBaCuO trilayer junctions, using a backing plate of Al and Cu stacked foils between substrate and sample holder to ensure uniform substrate temperature during deposition of trilayer films, and by controlling film composition precisely to reduce the precipitated particles on the surface of the films. All junctions with a barrier thickness of 35 nm and dimensions of less than 10 /spl mu/m/spl times/10 /spl mu/m showed RSJ-like current-voltage characteristics with some excess currents at 4.2 K. Typical values of Josephson critical current density J/sub c/ as measured using a magnetic modulation of total critical current density, and normalized junction resistance R/sub n/A were 2.4/spl times/10/sup 3/A/cm/sup 2/ and 1.8/spl times/10/sup -7//spl Omega/cm/sup 2/ at 4.2 K, respectively. For the 88 junctions on the substrate, the 1-/spl sigma/ spreads of J/sub c/ and R/sub n/A were obtained to be 34% and 25%, respectively.

Design and fabrication of an adder circuit in the extended phase-mode logic

We present the design and the fabrication of an adder circuit in the extended phase-mode logic family. The phase-mode logic is a single flux quantum (SFQ) logic which utilizes an SFQ as an information bit carrier. The single-bit adder circuit is made up of an INHIBIT gate which is the basic device of the phase-mode logic. The circuit has been designed and fabricated using Nb/AlO/sub x//Nb Josephson junctions with Josephson critical current density of 1.0 kA/cm/sup 2/. In order to confirm the circuit operation, the fabricated adder circuit has been tested at low speed. For investigating the possibility of a high-frequency operation, dc voltages generated by fluxon pulse trains have been measured. From the Josephson voltage-frequency relation, the result shows that the circuit has potential to complete the carry operation within 20 psec.

Low noise SIS mixer for 230 GHz receivers of plateau de bure interferometer

We present a SIS mixer developed for 200 – 250 GHz band receivers of Plateau de Bure Interferometer. We demonstrate the minimum DSB receiver noise of 20 K at 220 GHz. The average receiver noise of 25 K is possible in 200 – 250 GHz range. The receiver conversion gain and output noise instability of 10−4 on the time scale of 1 minute is comparable with the Shottky receivers performance. The minimum measured SIS mixer noise of about 10 K is close to the quantum limit. The waveguide SIS mixer with a single backshort has two junction array with inductively tuned junctions. The Nb/Al Oxide/Nb SIS junctions are 2.24 µm2 each with the Josephson critical current density of 3.2 KA/cm2. The thermal properties of the SIS mixer are studied. The mixer band of the low noise operation is in a good agreement with the design requirements.

Effects of an energy dependent gap parameter on thermodynamic properties of the superconducting state

Abstract In a previous paper [Z. Hao, Mod. Phys. Lett. B7 , 1439 (1993)] we have derived an energy dependent gap parameter, with the magnitude ¦Δ k ¦ being appreciable within only a few k B T c around the Fermi level. In this paper we use this energy dependent ¦Δ k ¦ to calculate the critical magnetic field H c , electronic specific heat C , and Josephson critical current density I c of an SIS junction, and compare the results with those obtained by using the cutoff approximation. The main significant difference is that the present result for I c is much smaller than that of the cutoff approximation, with the ratio between the former and the latter at T = 0 being only 0.35 for symmetric junctions and even less for asymmetric junctions. As to the temperature dependences of H c , C and I c , the differences between the present results and those of the cutoff approximation are much less significant; i.e., there exist only minor numerical modifications. A qualitative discussion is also given on the physical significance of the phase θ k of the gap parameter ( Δ k = ¦Δ k ¦e iθ k ).

Low noise submillimeter SIS receiver with niobium nitride quasiparticle tunnel junctions

We have developed and tested a submillimeter waveguide SIS mixer with NbN-MgO-NbN quasiparticle tunnel junctions. The two junction array is integrated in a full NbN printed circuit. The NbN film critical temperature is 15 K and the junction gap voltage is 5 mV. The size of the junctions is 1.4 × 1.4 µm and Josephson critical current density is about 1.5 KA/cm2 resulting in junction RNωC product about 40. The inductive tuning circuit in NbN is integrated with each junction in two junction array. A single non contacting backshort was tuned at each frequency in the mixer block.

