Applied Superconductivity最新文献

We performed transient noise simulations of a Rapid Single Flux Quantum RS Flip-Flop and compared computer results with predictions of a previously developed theory. The probability of error switching in dependence on the Flip-Flop inductance was calculated. The results show that noise causes essential narrowing of the tolerance margins. This effect depends on operation frequency.

Single electron devices are based on small normal metal electrodes called ‘islands’ separated by tunnel junctions. In such circuits, electrons can be transfered one by one when the energy of thermal fluctuations is less than the electrostatic energy necessary to add an extra electron to an island. We present here experimental results in single electron devices for which all normal-metal electrodes are replaced by superconducting ones. In such devices coherent Cooper pair tunneling (i.e. the DC-Josephson effect) combines with Coulomb blockade. These effects can be tuned in such a way that signatures of the macroscopic quantum coherence between two well-defined charge states become observable.

A comprehensive analytical theory of nonhysteretic rf SQUIDs operating in the adiabatic mode at elevated temperatures, around and above 77 K, is presented. When β≪1 (β is the hysteresis parameter) the theory is applicable also for rf SQUIDs operating in the nonadiabatic mode. In contrast to previous theories which are applicable only if βΓ≪1 (the case of small thermal fluctuations — 4 K), where Γ is the noise parameter, the present theory is valid for βΓ around unity or higher (the case of high thermal fluctuations, 77 K). A good qualitatively agreement with experimental data has been found. Based on this analytical investigation, the superiority at 77 K of rf SQUID over its counterpart, the dc SQUID, is theoretically predicted.

A SQUID measurement system is presented that was developed for analysis of antigen–antibody reactions in fluid samples. The detection method is based on the signal generated by the remanent or relaxing magnetization of nanoparticles which are attached as labels to the antibodies. The measurement system involves two 7.2-mm-diameter multiloop SQUID chip magnetometers (type W7A) with a typical field noise of √$\text{S}{}_{\mathrm{B}}$=1.5 fT/√$\text{Hz}$ arranged as an electronic gradiometer with a baseline of 160 mm. It is embedded in a liquid helium dewar with a tail bottom allowing 6 mm distance between SQUID and room temperature. A sophisticated direct-coupled read-out-electronics was realized with a bandwidth of 8 MHz. The read-out-electronics can be reset within τ_R ⩽0.5 μs. A 2 channel control unit was developed which enables environmental noise reduction. Data are acquired by a PC-controlled 8 channel system with 16-bit resolution and a maximum sampling frequency of f_s=196 kHz. First measurements are presented.

We fabricated YBa₂Cu₃O_7−x/PrBa₂Cu_2.8Co_0.2O_7−y/YBa₂Cu₃O_7−x ramp-edge Josephson junctions and measured their current–voltage (I–V) characteristics. Critical current (I_c) and conductance (G) were evaluated as a function of the barrier layer thickness, and both of them exhibited a nearly exponential dependence. The decay parameters for I_c and G were estimated to be 1.1 and 1.5 nm, respectively. We also examined superconducting quantum interference devices (SQUIDs) using this type of junction as a preliminary test for realizing oxide superconductive integrated circuits with high-speed operation. DC SQUIDs with direct-coupling control lines and no ground plane were fabricated and their loop inductance was evaluated. The obtained SQUID inductance ranged from 8 to 38 pH for a 4-μm-wide, 3- to 12-μm-long hole in a rectangular SQUID loop. In addition, a flip–flop circuit based on such a dc SQUID was fabricated. Both flux trapping and flux detrapping in the SQUID loop hole were confirmed when 1-ms-wide current pulses were injected into the SQUID loop.

The paper presents an overview of recent results for NbN phonon-cooled hot electron bolometric (HEB) mixers. The noise temperature of the receivers based on both quasioptical and waveguide versions of HEB mixer has crossed the level of 1 K·GHz⁻¹ at 430 GHz (410 K) and 600–650 GHz (480 K) and is close to this level at 820 GHz (1100 K) and 900 GHz (980 K). The gain bandwidth measured for quasioptical HEB mixer at 620 GHz reached 4 GHz and the noise temperature bandwidth was almost 8 GHz. Local oscillator power requirements are about 1 μW for mixers made by photolithography and are about 100 nW for mixers made by e-beam lithography. The studies in terahertz receivers based on HEB superconducting mixers now present a dynamic, rapidly developing field.

For “SNS step and gap”, and for “step-edge grain boundary”, Josephson-junctions steep steps in the substrate are needed to get either a break in the YBa₂Cu₃O₇-film (SNS) or to induce grain boundaries in the growth of the YBa₂Cu₃O₇-film across the step (step-edge).

We report on the influence of the profiles of ion-beam etched substrate steps in MgO and SrTiO₃ on the growth of YBa₂Cu₃O₇-films at the step-edges and on the electrical properties of the different Josephson-junction types. On MgO we have fabricated Josephson-junctions of the SNS-type (using Ag as “N”), while for the step-edge junctions SrTiO₃ was used. Both types of junctions exhibit Josephson-effects at 77 K. For the steps we use photoresist AZ 5214 E with steep edges as the mask. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis on differently fabricated steps are presented and compared to the electrical properties of patterned YBa₂Cu₃O₇-films grown on them. Especially the role of material redeposited at the edge of a step during ion-beam etching is discussed. The reproducible step edge profiles are reflected in reproducible electrical properties of the Josephson-junctions and dc-SQUIDs (on chip and chip to chip).

We present a review of the dynamical states of the long Josephson junction when dc and ac currents are considered as bias terms. When only the dc terms are present, depending on the value of the external magnetic field, the excitations consists of fluxon oscillations, cavity modes and travelling waves interactions with the Josephson effect. These three different modes generate well identified singularities in the current-voltage characteristics. We also report on the effect that ac current terms applied to the transmission line can have on these current singularities. In this case a theoretical analysis, based on the nature of dc excitations, has provided predictions for the ranges of locking of the external signal to junction’s oscillations. These predictions are in good agreement with experimental and numerical data that have been reported in the literature.

To explore the idea that weak links are responsible for power dependence, we have measured and modeled the microwave impedance of fabricated Josephson-junctions, in which we have observed evidence for Josephson-vortex creation and annihilation by the microwave currents. We believe that we have observed long-junction effects in fabricated high-T_c Josephson-junctions and that these are the first observation of microwave-frequency vortex creation. To explain the nonlinear surface impedance of thin films of the high-T_c materials, an extended-coupled-grain model is introduced and discussed. The model incorporates a series array of Josephson-junction defects with a distribution of I_cR_n products. Comparison with experiments shows good qualitative agreement.