Design of Relaxation Oscillator Based Ultra-wideband SFQ Amplifier for Chip to Chip Interconnection

In Rapid Signal Flux Quantum (RSFQ) logic circuits, on-chip interconnects and multichip module implementations for nearby distances have already been established. However, the flexible interconnection of two distant chips is still not achieved reliably due to impedance mismatching and attenuation. In this work, we propose a circuit that allows the usage of Passive Transmission Lines (PTLs) to transfer single-flux-quantum (SFQ) pulses between two distant chips which are separated by a distance greater than 10 cm by using 50 ?? transmission lines. For this purpose, we design an SFQ amplifier circuit to deal with impedance mismatch and attenuation problems. The circuit consists of two main parts: a relaxation oscillator (RO) circuit and an impedance transformer. The RO circuit utilizes relaxation oscillations occur in the underdamped Josephson junctions. The impedance matching circuit is an 8-section Chebyshev quarter-wave transformer and it eliminates impedance mismatching problem between the amplifier circuit and PTL. We performed circuit simulations and obtained voltage amplitude of about 600 ??V at the output of the circuit. The transformer has a broadband impedance matching with a fractional bandwidth (ratio of the bandwidth of a device to its central frequency) of 1.4 and a maximum Voltage Standing Wave Ratio (VSWR, the maximum voltage divided by minimum voltage on the transmission line) of 1.5.     

Complete Analysis of STJ Detector Performance via Absorption of Phonon Pulses

For the first time a complete analysis of the tunnel and loss parameters of superconducting tunnel junction photon detectors has been made solely by the use of nanosecond phonon pulse excitation. Previously only a partial characterization, requiring supplementary information from photo-excitation measurements, was possible. The present results have been achieved by a more realistic model for the energy spectrum of the phonon pulses and greatly improved (nanosecond) time resolution of the detected signal. The value determined for the tunnel rate is in good agreement with calculations based on the device layer structure. It is believed that the relatively high values of loss time obtained are the result of trap-enhanced recombination due to the high quasiparticle densities attained in the experiments.      

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.     

Quasi-static and dynamic switching of exchange biased micron-sized TMR junctions

We study the quasi-static and dynamical switching of magnetic tunnel junction patterned in micron-sized cells with integrated field pulse line. The tunnel junctions are CoFe/AlO/CoFe with an exchange biasing layer of MnIr. Quasi-static characterizations have been used to determine anisotropy, coercive as well as exchange bias fields. Dynamic switching measurements are done by applying fast-rising magnetic field pulses (178 ps–10 ns) along the hard axis of the junction with a quasi-static easy-axis applied field. We identify the field conditions leading to no-switching, to direct-writing and to toggle switching. We identify these field conditions up to the precessional limit, and construct the experimental dynamical astroïd. The magnetization trajectories leading to direct-writing and to toggle switching are well described by macrospin simulations.     

Single flux quantum signal transmission techniques in building practical RSFQ circuits

The practical implementation of rapid single flux quantum (RSFQ) technology requires much more complexity than the presently developed circuits. Multiple chips have to be integrated with a technology that is reliable at cryogenic temperatures. The interchip and intrachip data transmission speed of tens of GHz has to be supported. Also, the large RSFQ circuits need serial biasing to reduce the amount of the bias current. The test circuits were designed, simulated, fabricated with Nb technology, and tested at a temperature of 4.2 K. Test results at GHz frequencies showed that SFQ pulses can be successfully transmitted over an extensive distance in a chip, between chips, and over the circuits in different ground planes.     

