Indications of superconductivity in doped highly oriented pyrolytic graphite

We have observed possible superconductivity using standard resistance vs. temperature techniques in phosphorous ion implanted Highly Oriented Pyrolytic Graphite. The onset appears to be above 100 K and quenching by an applied magnetic field has been observed. The four initial boron implanted samples showed no signs of becoming superconductive whereas all four initial and eight subsequent samples that were implanted with phosphorous showed at least some sign of the existence of small amounts of the possibly superconducting phases. The observed onset temperature is dependent on both the number of electron donors present and the amount of damage done to the graphene sub-layers in the Highly Oriented Pyrolytic Graphite samples. As a result the data appears to suggest that the potential for far higher onset temperatures in un-damaged doped graphite exists.Comment: 7 pages, 1 table, 5 figures, 11 references, Acknowledgments section was correcte

Effectiveness of BaTiO3 dielectric patches on YBa2Cu3O7 thin films for MEM switches

A micro-electro-mechanical (MEM) switch built on a superconducting microstrip filter will be utilized to investigate BaTiO3 dielectric patches for functional switching points of contact. Actuation voltage resulting from the MEM switch provokes static friction between the bridge membrane and BaTiO3 insulation layer. The dielectric patch crystal structure and roughness affect the ability of repetitively switching cycles and lifetime. A series of experiments have been performed using different deposition methods and RF magnetron sputtering was found to be the best deposition process for the BaTiO3 layer. The effect examination of surface morphology will be presented using characterization techniques as x-ray diffraction, SEM and AFM for an optimum switching device. The thin film is made of YBa2Cu3O7 deposited on LaAlO3 substrate by pulsed laser deposition. For this work, the dielectric material sputtering pressure is set at 9.5x10-6 Torr. The argon gas is released through a mass-flow controller to purge the system prior to deposition. RF power is 85 W at a distance of 9 cm. The behavior of Au membranes built on ultimate BaTiO3 patches will be shown as part of the results. These novel surface patterns will in turn be used in modelling other RF MEM switch devices such as distributed-satellite communication system operating at cryogenic temperatures

Generation–recombination noise in gallium nitride-based quantum well structures

Electronic noise has been investigated in AlxGa1−xN/GaN modulation-doped field-effect transistors of submicron dimensions, grown by molecular beam epitaxy techniques. Some 20 devices were grown on a sapphire substrate. Conduction takes place in the quasi-two-dimensional (2D) layer of the junction (xy plane) which is perpendicular to the triangular quantum well (z direction). A nondoped intrinsic buffer layer separates the Si-doped donors in the AlxGa1−xN layer from the 2D transistor plane. Since all contacts must reach through the AlxGa1−xN layer to connect internally to the 2D plane, parallel conduction through this layer is a feature of all modulation-doped devices. The excess noise has been analyzed as a sum of Lorentzian spectra and 1/fαnoise. The Lorentzian noise is ascribed to trapping of the carriers in the AlxGa1−xN layer. The trap depths have been obtained from Arrhenius plots of log(τT 2)versus 1000/T. Comparison with previous noise results for GaAs devices shows that: (a) many more trapping levels are present in these nitride-based devices and (b) the traps are deeper (farther below the conduction band) than for GaAs, as expected for higher band-gap materials. Furthermore, the magnitude of the noise is strongly dependent on the level of depletion of the AlxGa1−xN donor layer. We also note that the trap-measured energies are in good agreement with the energies obtained by deep level transient spectroscopy

YBa2Cu3O7 on Y-Stabilized ZrO2 buffered (100) Si – "T " Resonator Microwave Characteristics

Abstract—We have successfully fabricated high quality YBa 2Cu 3O 7 microwave "T " resonators on YSZ buffered p-type (100) oriented 53-56 W cm silicon. The 40 to 50 nm thick yttriastabilized zirconia buffer layer was deposited by pulsed dc magnetron sputtering at 830 C. The 250 to 400 nm thick YBa 2Cu 3O 7 film was deposited using laser ablation at 760 C. Results include Q values in excess of 20,000 at 3.8 GHz on a coplanar "T " resonator at temperatures below 50 K. Index Terms—Superconductor, microwave, Si, YBa 2Cu 3O 7. I

Lorentzian noise in the two-dimensional electron gas of AlxGa1–xAs/GaAs quantum wells

Current noise spectra SI() are reported on samples grown by the molecular beam epitaxy technique, with current-carrying contacts, acting as source and drain, and two probes extending into the two-dimensional electron gas (2DEG) of the AlGaAs/GaAs quantum well, in the range 77–295 K for frequencies of 10 Hz to 1 MHz. The time constants are almost independent of temperature and the current dependence is close to linear. The noise is interpreted as Lorentzian-modulated shot noise of the 2DEG current

Superconducting Half-Wave Microwave Resonator on YSZ Buffered Si (100)

Abstract—We have produced high quality superconducting half-wave microwave resonators on YBCO/YSZ/Si heterostructures. The YSZ was grown on (100) Si by pulsed dc magnetron sputtering at 820°C in 37 mTorr Ar/O 2. YBCO was then deposited on the YSZ/Si by pulsed laser ablation at 760°C in 0.5 Torr of O 2. A coplanar meander-line structure was patterned and its microwave characteristics measured. The results indicate Si to be a definite candidate for microwave applications producing Q values exceeding 50,000 at 4 GHz for T < 30 K. The microwave properties compare favorably to those obtained from identical resonators patterned on normal meta