Critical Current Of Junction Research Articles

LTS Josephson junction critical current uniformities for LSI applications

Manufacturing yields of large scale superconducting circuits depend strongly on the uniformity of junction critical currents. We report on junction manufacturing tolerances based on extensive measurements of Nb- and NbN-based junction arrays and individual junctions. Transient waveforms induced by switching of a single junction have sufficient amplitude to switch other junctions in a series array. We have measured the effect of sympathetic switching and developed damping structures to dissipate switching transients. Comparisons of critical current distributions measured on individual junctions with critical current distributions determined from series junction arrays are presented. In addition, the validity of using series arrays of large numbers of junctions to assess the critical current uniformity is discussed. >

CMOS self-calibration of Josephson flash-type A/D circuits

We present CMOS self-calibration and bias circuits for Josephson circuits. In Josephson technology, process variations in critical circuit parameters such as junction critical current, resistance, and inductance adversely affect the performance. For example, process deviations in junction critical currents leads to low dynamic range and nonuniform quantization steps in flash-type A/D converters. While CMOS technology has a speed disadvantage with respect to Josephson technology, it does make available highly accurate current sources that are relatively process independent and difficult to obtain in Josephson technology. Here, we discuss how the 4 K CMOS self-calibration of a Josephson flash type A/D converter can be used to improve dynamic range. >

Josephson behaviour and flux penetration effects in YBCO double tilt-angle step-edge junctions

YBCO thin-film step-edge junctions have been prepared whose characteristics show features corresponding to the two tilt-angle grain-boundary junctions formed at the step. The I-V characteristics of these junctions are fitted to high accuracy by a model consisting of two noisy resistively-shunted Josephson junctions. In applied magnetic fields up to a few hundred microtesla, both junction critical currents show reversible flux modulation, in one case indicating excellent uniformity of the critical current distribution along the step. In stronger magnetic fields, the junction critical current modulation shows evidence of flux penetration into the film, i.e. the mixed state. Taking demagnetising effects into account the lower critical field of YBCO at 77 K, /spl mu//sub 0/H/sub e1,e/, is found to be 12 mT. >

Excess low-frequency noise in YBCO thin-film devices

Excess low-frequency noise degrades the performance of many of the high T/sub c/ superconducting devices that have been proposed or demonstrated. We report measurements of noise in grain-boundary junctions and flux-flow amplifier devices that have been fabricated in several different laboratories. The system is calibrated in terms of absolute noise temperature, and its voltage noise sensitivity is below 0.1 nVHz/sup -1/2/. Device characteristics and noise have been measured as a function of junction area, critical current, temperature, bias current and magnetic field. Noise levels in a fixed frequency band of 40-60 kHz are reported, as well as the frequency dependence between 0.1 Hz and 60 kHz. Noise temperatures exceeding the device physical temperature at certain levels of bias current are attributed to fluctuations in critical current and normal tunnelling resistance. Noise due to magnetic flux motion is also seen. We compare results with available theory and other relevant data in the literature, and assess the practical effects of noise on the performance of high T/sub c/ devices such as SQUIDs, flux flow amplifiers and bolometers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High T/sub c/ SNS step-edge junctions and SQUIDs by using; YBa/sub 2/Cu/sub 3/O/sub 7//CaRuO/sub 3/ double layer

Superconductor-normal-superconductor (SNS) step-edge junction dc SQUIDs were fabricated from YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films with CaRuO/sub 3/ (CRO) on top as the normal metal. The films were made by pulsed laser deposition (PLD) and patterned by photolithography and ion milling. The junctions and SQUIDs had to be annealed in oxygen environment for a much longer period of time (5-48 hours) compared with standard YBCO film preparation (as deposited or 1 hour), indicating that the top CRO film blocks oxygen in and out of the YBCO film. Before annealing T/sub c/'s of CRO/YBCO double layers and junctions made from them were <70 X and <50 K, respectively, and increased to >88 K and >77 K, respectively after long time annealing. Current-voltage curves of the junctions showed well-behaved resistively-shunted-junction (RSJ) properties and the temperature dependence of the junction critical current was roughly linear. SQUIDs made of those junctions showed voltage modulation in response to applied fields at 77 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High T c superconductor-normal-superconductor step-edge junction dc SQUIDs with CaRuO3 as the normal metal

