Susanne Lomatch

Multilayered Josephson junction logic and memory devices

Flux quantum logic and memory circuits using superconducting Josephson tunnel junctions have high-speed switching times (approximately 1 ps), low power dissipation (< 1 (mu) W per circuit) and low levels of thermally induced electrical noise. Current designs of such circuits employ single trilayer junctions, which impose circuit size and logic threshold limitations. A new design component, the multilayered tunnel junction, consists of a vertically stacked array (a 1D superlattice) of Josephson tunnel junctions. The introduction of multilayered junctions into superconducting electronic circuitry offers a reduction in the current device size, fault tolerances, and new device applications. We present numerical simulations of simple circuits employing multilayered Josephson junctions as design components. Comparison with conventional single flux quantum circuitry is discussed. We also present preliminary measurements of multilayered Josephson junctions fabricated for use in flux quantum devices.

Multilayer Josephson junction flux quantum devices

We describe the properties of flux quantum circuitry employing the relatively young technology of multilayer Josephson junctions with n superconductor-insulator (SI) layers. Multilayer junctions can be employed as both passive and active devices to increase circuit integration density, allow for new logic/voltage thresholds and higher impedances, and improve thermal noise stability. We present the results from numerical simulations of a conventional RSFQ circuit and two novel circuits with multilayer junction designs. Neural circuitry is a focus of our novel multilayer designs. We also discuss layout and fabrication issues, considering the recent progress in the fabrication of Nb multilayer junctions with AlN tunnel barriers, which exhibit intrinsic overdamping at the level of each SI layer. Included in this discussion is a long term assessment of a multilayer approach in view of deep sub-micron and high T/sub c/ technologies.<<ETX>>

Intrinsically damped multilayered (stacked) Nb/Al-AlNx/Nb superconducting tunnel junctions

Abstract Single and stacked Nb/Al-AlNx/Nb superconducting tunnel junctions with both hysteretic (underdamped) and non-hysteretic (overdamped) current-voltage relationships have been produced utilizing reactively sputtered aluminum nitride tunnel barriers. Standard multilayer deposition and lithographic processing techniques, compatible with existing Nb/Al-AlOx/Nb fabrication techniques, are used in fabrication. The degree of damping in the junctions is controlled through the deposition parameters. Critical current dependence on applied magnetic field indicates that the overdamped junctions have a distributed Josephson coupling and are not simple microshorts. The shorter deposition time to grow reactively sputtered AlNx barriers makes this system a promising alternative to fabricate stacked Josephson junctions.

Role of engineered materials in superconducting tunnel-junction x-ray detectors: suppression of quasi-particle recombination losses via a phononic bandgap

While much progress has been made towards improved energy reso1utvn in superconducting tunnel junction (STJ) detectors recently, results are still more than an order of magnitude worse than the theoretical limit. Several factors have been identified as contributing to degradation of energy resolution in STJ devices: recombination losses, parasitic quasiparticle trapping and quasiparticle diffusion into current leads. In addition, STJ detectors tend to have poor photon capture efficiency. Semiconducting detectors achieve their near theoretical energy resolutions and high efficiencies via doping and/or applying an external field to a pure substance. These methods are ineffective for STJ detectors, therefore such alternatives as engineered materials, consisting of multiple materials artificially patterned on the microscopic level, should be considered. The most common engineered structures in use are quasiparticle trapping configurations, which alleviate lead diffusion and detection efficiency problems. We have previously proposed a multilayered approach which addresses parasitic trapping, along with diffusion and efficiency. We now propose the possibility of an engineered structure which will alleviate quasiparticle recombination losses via the existence of a phononic band gap that overlaps the 2i energy of phonons produced during recombination of quasiparticles. We will present a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example

Applications for superconductor-insulator multilayers

Superconductor/insulator (SI) superlattices may be viewed as repeated (or stacked) SIS junctions connected in series. Such superlattices have a number of promising applications which we will survey. In the weakly coupled (tunnel junction) limit the possibility of fabricating high resolution X-ray detectors is being studied. For more strongly coupled junctions involving Josephson coupling, possible applications are numerous. The suggestion of Auvil and Ketterson that an SI multilayer would function as an efficient radiation source (due to coherent radiation from the layers and an increased junction phase velocity) has yet to be studied experimentally. Rippert and Ketterson have proposed that a phonon maser might be realized involving 2(Delta) recombination phonons and internal feedback via the superlattice Bragg mirror. Lomatch et al. have suggested that the added kinetic inductance of a multilayer could be used to eliminate stripline inductors from a Josephson transmission line. Finally multilayer SI tapes have been shown to have an enhanced critical current density, Jc. The mechanisms involved in the above devices and the status of efforts to fabricate them are reviewed.

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.

Transient dynamics of Josephson-coupled multilayers

Abstract A novel model is proposed to allow the exploration of the short-time (transient) dynamics of a superconductor–insulator multilayer with Josephson coupling between the layers. This model treats the charge on the layer interface planes as a dynamic variable, whose evolution is determined via the interlayer charge-current continuity equations. The high frequency current responses of both the superconducting and the insulating layers are included in the proper time domain form, derived for the general case of nonuniform layers from standard BCS theory. We present the model equations for the response of the system to an initial charge distribution, with the focus of determining the Josephson switching properties of an overdamped, nonuniform multilayer. Such structures may have potential applications in superconducting flux quantum electronics.

Transient response of Josephson-coupled multilayers

We investigate the response of a Josephson-coupled multilayer to an ultra-short voltage pulse. The response is understood in terms of the dynamics of the phase differences across the layers, calculated from both the microscopic theory and the resistively shunted junction (RSJ) model. We discuss how such response might play a role in novel multilayered switching devices for use in electronics applications, such as flux quantum digital circuitry.

Theory and application of Josephson-coupled multilayers as switching devices in superconducting electronics

The design and application of Josephson-coupled multilayers (JCMs) as active switching devices in superconducting electronics requires an understanding of the transient dynamics of these structures. We discuss the theoretical issues surrounding the switching dynamics of JCMs, and describe our own theoretical model which includes the high frequency behavior of the superconducting layers through a nonlocal treatment involving the charge-current conservation equations. We cast our discussion in the context of materials system, from that of conventional low Tc engineered JCMs to that of intrinsic high Tc compounds. The possibility of coherent stable state switching in JCM structures is explored.

Multilayered superconducting tunnel junction x‐ray detectors

Abstract Superconducting tunnel junction X‐ray detectors have potential theoretical energy resolutions on the order of eVs. Such high energy resolutions would be of great use for astrophysical observations in the discrimination of spectral lines in hot plasmas and the determination of the Doppler shifts of these lines. Current results for these devices, however, yield energy resolutions more than an order of magnitude worse than the theoretical limit. In addition, low efficiency and breakdown under thermal cycling are also common problems. We propose a new approach based on multiple (10s to 100s) layers of vertically stacked tunnel junctions. This approach holds promise for the alleviation of many of the mechanisms that have been suggested as leading to the degradation of energy resolution and providing built in redundancy against barrier shorts. It should also be possible to make use of different materials which would yield higher operating temperatures and more robust devices. Also, the artificial 1‐...