Josephson shift register design and layout

Integrated circuit chips were designed and fabricated, based on a Josephson shift register circuit that simulated operation at 25 GHz using the SPICE program. The 6.25-mm² chip featured a twelve-gate, four-stage shift register fabricated with Nb/AlO_x/Nb Josephson junctions with a design value of 2000 A/cm² critical current density. SUPERCOMPACT, a general program for the design of monolithic microwave integrated circuits, was used to model the effects of layout geometry on the uniformity and phase coherence of logic gate bias currents. A layout geometry for the superconductive transmission lines and thin-film bias resistors was developed. The original SPICE-designed circuit was modified as a result of these calculations. Modeling indicated that bias current variations could be limited to 3% for all possible logic states of the shift register, and phase coherence of the gates could be maintained to within 2° at 10 GHz. The fundamental soundness of the circuit design was demonstrated by the proper operation of fabricated shift registers     

Direct Comparison of Josephson Waveforms Using an AC Quantum Voltmeter

A new method for calibrating ac voltage waveforms based on measuring the difference relative to a synchronously synthesized waveform supplied from a programmable Josephson array is presented. In order to test the accuracy of this approach, the differences between two Josephson waveforms have been measured at the 1.2-V level for frequencies up to 5 kHz. The uncertainties achieved are at least a factor of ten lower than those from the direct sampling method. Below 400 Hz, this new method gives an uncertainty of 5 10^-8 (k = 1), while at 1 kHz, the uncertainty is about 2 10⁷ (k = 1)     

Y-Ba-Cu-O/Nb Josephson tunnel junctions

The authors fabricated Y-Ba-Cu-O/Au/AlO_x/Nb and Y-Ba-Cu-O/AlO_x/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Sintered Y-Ba-Cu-O was used as the base electrode. Superconducting Josephson current and hysteresis of the current-voltage characteristics, which are typical features of Josephson tunnel junctions, have been observed at 4.2 K. RF-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed, and RF-induced subharmonic steps have also appeared. The superconducting Josephson current was modulated by the magnetic field     

First Direct Comparison of a Cryocooler-Based Josephson Voltage Standard System at 10 V

A commercially available and fully automated 10-V Josephson voltage standard system with a liquid helium free cooling has been developed as a result of the cooperation between the Institute of Photonic Technology and Supracon AG, both in Jena, Germany. The system operates with an array of 19 700 superconductor–insulator–superconductor Josephson tunnel junctions installed in a pulse tube cooler. A stable operation is achieved by the proper integration of the voltage standard circuit to the cold stage of the cryocooler. Different operation setups are discussed. A direct comparison of a cryocooler-based Josephson voltage standard system versus a liquid-helium-based system was performed at a voltage level of 10 V. We obtained a voltage difference of 1.3 nV with a total combined uncertainty of 2 nV. This corresponds to a relative uncertainty of $2 times 10^{-10}$.      

Mg4Nb2O9/CaTiO3复合陶瓷的微波介电性能

采用传统固相法制备Mg4Nb2O9/CaTiO3复合陶瓷,研究Li2CO3-V2O5(LV)共掺杂对其烧结特性、微观结构和微波介电性能的影响.实验结果表明:一定量的LV掺杂能够使Mg4Nb2O9/CaTiO3复合陶瓷的烧结温度降至1200℃;XRD和EDX综合分析表明,样品由(Mg4-xCax)Nb2O9/(Ca1-x...     

约瑟夫森结阵器件的研究进展

介绍了约瑟夫森效应在电压基准方面的应用,综述了目前国内外对用于电压基准的约瑟夫森结阵的研究和发展过程,重点介绍了国内用于电压基准的Nb/NbxSi1-x/Nb单结的研究进展。     

Direct comparison of two independent Josephson voltage standards at1 V and precision measurements up to 10 V

Two Josephson voltage standards have been compared using a room-temperature electronic nanovoltmeter with a peak-to-peak noise of about 2 nV at the 1-V level corresponding to a RMS uncertainty of 4×10^-10. The excellent stability in maintaining the desired voltage steps makes it possible to obtain recorder traces comparing Nb/Al₂O₃/Nb Josephson standards with Weston cells and Zener reference standards at 1 V and 10 V. At 10 V the best result shows a peak-to-peak noise of 250 nV corresponding to a RMS uncertainty of 5×10^-9 for a Zener reference and 50 nV corresponding to 1×10^-9 for a series connection of nine Weston cells. As an example for the application of the Josephson standard as a potentiometer the deviation in the linearity of a digital voltmeter is confirmed to be on the order of 0.1 p.p.m. in the range from -10 V to +10 V     