Superconductor-normal-superconductor (SNS) type step-edge junction dc SQUIDs were fabricated from YBa2Cu3O7−δ (YBCO) films with CaRuO3 as the normal metal by pulsed laser deposition and Ar+ beam patterning techniques. CaRuO3 films were grown epitaxially on YBCO and had a metallic conduction property. Current-voltage curves of the junctions showed a resistively shunted-junction behavior. Temperature dependence of the junction critical current was linear in agreement with the theory for junctions with the junction length shorter than twice of the normal coherence length (dN&amp;lt;2ξN). A well-behaving SQUID modulation signal was observed up to 52 K with a focusing parameter of 8.6.

Fabrication of a quasi-three-dimensional Josephson-junction array

In a first step towards fabricating a truly 3-dimensional (3- d) Josephson-junction array, we have fabricated a quasi-3- d one, i.e. an array consisting of two 2- d arrays stacked on top of and capacitively- and inductively-coupled to one another. We discuss in detail the fabrication techniques we have employed to create this novel type of an array. Such techniques include alignment using topological markers, electron-beam lithography, shadow evaporation, and planarization. We have fabricated different samples whose 2- d arrays consist of underdamped Al-Al 2O 3-Al junctions in which either the Josephson-coupling energy E J=ℏi c/2e , where i c is the junction critical current, dominates the single-junction charging energy E c=e 2/2C , where C is the junction capacitance, or vice versa. With these arrays, we plan to study the interaction between vortices and/or charges.

Influence of the Josephson junction on the impedance and noise of a resistive superconductive quantum interference device

The impedance and noise of a resistive superconductive quantum interference device (R-SQUID) have been measured as a function of the dc and rf currents applied to it. The Josephson junction was adjustable so that data were also taken for several values of the junction critical current. The results were compared with the predictions of a resistively shunted junction (RSJ) model which takes into account the influence of the Josephson junction on the impedance and noise. The agreement was found to be quite good and demonstrates that the noise in the circuit is well understood. Use of the R-SQUID as a noise thermometer below 1 K is assessed in terms of corrections due to the RSJ model. It is demonstrated how the dc and rf currents may be adjusted so that the total noise of the R-SQUID is reduced to within 0.1% of the Johnson noise generated by the resistor alone. Under these conditions, the R-SQUID may be used as a noise thermometer to determine thermodynamic temperature to this inaccuracy from 6 to 700 mK.

Dynamics of quasioptical Josephson junction arrays for submillimeter coherent sources

The dynamics of the quasi-optical two-dimensional Josephson junction arrays for oscillators is investigated. Numerical time-domain simulations of small-scale parallel arrays were performed. It is found that the DC biasing circuit determines the states of these arrays. It is also found that other states exist in which parts of the array operate in phase. The authors investigate the sensitivity of in-phase states to variations in junction critical currents and to nonvanishing fluctuations. It is found that the DC self-field effects do not provide a stable phase-locking mechanism and that RF interactions are necessary for locking the array in phase. >

Detection of magnetic flux with superconducting quantum interference gratings

The authors have carried out finite-inductance calculations of the critical current vs. flux (I/sub c/- Phi ) and voltage vs. flux (V- Phi ) characteristics of superconducting interferometers with many Josephson junctions in parallel. At least two features of the calculations suggest that many junction interferometers, called superconducting quantum interference gratings, might be advantageous for the detection of magnetic flux. First, the voltage noise can be reduced significantly for a given flux-to-voltage transfer coefficient, a feature which is likely to improve the magnetic flux sensitivity of both low- and high-T/sub c/ superconducting devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction grating such as enhancement and narrowing of the peaks in the I/sub c/- Phi characteristic. Specific schemes for efficiently coupling flux into the device are proposed. >

Arrays of high temperature superconductor Josephson junctions

The authors present results on parallel, series, and two-dimensional arrays of high-temperature superconductor junctions. The parallel arrays show the expected field dependence of critical current and can function as vortex flow transistors. The series arrays give data about the spread in junction critical currents for the present process. The two-dimensional arrays show many of the phenomena previously observed in low-T/sub c/ technology, such as field frustration of the phase transition, some evidence for superlattice states, and the tentative indications of giant Shapiro steps, showing large-scale cooperative behavior in the device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

