A 4K Josephson memory

The authors describe the design and experimental performance of a 4 K×1-bit Josephson (RAM). For high-speed memory operation, the authors have developed a compact AND gate for the decoder, a high-voltage driver gate, and a capacitively coupled single-flux quantum memory cell. The 4 K memory was designed using these gates and cell and was fabricated with Nb/AlO_x/Nb junctions. The minimum access time was 590 ps, and the total power dissipation was 19 mW     

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     

A new 10-V array of Josephson junctions based on low-frequencydesigns scaled up to 94 GHz

A novel approach is described for the design of improved coupling of millimeter-waves in a 10-V array of Josephson junctions. In this approach, microwave structures were fabricated and tested in the range 2.8 GHz to 3.6 GHz, and a scaling calculation was then used to transform them to 94 GHz. By optimizing the trade-off between antenna size and efficiency, it was possible to make more space available on the device chip to increase the number of Josephson junctions. Simulation results showed that the optimal antenna is a short double-fin-line type. This antenna type allowed us to fabricate a Josephson junction array that had 25944 Nb/Al-AlO_x/Nb junctions. For this new array, zero-crossing steps were observed up to 18.5 V. The new array was then successfully used to calibrate a Zener voltage standard that had a 10-V output     

Near-Zero Bias Arrays of Josephson Tunnel Junctions Providing Standard Voltages up to 1 V

Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.     

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     

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.     

Half-harmonic parametric oscillations in Josephson junctions

Half-harmonic generation in Josephson tunnel junctions and microbridges is investigated theoretically, numerically, and experimentally. It is found that such generation may take place both at zero voltage and on rf-induced steps in tunnel junctions, but not in microbridges. A simplified theory, which leads to the definition of a plasma resonance on rf-induced steps, is presented. The experiments are made on junctions with different current densities and with pump frequencies at 18 and 70 GHz.This work was supported by The Danish Natural Science Research Council, grants 511-8048, 511-10098, 511-10279, and 511-15059.     

Spin-Polarized Transport Phenomena in Double Magnetic Tunnel Junctions Caused by Ferromagnetic CoFe Nanoparticles

We present some unusual spin-polarized transport phenomena in asymmetric double barrier magnetic tunneling junctions (ADBMTJs) with CoFe/AlO_x/ferromagnetic nanoparticle (FM-NPs)/AlO_x/Ta structures. The conductance curves and the magnetoresistance ratio clearly oscillate with applied bias voltage, indicating the presence of Coulomb blockade effects due to isolated ferromagnetic nanoparticles in the parallel configuration in the ADBMTJ. The oscillation period is about 1.5 mV at 2 K.     

Subharmonic gap structure and subharmonic Josephson steps

Subharmonic microwave-induced Josephson step structure is often observed in superconducting junctions that exhibit subharmonic gap structure. The two theories, multiparticle tunneling and self-coupling due to an electromagnetic field set up by the ac Josephson current, which have been suggested to explain the subharmonic gap structure are investigated with respect to the possible existence of subharmonic Josephson steps. Also treated is a Ginzburg-Landau calculation of the current-phase relation for Josephson junctions. All three theories give subharmonic Josephson steps with the same microwave-power dependence. The temperature dependence of the subharmonic step structure and the subharmonic gap structure is discussed. It seems the self-coupling is the main cause for both effects in tunnel junctions and for the subharmonic gap structure in Dayem bridges, while the subharmonic step structure in the Dayem bridge seems to be a geometrical effect related to “multiparticle” transfer processes.     

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.     

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     

A fast Josephson SFQ shift register

A Josephson SFQ (single flux quantum) shift register circuit operating under a two-phase power was designed and tested. The test cell was fabricated using the Nb/AlO_x/Nb junction SNEAP process. The main data latch is DC biased; the data storage is accomplished by the transfer of a single flux quantum in or out of a SQUID (superconducting quantum interference device) superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended     

Magnetic field behavior of Fiske steps in long Josephson junctions

The magnetic field dependence of the maximum current of Fiske steps for long, one-dimensional, Josephson tunnel junctions has been measured. Experimental data obtained from junctions of various normalized lengths are presented; for slightly long junctions a comparison with the theory is made.     

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.     

Simulation Study of Noise Effect on Shapiro Steps in High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Josephson Junctions Using RCLSJ Model

The current voltage characteristics of Josephson junctions at finite temperatures under microwave irradiation are simulated in the resistively, capacitively, inductively shunted junction (RCLSJ) model including white noise. The simulated results as function of microwave power clearly show the appearance of Shapiro steps. It is shown that higher steps are suppressed for low microwave amplitudes. The rounding of the Shapiro steps observed experimentally has been reproduced by taking noise into consideration in RCLSJ model.     

Studies on microbridges of superconducting YBCO thin film

Critical current densities of the superconducting Y-Ba-Cu-O (YBCO) films have been observed to decrease with the increase of power of microwave radiation. Presence of Josephson type of junctions in the microbridges has been established from the microwave irradiation and magnetic field studies. BCS energy gap parameter (2/kT _c ) has been calculated from thedI/dV characteristics and found to be 3.7 at 13 K.     

Alternating current Josephson effect and resonant superconducting transport through vibrating Nb nanowires

In 1962, Josephson made a celebrated prediction: when a constant voltage is applied across a thin insulator separating two superconductors, it will generate an oscillating current. These oscillations are ubiquitous in superconducting weak links of various geometries, and analogues have been found in other macroscopic quantum systems, such as superfluids and gaseous Bose-Einstein condensates. The interplay between the oscillating current and external microwave radiation of matching frequency (Shapiro steps) or with internal electrodynamic resonances (Fiske effect) appear as changes in the current-voltage characteristics of superconducting tunnel junctions and provide further insight into the phenomenon. Here, we report measurements and theoretical studies suggesting that Josephson current oscillations interact with atomic-scale mechanical motion as well. We formed a niobium dimer nanowire that acts as a weak link between two superconducting (bulk) niobium electrodes. We find features in the differential conductance through the dimer which we believe correspond to excitations of the dimer vibrational modes by Josephson oscillations and support our results with theoretical simulations.     

Propagation of single flux quantum pulse of superconducting transmission line

Josephson tunnel junction switching by single flux quantum (SFQ) pulses generated by a two-junction interferometer and propagated on a superconducting transmission line have been investigated experimentally and by numerical simulation. High current density tunnel junctions have been used to enhance voltage SFQ pulse amplitudes and reduce junction damping parameters. The measured effective amplitudes of SFQ pulses allow an evaluation of the applicability of transmission lines to SFQ logic circuits and Josephson samplers.     

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。     

Investigations onNb/Al-AlO x -Al/Nb proximity tunneling junctions for X-ray detection

A new Nb/Al-AlO_x-Al/Nb trilayer process using a movable mechanical slit for junction-detector applications is described. Best junctions have a quality factor of 42 mV with a Josephson current density of 1200 A/cm ² at 4.2 K. The temperature dependence of the sub-gap current in the range of 0.45 to 4.2 K has been measured. Deviations from the thermally activated behavior due to imperfections in the tunneling barrier are observed. We compare our simple method with the usual whole wafer processes and we discuss the influence of proximity effects in these junctions in terms of the models proposed by Golubov et al. and by McMillan.