Experimental operation of an RS flip-flop composed of nonlatching Josephson gates

We present the experimental implementation of an RS flip-flop (RS-FF) composed of dc-biased coupled-SQUID (C-SQUID) gates. The C-SQUID gate is a combination of a single-junction SQUID and a double-junction SQUID. This gate utilizes nonhysteretic Josephson junctions and it is operated in nonlatching mode with dc-biasing. Several logical functions are able to be realized with a C-SQUID gate by adjusting the input bias and the input signal levels. The speed performance of the gate is evaluated by simulation for ring oscillators, and the minimum switching delay of 6.5 ps/stage is obtained under Josephson critical current density of 10 kA/cm/sup 2/. We have fabricated the RS-FF composed of two C-SQUID NOR gates. The circuit is integrated using a Nb/AlO/sub x//Nb junction technology and its operation is demonstrated experimentally.

Properties of superconducting Nb/Al/Nb/Al−AlOx−Al−AlOx−Al/Nb/Al/Nb tunnel junctions

Modified geometry (MG) devices, Nb/Al/Nb/Al−AlOx−Al−AlOx−Al/Nb/Al/Nb, have been fabricated and investigated in comparison with the basic geometry (BG) double-barrier Nb/Al−AlOx−Al−AlOx−Al/Nb devices. The enhancement of the critical temperature in the Al film is found to be weaker for the MG devices as compared with the BG devices at temperatures nearT=4.2 K but stronger at lowT. Indication of an enhancement of dc Josephson critical current density,j c , at bias voltageV≠0 as compared withj c (V=0) has been observed in the MG devices for the first time.

Two-particle structures in high quality Nb/AlO x/Nb Josephson tunnel junctions

We have investigated both theoretically and experimentally the two-particle structures, which appear at low temperature in high quality Nb/AlO x/Nb Josephson tunnel junctions in the subgap region of the current-voltage characteristics. We performed measurements on low Josephson critical current density junctions and the results are discussed in the framework of the multiparticle tunnel theory.

Degradation of superconducting tunnel junction characteristics with increasing barrier transparency

Measurements are reported of the single-particle characteristics of Nb-Al oxide-Nb superconducting tunnel junctions. Subgap leakage, subharmonic gap structure, and excess current are observed to increase systematically with increasing Josephson critical current density above a few kA/cm2. These phenomena are attributed to multiple Andreev reflections in high transmittance regions of the barrier. Such effects may significantly impact the applications of high critical current density junctions in advanced circuits.

NbN/MgO/NbN Josephson Junctions for Integrated Circuits

Characteristics of the NbN/MgO/NbN Josephson junction for integrated circuits have been investigated. The deposition process of the MgO film for the tunnel barrier was systematically investigated to obtain the controllability of the Josephson critical current density. The reproducibility of the Josephson critical current density was improved to ±28% with the critical current density of 610 A/cm2 and ±20% with the critical current density of 16 kA/cm2. The uniformity of the Josephson critical current density across a 3 cm-square area on the substrate was improved to ±12% with the critical current density of 650 A/cm2.

Fabrication process for Josephson computer ETL-JC1 using Nb tunnel junctions

The fabrication process that was used to develop a multichip Josephson computer named ETL-JC1 is described. The ETL-JC1 consists of four Josephson LSI chips: a register arithmetic logic unit chip (RALU), a sequence control unit chip (SQCU), a 1280-b read-only memory chip (IROU), and a 1-kb random access memory chip (DRAU). The fabrication process, based on a 3- mu m Nb/AlO/sub x//Nb junction technology, has been developed to make a complete set of the Josephson LSI chips. The present fabrication process includes a trilayer tunnel junction formation, a Nb underlayer method, a self-aligned insulation method, a reactive ion etching (RIE) process, an etching stopper layer formation, and a superconducting contact formation. The Josephson critical current density was controlled by the oxidation time within the fluctuation of +or-20% in the LSI fabrication runs. The resistors were made of palladium metal film on the LSI chips. The sheet resistance was controlled within the fluctuation between -12.5% and +19% in the LSI runs. It was found that the Josephson LSI chips fabricated by this process showed a high reliability for a long-term storage at room temperature and thermal cyclings between 4.2 K and room temperature without any passivation layers on the LSI surface.