DC-SQUIDs with low noise and large beta /sub L/-values on (Y)ZrO/sub 2/ bicrystal substrate

High-quality YBa/sub 2/Cu/sub 3/O/sub 7/ DC-SQUIDs (superconducting quantum interference devices) fabricated on bicrystal (Y)ZrO/sub 2/ substrates have been operated at temperatures from 10 K to the critical temperature at 89 K. Stable operation with low white noise levels is found even for beta /sub L/-values as large as 60 for these nonhysteretic junctions ( beta /sub c/<<1). In addition, two noise sources were identified in the 1/f noise: (1) a junction critical current dependent noise S/sub v/ approximately I/sub 0/(T)/sup 2.7/ and (2) a narrow peak in S/sub v/(T) centered at 87 K with its origin in vortex motion within the superconductor. Optimum low-noise operation was obtained in a minimum at 85 K with a flux noise S/sub Phi /(10 Hz)=1.5*10/sup -9/ Phi /sub 0//sup 2//Hz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reproducibility of niobium junction critical currents: statistical analysis and data

The authors present a statistical model to account for Josephson-junction critical current variability produced in integrated circuit processing. On the basis of data gathered from over a thousand junctions from the TRW superconducting integrated-circuit foundry line, it is shown that present niobium technology can produce junctions with critical currents identical to within a few percent, depending on junction size. Less than 2% (1 sigma ) spread critical currents were measured for 10 mu m junctions. >

Critical current and Josephson properties of single grain boundary in BaPb 1- xBi xO 3 bicrystals

The critical current and transport properties of the single grain boundary in bulk bicrystals of the oxide superconductor BaPb 1− x Bi x O 3 were measured. The I − V characteristic clearly shows a Josephson supercurrent and a system of self-excited current resonances, and the measured critical current oscillates in an applied magnetic field. Estimates of the Josephson junction size, based on the period of these oscillations, gives values close to the geometric size of a single grain boundary and we conclude that the grain boundary is single and an unbroken Josephson barrier exists, with some inhomogeneities in the spatial distribution of the critical current. The temperature dependence of the grain boundary junction critical current was investigated, including effects of thermally activated phase slippage. The temperature dependence of the critical current (without reduction due to fluctuations) in terms of the power law I c ≈ (1 − T/ T c) 1.6 near the critical temperature was evaluated by resistive transition analysis.

Resistance steps in underdamped Josephson-junction arrays.

The response of N\ifmmode\times\else\texttimes\fi{}N underdamped Josephson-junction arrays to dc-current bias and dc-voltage bias is studied numerically. The current-voltage (I-〈V〉) relation reveals a substantial hysteresis. If a dc-current bias is applied to inhomogeneous underdamped arrays (i.e., arrays with a random distribution of junction critical currents), the I-〈V〉 curve exhibits many (but no more than N) steplike increases of resistance, in agreement with experiments of Tighe, Johnson, and Tinkham. These steps are shown to arise from row-switching behavior of the arrays, in which one or several rows of junctions across the width of the array switch simultaneously from a supercurrent state to a resistively dissipative state. If the array is voltage biased rather than current biased, this row-switching behavior is observed even in homogeneous underdamped arrays, in qualitative agreement with the experiments of van der Zant et al. The resistive steps persist in an applied magnetic field and melt at sufficiently high temperature. We find no evidence for row switching in overdamped arrays, whether they are current biased or voltage biased. We speculate briefly about the physical origins of this behavior.

Applications of Parallel Josephson Junction Arrays for the Detection of Magnetic Flux

ABSTRACTWe have carried out finite-inductance calculations of the critical current vs, flux(Ic-φ) and voltage vs, flux (V-φ) characteristics of superconducting interferometers with many Josephson junctions (JJ's) in parallel. At least two features of our calculations suggest that interferometers with many junctions, which we call Superconducting QUantum Interference Gratings, or SQUIG's, might be advantageous for the detection of magnetic flux. First, the voltage noise can potentially be reduced significantly as compared to a dc SQUID with the same overall voltage-to-flux transfer coefficient - a feature which might reduce 1/f noise and enhance the magnetic flux sensitivity of both low and high Tc superconducting (HTS) devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction-grating like enhancement and narrowing of the peaks in the Ic-φcharacteristic, suggesting that flux uniformity, rather that critical current uniformity, is of primary importance.