Optimization of Single Flux Quantum Circuit Based Comparators Using PSO

In the recent years, a number of groups have shown that 1-bit comparators are suitable candidates for a number of circuits such as front-end of the low temperature detector readout circuits and analog to digital converters. Even though a number of tools exist for the development of digital logic gates, we are not aware of any optimizer for analog circuits that takes into account the stochastic effects in the Josephson junctions. Comparator circuits contain just a few parameters and have been analyzed extensively by a number of groups for over a decade. However, designing the comparator cells by hand is tedious work since it is based on the statistical analysis of the input–output relations of the cell. For the result to be reliable enough, statistically meaningful number of input SFQ pulses should be generated and corresponding output pulses should be analyzed. In addition, the design parameters should be suitable for fabrication and compatible for the rest of the cell library. In this work, we report an optimization tool to find the possible minimum gray-zone width, limited by the process design rules, for single flux quantum circuit based comparators. We used Particle Swarm Optimization (PSO) algorithm to determine the minimum possible gray zone width of a 1-bit comparator. To test the reliability of the PSO algorithm, we made a brute force sweep for a quasi-one-junction SQUID (QOS) with five parameters of the comparator, namely, three junction critical currents, one inductance value, and bias current. We find that the PSO gray zone results closely matches, even exceeds, the brute force design values and takes at least two orders of magnitude less computation time.     

Theory of the cos ø term in the Josephson tunneling current

The current-phase relation for the Josephson tunnel junction is investigated by using the time-dependent Ginzburg-Landau (TDGL) equations. The cos ø term is shown to have a negative coefficient and the same order of magnitude as the phase-independent term. The effect of the tunneling barrier is taken into account in terms of the boundary conditions of the order parameter and the electric potential at the junction; the new boundary conditions are derived by generalizing de Gennes' discussions for the dc Josephson effect to the time-dependent phenomena. The TDGL equations are solved on the assumption that the transmission coefficient across the junction is small enough and that the processes involved are quasi-stationary. It is concluded that the spatial and temporal variations of various quantities are of vital importance to an understanding of the nonequilibrium characteristics in the tunnel junction.     

Single flux-quantum memory cells

Detailed investigations have been carried out on two-junction interferometers. These devices have potential as memory elements. Information is stored as single-flux quanta (SFQ cells) in overlapping vortex modes and is destructively read out by switching from a vortex to the voltage state. The devices are fabricated with a lead alloy and the junction oxide is formed by rf oxidation. Most investigations have been done on devices with an area of about 1000 μm², but storage and reading have also been demonstrated in our smallest interferometers having a size of about 150 μm². Computer studies of cell properties, especially of the vortex transitions, have given good agreement with experiments. It has also been found that the cell behavior is little affected by loads such as would exist in an array environment.     

Superelliptic Josephson Tunnel Junctions

The most important practical characteristic of a Josephson junction is its critical current. The shape of the junction determines the specific form of the magnetic field dependence of its Josephson current. Here we address the magnetic diffraction patterns of specially shaped planar Josephson tunnel junctions. We focus on a wide ensemble of generalized ellipses, called superellipses, which retain the second-order symmetry. We analyze the implications of this type of isometry and derive the explicit expressions for the threshold curves of superelliptic Josephson junctions. A detailed study is made of their magnetic patterns with an emphasis on the rate of decay of the sidelobe amplitudes for large field amplitudes.     

The role of epitaxy in superconducting tunnel junction detectors

We have experimentally analysed the use of epitaxial and polycrystalline tantalum trapping layers in tunnel junction detectors, using epitaxial niobium based double tunnel junction devices. We have shown that the trapping rate is enhanced by having a low mean-free-path in the Ta trap film and an epitaxial Nb absorber film. This effectively increases the proportion of time spent by a quasiparticle in the trap film. Phonon-quasiparticle scattering, which reduces the effective trapping rate, has been observed.     

Multilayered Josephson transmission line based photon counting detector with ultra-high temporal and high spatial resolution

A superconducting Josephson transmission line (JTL) fabricated with multilayered tunnel junctions with thin (100 Å) superconducting layers may be used as an ionizing radiation detector. The suppression of the superconducting energy gap in the layers of such a JTL, due to the local deposition of energy by incident radiation, will initiate the propagation of one or more fluxons in the device. These fluxons represent digital information in the form processable by single flux quantum (SFQ) superconducting digital circuitry. Designs for JTL based detectors with temporal resolutions on the order of picoseconds and spatial resolution on the order of microns, along with numerical simulation results, are presented.     

