Edward D. Rippert

Preparation of Y‐Ba‐Cu‐O thin films on MgO by dc magnetron sputtering from a stoichiometric Y1Ba2Cu3O7−δ target

Y‐Ba‐Cu‐O thin films have been deposited on MgO substrates by dc magnetron sputtering from a stoichiometric YBa2Cu3O7−δ target, after which they are subjected to a short heat treatment. Zero resistance is routinely achieved near 60 K, and one film shows zero resistance at 68.0 K.

ULTRAVIOLET SECOND HARMONIC GENERATION IN RADIO-FREQUENCY SPUTTER-DEPOSITED ALUMINUM NITRIDE THIN FILMS

Optical second harmonic generation in radio‐frequency sputter‐deposited AlN thin films has been studied for harmonic wavelengths from ultraviolet to near infrared. The effective second‐order nonlinearity χ(2)(ω) was determined to have a nonresonant background value of ∼5×10−9 esu for second harmonic wavelengths longer than 400 nm, and it increases dramatically as the second harmonic frequency approaches the bandgap of 6.2 eV. This is likely due to resonance of the second harmonic frequency with the critical point transition associated with the direct bandgap of AlN.

Second order optical nonlinearities of radio frequency sputter‐deposited AlN thin films

Polarized second harmonic generation measurements were performed on AlN films deposited on (100) sapphire substrates by the reactive rf sputtering technique. The bulk effective second order nonlinearity observed in these films is typically about 6×10−9 esu at 1.06 μm, several times larger than that of quartz or KTP. The tensorial properties of the nonlinearity are consistent with the crystal symmetry of AlN and the microcrystallinity of these films.

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>>

Multilayered superconducting tunnel junctions for use as high-energy-resolution x-ray detectors

X-ray detectors based on superconducting tunnel junction technology are desirable due to their potential for higher energy resolution and greater charge carrier production than conventional semiconductor devices. Single junction devices fabricated by thermal or plasma oxidation of elemental, soft metal, superconductors have shown some promise, but tend to suffer from poor opacity, performance degradation upon thermal recycling, and unequal total charge collected for absorption of x rays in different layers due to differing tunneling characteristics of each layer. More complex designs, such as quasi-particle trapping systems and all refractory materials, can help with the unequal charge and thermal cycling problems, respectively. However, new problems in complexity of fabrication and short quasi-particle lifetimes arise. Other potential difficulties include resolution degradation due to trapping of quasi-particles at the photoabsorption site or near defects and tolerance of the devices to faults induced by their environment. The use of multilayers of superconducting tunnel junctions, consisting of tens to hundreds of identical tunnel junctions stacked on top of one another, as a design can address these problems. The multilayer can, in principle, be made as thick as desired in order to increase opacity, while the individual superconducting layers can be made very thin in order that the tunneling time of the quasi-particles be short compared with the quasi-particle lifetime for even refractory superconductors. In addition, the layer thinness helps to alleviate undesirable quasi-particle trapping. The inherent redundancy of junctions in a multilayered device allows for continued operation even after multiple single layer failures. The unique physical and technological possibilities of multilayered devices, such as resonant tunneling and built in preamplification, make them structures worthy of study even without the more pragmatic inducements described above.

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

A multilayered superconducting neural network implementation

We present the results of numerical simulations of a novel neural networking implementation utilizing multilayered Josephson junction (or series array) based synaptic circuits with local memory. These synaptic circuits utilize single flux quanta for both neural information and synaptic weight programming, and we present a simple circuit that can implement Hebbian learning at a completely local level, with global control over the rates of both learning and forgetting in synapses.

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.