Enhanced quantum interference effects in parallel Josephson junction arrays

We have calculated the total critical current as a function of applied magnetic flux for a superconducting interferometer consisting of many Josephson junctions in parallel. An enhancement and narrowing of the periodic principal maxima in the critical current versus flux characteristic is predicted as the number of junctions in parallel increases, even when the effects of finite self- and mutual inductances and nonuniformity of the junction critical currents are included in the calculations. The possible application of the superconducting quantum interference grating, or SQUIG, for the detection of magnetic flux is discussed.

Studies of the superconducting properties of YBCO/Ag/YBCO proximity junctions using a ceramic composite ring

We have studied the properties of YBCO/Ag/YBCO proximity interfaces in a Y 1Ba 2Cu 3O 7−δ composite ring close to T c; the junction critical current as a function of temperature (76–85K) and magnetic field (up to 50G). If a SNS junction is a part of a composite superconducting ring, the magnitude and relaxation of a macroscopic persistent current circulating around the ring provides a direct evidence that the junctions are really superconducting. We manufactured a SQUID-like ceramic ring containing two parallel SNS junctions and measured profiles of the trapped magnetic field due to the persistent current generated in the zero-field-cooled ring. The junction critical current was found to be in reasonable agreement with a formula for a SNS junction: I c(T)= I c ( O) [1−(T/T c)] 2.

Two-dimensional arrays of Josephson junctions as voltage-tunable oscillators

The authors have detected coherent emission from two-dimensional (2D) arrays of superconductor-insulator-superconductor (SIS) Josephson junctions. 2D arrays emit coherent radiation over a frequency range of 60 to 210 GHz, when coupled to detector junctions through DC-blocking capacitors. The detector junctions exhibit Shapiro steps at frequencies corresponding to the voltage across single-array junctions. The maximum power from a 10-by-10 junction-array coupled to a detector junction occurs at 150 GHz and is estimated to be 0.4 mu W, based on simulations of the detector circuit. By varying the number of array junctions, the array geometry, the junction critical current, and the coupling circuit, they have begun determining the essential conditions for observing coherent emission.

Flux quantization in single-crystal YBa2Cu3O7- delta as a function of dc and microwave magnetic fields.

Using the modulated-microwave-absorption technique, we have studied the flux quantization in single-crystal samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as a function of an externally applied dc magnetic field (dc scan) at constant amplitude of the microwave magnetic field, and as a function of the microwave-field amplitude (microwave scan) at nominally zero dc field. Periodic oscillations in the modulated microwave absorption due to flux quantization are observed in both scans. In the dc scan an increasing number of junctions are activated as the temperature is increased at a fixed microwave power level or as the power level is increased at a fixed temperature. Surprisingly, only a single junction is activated in the microwave scan in the temperature range 0.1t=0.92, where t=T/${\mathit{T}}_{\mathit{c}}$ is the reduced temperature. It is shown that different junctions are activated in the two scans. From the temperature dependence of the microwave scan spectra, the superconducting penetration depth ${\ensuremath{\lambda}}_{\mathit{L}}$ is shown to obey the empirical two-fluid temperature dependence ${\ensuremath{\lambda}}_{\mathit{L}}$(T)/${\ensuremath{\lambda}}_{\mathit{L}}$(0)=(1-${\mathit{t}}^{4}$${)}^{\mathrm{\ensuremath{-}}1/2}$ in the temperature range 10--85 K, which is consistent with the s-wave-pairing mechanism. In the same temperature range the temperature dependence of the junction critical current can be described by ${\mathit{J}}_{\mathit{c}}$(T)/${\mathit{J}}_{\mathit{c}}$(0)=(1-t${)}^{1.9\ifmmode\pm\else\textpm\fi{}0.2}$.