High-frequency impedance of superconducting tunnel junctions

Comparison is made between the high-frequency impedance of a Josephson tunnel junction predicted by the RSJ model and that predicted by the Werthamer theory. Results obtained analytically, numerically, and by electronic simulation illustrate several fundamental differences between the two models. In particular, the small-signal rf resistance of a tunnel junction described by the Werthamer theory exhibits strong variations with temperature and frequency. In addition, the Werthamer theory predicts that a tunnel junction will have a plasma-like resonance in the absence of any electrode capacitance, and that the junction can have a nonzero reactance when current-biased outside of a constant-voltage step.     

Experimental study of laser-generated shear waves using interferometry

The shear displacements generated by short laser pulses have been measured in aluminum semicylindrical samples, both in the thermoelastic and ablation regimes. We measured the waveforms at different angles and obtained the angular distribution pattern of the amplitudes. For the thermoelastic regime good agreement has been found between the measured and the theoretically predicted shear waveforms. In the ablation regime, the absolute values of the amplitudes are comparable to the ones of compressional waves. The shear waveforms are difficult to interpret, particularly in the case where both thermoelastic and ablative effects play a role, because the phases of the shear pulses are opposite to one another in these two regimes. To measure the in-plane displacements, and hence the shear displacement field generated by the pulsed laser, a speckle heterodyne interferometer was used.This article is dedicated to Professor Dr. Paul Höller on the occasion of his 65th birthday.     

Field dielectric measurements of frozen silt using VHF pulses

Radiowave propagation experiments utilizing short pulses in the VHF band were conducted in the permafrost tunnel at Fox, Alaska. The purpose was to measure dielectric properties of this naturally occurring, perennially frozen organic silt which is common to much of interior Alaska and for which ice content varies between about 54 and 79% by volume. Transmissions across a septum dividing two drifts gave relative dielectric permittivity values between 3.9 and 7.3. The low values resulted when transmission was predominantly through an ice wedge. Propagation along the septum gave values of 3.3 and 5.0 depending on antenna polarization. This propagation was influenced by the dry, surface silts, as was propagation along a ceiling section, which also gave an approximate value of 3.3. The data from attempted transmissions from the ground surface directly above the tunnel to the tunnel ceiling (approximately 12 m distance) are ambiguous, as signals that propagated indirectly along the transmitter cable through a nearby ventilation shaft may or may not have masked direct transmission through the permafrost. The results agree with previous laboratory investigations conducted at temperatures well below that of naturally occurring materials in interior Alaska suggesting that winter refrigeration of the tunnel by circulated outside air greatly affected the natural conditions at this site.     

Experimental Study of Laser-Generated Shear Waves Using Interferometry

Abstract

The shear displacements generated by short laser pulses have been measured in aluminum semicylindrical samples, both in the thermoelastic and ablation regimes. We measured the waveforms at different angles and obtained the angular distribution pattern of the amplitudes. For the thermoelastic regime good agreement has been found between the measured and the theoretically predicted shear waveforms. In the ablation regime, the absolute values of the amplitudes are comparable to the ones of compressional waves. The shear waveforms are difficult to interpret, particularly in the case where both thermoelastic and ablative effects play a role, because the phases of the shear pulses are opposite to one another in these two regimes. To measure the in-plane displacements, and hence the shear displacement field generated by the pulsed laser, a speckle heterodyne interferometer was used.     