Microwave-Induced Constant Voltage Steps at Series Arrays of Josephson Tunnel Junctions with Near-Zero Current Bias

Microwave-induced, constant-voltage steps in the current-voltage characteristics of Josephson tunnel junctions are used as a reference voltage in cryogenic voltage standards. This relatively low reference voltage of a few millivolts could be increased by using series arrays of tunnel junctions instead of a single junction. Series arrays consisting of 2, 6, 14, and 54 tunnel junctions were tested at a frequency of 70 GHz. The use of this relatively high frequency and of lead-alloy base electrodes allows zero crossing steps of up to 34 mV to be observed for an array with only 54 junctions when irradiated by an RF power of 32 mW. The measurements were performed at 4.2 K.     

New wave phenomena in series Josephson junctions

The use of electromagnetic waves to obtain coherence between widely spaced Josephson tunnel junctions is explored. A pair of coupled non-linear differential equations is developed for the case of junctions used as series elements in a microstrip transmission line. We use analysis to demonstrate the existence of oscillatory traveling waves, solitary waves, and standing waves on such a transmission line. We report simulations that demonstrate these waves, and an observation of standing waves in a real series array. It appears that substantial power at frequencies above 100 GHz may be obtained from these series arrays.     

An Effective Method for Improving Emissions from Intrinsic Josephson Junction Arrays of Misaligned Tl2Ba2CaCu2O8+δ Thin Films

In this paper, an effective method for improving emissions from Josephson junction arrays was proposed. We took the substrate as a dielectric resonance antenna to improve the coupling of intrinsic junctions to electromagnetic waves. Electromagnetic simulations were performed in parallel with experiment investigations. Sharp radiation peaks at 76.9?GHz were detected from intrinsic Josephson junction arrays in misaligned Tl₂Ba₂CaCu₂O_8+?? thin films at 78?K. Experimental results proved the effectiveness of this method, which offered a competitive choice for coherent emission from intrinsic Josephson junction arrays.     

Josephson Bicrystal Junctions on Sapphire Substrates for THz Frequency Application

Thin film metal oxide superconducting bicrystal junctions on sapphire substrates with I _c R _N products up to 2.5 mV at 4.2 K for width 4 μm and normal-state junction resistance 10–60 Ω were fabricated and characterized at dc and THz frequency. Three types of samples—one with broadband log-periodic antenna, another with double-slot antenna for 300 GHz and third one with double-slot antenna for 400 GHz—have been investigated at THz frequency. New design of antenna coupling with Josephson junction was elaborated for minimization of THz frequency losses in superconducting film. For a particular case of f=320 GHz double-slot antenna, a ratio for bandwidth Q = f/Δf ≈ 10 was measured.     

Comparison of a Multichip 10-V Programmable Josephson Voltage Standard System With a Superconductor–Insulator–Superconductor-Based Conventional System

We developed a 10-V dc programmable Josephson voltage standard (PJVS) using a multichip technique. The PJVS was based on $hbox{NbN/TiN}_{x}/hbox{NbN}$ junctions and operated using a 10-K compact cryocooler. We carried out an indirect comparison with a superconductor–insulator–superconductor-based conventional Josephson voltage standard (JVS) by measuring the voltage of a 10-V zener diode reference standard. The combined standard uncertainty of the comparison was $u_{c} = 0.03 muhbox{V}(k = 1)$, and the relative combined standard uncertainty was $3 times 10^{-9}$.      

The new BIPM 1-V reference standard based on an array of Josephsonjunctions

The authors report the first measurements at the Bureau International des Poids et Mesures (BIPM) with a 1-V reference standard based on an array of Josephson junctions. A novel procedure for standard cell measurements is described. It allows the measurement of a cell electromotive force (EMF) with a random uncertainty of less than 1 nV and a total uncertainty of about 2 nV while avoiding direct connection of the cell to the array. Comparisons with an older Josephson reference standard, which is based on a junction output of 10 mV, were made both indirectly by means of standard cells and directly by connecting the instruments together. The results and the random uncertainties imply that the value assigned to a standard cell using the array minus that given by the older apparatus is -18.8±3.4 nV for the indirect method and -17.1±1.5 nV for the direct method. Taking into account the estimated total uncertainty of 23.5 nV for the old system, the agreement is excellent     