Josephson LSI fabrication technology using NbN/MgO/NbN tunnel junctions

Josephson LSI fabrication technology using NbN/MgO/NbN tunnel junctions has been developed. The deposition process of the NbN electrode was investigated to obtain high uniformity of the electrical properties. The deposition process of the MgO tunnel barrier was investigated to obtain high reproducibility of the Josephson critical current density. The NbN film with high T/sub c/ of 15 K was obtained. The reproducibility of the MgO deposition rate was improved. The 10-b instruction 128-word ROM unit chip was successfully fabricated using the NbN/MgO/NbN junction LSI technology with the 3- mu m design rule. The READ operation test was performed for a few 10-b words. The total access time was measured to be 710 ps. The uniformity and reproducibility of the critical current density in the LSI chip were improved.

Electrical properties of grain boundaries in Ba1−xKxBiO3 polycrystalline thin films

Ba1−xKxBiO3 polycrystalline thin films are prepared on sapphire substrates by evaporation. In these films, grain boundaries affect the film resistivity in the normal state and I-V characteristics in the superconducting state. The temperature dependence of the resistivity is metallic for low-resistivity samples. On the other hand, that for high-resistivity is semiconductive and a bend is observed near 100 K. The I-V characteristics show series-connected tunneling Josephson junctions formed along a grain boundary. The ratio between the superconducting energy gap and Tc is estimated to be 3.5, equal to that of BaPb1−xBixO3. Using the resistively shunted junction model and Ambegaokar and Baratoff’s theory [Phys. Rev. Lett. 10, 486 (1963)], the tunneling resistance in a superconducting state is derived from the Josephson critical current densities and the hysteresis of the I-V curves. It increases with decreasing temperature at low temperatures and changes the junctions from weak link to tunneling.

The formation of a multiquantum vortex in the Josephson junction

It has been shown that in low magnetic fields the formation of a single quantum vortex inside the Josephson junction is not, in general, favourable. The field for the first multiquantum flux penetration has been derived. The field dependences of the flux-flow resistivity and the Josephson critical current density are also discussed.

Tunnel barriers on Pb–In–Au alloy films

Tunnel barrier oxides consisting primarily of In2O3 formed on Pb–In–Au alloys by various thermal and rf-plasma oxidation techniques were characterized in situ using ellipsometry and Auger Electron Spectroscopy (AES). The results were compared to measurements of the Josephson critical current density j1 and the specific capacitance of tunnel junctions fabricated on the same wafers. It was found for junctions with oxides consisting entirely of In2O3 that j1 exceeded 3 KA/cm2 and did not depend strongly on oxide thickness from about 3 nm (for thermally-grown oxides) to 4.5–7 nm (for rf-grown oxides). The addition of a thin layer of PbOx at the top of the barrier, either by backscattering during rf oxidation or by deposition onto a thermal oxide, caused by a substantial decrease in j1, while the presence of PbO in the bulk of the oxide had little effect. The specific capacitance of the lower current density junctions was essentially the same for either 3- or 4.5–6-nm thick oxides and was consistent with a barrier thickness of order 2 nm. These results are interpreted as evidence that tunneling occurs through a Schottky barrier in the indium oxide at the interface with the counterelectrode. Such a barrier, which is consistent with the known behavior of In2O3 as a degenerately-doped semiconductor, would account for effective tunneling and capacitive thicknesses less than the physical oxide thickness.

The dc Josephson current in cylindrical junctions

The dc Josephson current between co-axial cylinders is calculated for arbitrary Josephson penetration depth. The results support the interpretation of recent experiments which determined the magnetic-field dependence of the Josephson critical current density. The interpretation centers on the conclusion that the Josephson current vanishes when the fluxoid quantum number differ.