Superconducting NbN Thin-Film Nanowire Detectors for Time-of-Flight Mass Spectrometry

Superconducting nanowire detectors (SND) have been applied for time-of-flight mass spectrometry (TOF-MS) for the first time. In this study, we used the SND, which consists of a very thin niobium nitride (NbN) film having a nanowire meander pattern with a thickness of 6.8 nm and a width of 200 nm on a MgO substrate. The experiments were carried out for Angiotensin I and bovine serum albumin (BSA). These biomolecules were ionized by laser radiation with matrix-assisted laser desorption ionization (MALDI). The ions were accelerated by a static high voltage of 17.5 kV, and incident on the NbN meander, which is dc-biased below a superconducting critical current (I _c). It was found that the output pulses have a rise time of about 640 ps, which is extremely faster than superconducting tunnel junction (STJ) detectors, and a fall time of about 50 ns. Moreover, we investigated the bias current dependence of output pulses, and confirmed that molecules can be detected even for bias currents of about 50% of I _c.      

Optimized structure of AlGaAs/GaAs double junction solar cells

In this paper, the sub-layers of AlGaAs/GaAs double junction (DJ) solar cell have been redesigned in order to achieve an optimum cell structure. It has been deduced with cooperation of detailed balance limit theory and structural behaviour of AlGaAs, that the Al_0.45Ga_0.55As is the best choice for top cell’s material in AlGaAs/GaAs DJ solar cell. Also, there is a trade-off between peak tunnelling current and transparency in tunnel junction which makes Al_0.07Ga_0.93As as the optimum tunnel junction of AlGaAs/GaAs cell. Finally, a smoothed reflectance senary-layer structure based on modified-DBR has been proposed to be used as anti-reflection coating of proposed structure. Also, the thickness and doping concentration level of different layers have been optimized.     

Features of the Time Shape of the Pressure Pulses of Optical Air Breakdown

The time shape and amplitude of pressure pulses initiated by surface laser air breakdown for different energies of laser pulses (1–180 mJ) has been compared to the results of numerical gasdynamic calculations of unsteady explosive motions with allowance for counterpressure at distances of 0.2 to 30 cm from the breakdown region. It has been established that the experimental pressure pulse has the character of slowly damped quasiperiodic vibrations, whereas the calculated pulse is a bipolar single pulse of a much shorter duration. Good agreement between the experimental and calculated amplitudes of a positive pressure phase has been found throughout the investigated range, whereas the agreement between the corresponding amplitudes and durations of a negative pressure phase is limited in character. The differences observed in the experimental and calculated data have been attributed to the transformation of the shockwave motion to acoustic radiation.     

Crossover from Kondo-assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers

The dependence of the tunneling magnetoresistance (TMR) of planar magnetic tunnel junctions on the size of magnetic nanodots incorporated within MgO tunnel barriers is explored. At low temperatures, in the Coulomb blockade (CB) regime, for smaller nanodots the conductance of the junction is increased at low bias consistent with Kondo-assisted tunneling and the TMR is suppressed. For slightly larger nanodots but within the CB regime, the TMR is enhanced at low bias, consistent with co-tunneling. Magnetic tunnel junctions (MTJ) exhibit giant magnetoresistance in small magnetic fields that arises from the flow of spin-polarized current through an ultrathin tunnel barrier separating two magnetic electrodes. The current through an MTJ device depends on the magnetic orientation of the electrodes and is typically higher when the electrode moments are parallel than when they are antiparallel. It has recently been demonstrated that the spin polarization of the tunneling current can be greatly enhanced by using crystalline tunnel barriers formed from MgO as compared with conventional amorphous barriers formed from alumina, due to spin filtering across the MgO layer. The magneto-transport properties of magnetic granular alloys and magnetic tunnel junction devices with magnetic nanodots embedded in amorphous dielectric matrices, and tunnel barriers, respectively, have been studied by several groups, but no systematic studies of the dependence on these properties on the nanodot size have been made.     

Tunnel junction used as thermometer for ions detection

We propose the use of a superconductor — normal metal tunnel junction as thermometer for the energy measurement on MeV ions, using a metallic bolometer. The direct measurement of electronic temperature leads to a small time constant even at very low temperature in contrast with systems where the time constant is limited by electronphonon interaction. Experiments with heat pulses obtained by LED and optical fibre confirm the calculated time constants. Sensitivity calculations using the measured data gives an energy resolution better than 1/1000 for 1MeV ions with a time constant of 50 µs.