Design and optimisation of a high-frequency EMC wideband horn antenna

A new approach for the design of high-frequency electromagnetic compatibility (EMC) broadband double-ridged horn (DRH) antennas is presented. In this approach, first a conventional DRH antenna at 1-18 GHz frequency band is considered. Using a thorough sensitivity analysis of different structural parameters of the 1-18 GHz DRH antenna, several modifications are applied to this antenna to overcome its deficiencies especially in its radiation pattern at higher frequencies. The final achieved design is then scaled up in the frequency to arrive at a design suitable for higher frequency ranges. A wideband DRH antenna for 18-40 GHz frequency band is then designed using this approach. The lower frequency ratio of 1:2.2 in the new antenna as opposed to the 1:18 ratio in the conventional antenna permits us to choose the best frequency window for the scaling up process. Besides, an optimisation technique is employed to further improve the antenna performance to meet the design goals over the entire new frequency band, that is, to have a single main lobe directed along the horn axis without any deterioration, and to have acceptable broadband gain with the minimum of 10 dB, and voltage standing wave ratio (VSWR) of less than 1.5. The final design which is more compact compared with the other commercial antennas has been used to make a prototype antenna. Measurements show that the built prototype meets the design goals very well     

Monitoring of the characteristics of Josephson microcircuits used in standards of the volt

The results of investigations of microcircuits based on Nb−AlO_x−Nb Josephson tunnel junctions, used in modern standards of the volt, are presented. The measured static characteristics of the microcircuits, their temperature dependences in the 4.2–1.7 K temperature range, the frequency response in the 54–80 GHz frequency band, and the attenuation in a microstrip line are used to optimize the construction of microcircuits and their manufacturing technology at PTB (Physikalisch-Technische Bundesanstalt, Germany). It is shown that microcircuits of a new construction, made using a modified PTB technology, have better characteristics and can be used as the unit of voltage at the level of both 1 V and 10 V. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 43–48, November, 1997.     

Josephson semiconductor interface circuit

This paper describes an output interface circuit which allows Josephson circuits to communicate with semiconductor circuits. The circuit combines Josephson and GaAs drivers to drive a 50 μ load at a signal level of semiconductor circuits. The output voltage of 2.8 mV (usual for Josephson gates using Nb/AlO_x/Nb junctions) was increased to 1.7 V. The interface circuit has been operated up to 800 MHz.     

10 V SINIS Josephson junction series arrays for programmablevoltage standards

A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al₂O₃ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed     

Flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependent current-voltage characteristics of stackled Nb/(Al−AlO_x/Nb)_n long Josephson junctions are investigated experimentally. The thickness of their common superconducting electrodes provides the magnetic coupling between the junctions. For stacks of n=7 Josephson junctions the current-voltage characteristics display collective flux-flow behaviour of Josephson vortices. In the interior layers Josephson vortices move simultaneously under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show broad range of characteristic frequencies. The measurements can be qualitatively explained by the Kleiner model for the resonances in stacks. Mutual locking of junctions in the stack is indicated by pronounced cavity resonances with large voltage spacing.     

Frequency Characteristics of Misaligned Tl2Ba2CaCu2O8 Thin-Film Intrinsic Josephson Junction Arrays Irradiated by Millimeter-Wave

We have investigated the millimeter wave irradiation characteristics of misaligned Tl₂Ba₂CaCu₂O₈ thin-film intrinsic Josephson junction arrays at a liquid nitrogen temperature in an antenna system by both simulation and experiments. The dielectric substrate was regarded as a dielectric resonance antenna to improve high-frequency electromagnetic coupling between the intrinsic Josephson junction array and a horn antenna. A useful model for simulating the microwave system was devised to demonstrate and analyze the optimization of the irradiation in the antenna system. The electric field distribution in the antenna system was computed and displayed. Also, the near-field and far-field frequency characteristics of the receiving antenna and transmitting antenna were calculated and analyzed to study the mechanism of the irradiation. In the experiment, by detecting the suppressed critical current of the IJJA, an optimum transmission frequency range was measured and the frequency characteristic was studied in the transmitting and receiving antenna system. The critical current of the IJJA was suppressed to 37 % and the optimum coupling effect was achieved at 74.8 GHz. And the electromagnetic simulation matched up well with the experimental result. Potential reasons of the acceptable nuances between the simulation and experiment results were also taken into account. The influence the size of the microbridge had on the frequency characteristic of the dielectric antenna were elaborately discussed.     

Coupling of Josephson Radiation to Voltage Standard Arrays

We coupled the radiation emitted by arrays of Josephson junctions oscillators to detector arrays of small Josephson junctions. The number of junctions in the detector array ranges up to 1536, which is typical for a 1V standard array operation. Evidence is presented that both uniform coupling of the emitted radiation over all the small junctions arrays and coherent emission of the Josephson oscillators can be achieved. PACS numbers: 74.50. + r, 74.40. + k.