Search Results - (Author, Cooperation:B. Hunt)

2011-05-14

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2013-12-24

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2013-05-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-08-23

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

2018-02-10

American Physical Society (APS)

0556-2813

1089-490X

Physics

Nuclear Structure

2018-12-07

American Society of Hematology (ASH)

0006-4971

1528-0020

Biology

Medicine

Thrombosis and Hemostasis, Blood Spotlight, Clinical Trials and Observations

1077-3118

AIP Digital Archive

Physics

Treatment of YBa2Cu3O7−x films with a nonaqueous solution of HF in absolute ethanol results in the formation of an oxyfluoride with relative Y:Ba:Cu concentrations of 1:4:3 on the surface, as determined by x-ray photoelectron spectroscopy. The passivation properties of chemically treated films were tested by monitoring the growth of the high binding energy O 1s peak, associated with nonsuperconducting surface species, as a function of air exposure time, for both HF-treated and untreated films. The native oxyfluoride is shown to reduce the reactivity of the superconductor to air.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the first observation of phonon density of states effects in a superconductor using a low-temperature scanning tunneling microscope (STM). The phonon effects were observed using a STM spectroscopy method to measure dItunneling /dV vs V for the tunnel junction formed by the Au STM probe and a superconducting Pb sample.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A nonaqueous chemical etch, with Br as the active ingredient, is described which removes the insulating hydroxides and carbonates that form on high-temperature superconductor surfaces as a result of atmospheric exposure. X-ray photoemission spectra have been recorded before and after etching YBa2Cu3O7−x films. It is found that, after the etch, the high binding energy O 1s and Ba 3d peaks associated with surface contaminants are greatly reduced, the Y:Ba:Cu ratio is close to the expected 1:2:3, and the oxidation state of the Cu(2+) is not affected. The resistance of an etched film reaches zero at 78 K, compared to 81 K for a similar unetched film. The suitability of other nonaqueous halogen-based etches is discussed, as is the applicability of this etch to other high Tc superconductors.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Superconducting thin films of Y-Ba-Cu-O near the 1:2:3 stoichiometry were produced by simultaneous (coevaporation) and sequential (multilayer) evaporation in the same evaporator. The best film obtained on yttria-stabilized zirconia (YSZ) had a superconducting onset temperature of 104 K, a midpoint Tc of 92 K, and zero resistance at T≤74 K. Stoichiometry was deduced by inductively coupled plasma emission spectroscopy, and elemental depth profiles were obtained by x-ray photoelectron spectroscopy. Film stoichiometry changes only near the film/substrate boundary for films on YSZ. Films on Si/SiO2 were not superconducting; depth profiling shows severe changes of film composition with depth. A major theme of this work is process reproducibility, which was found to be poor for coevaporation but improved considerably for sequential evaporation.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have used the x-ray standing-wave technique and bulk x-ray diffraction to investigate the structural properties of thin CoSi2 layers grown epitaxially on Si(111). The perpendicular lattice mismatch with respect to the Si substrate was found to be −0.0152±0.0003 and −0.016±0.001 for 6-nm-thick and 16-nm-thick layers, respectively. The distance between Si(111) and the first Co layer was measured to be (0.288±0.005) nm and is thus stretched by (0.014±0.005) nm compared with a value determined by Si-like bulk bond length. The Co atoms are attached to the Si(111) dangling bonds in agreement with the model of fivefold coordinated metal atoms at the interface.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Results on electrical characterization of YBa2Cu3O7−x thin-film surfaces treated with a Br/ethanol chemical etch are presented. Electrical measurements of YBa2Cu3O7−x/Au/Nb device structures fabricated using polycrystalline, post-annealed YBa2Cu3O7−x films with Br-etched surfaces, show improvements of approximately one or two orders of magnitude in current densities and resistivities (resistance-area products) relative to unetched devices. The existence of supercurrents in these structures has been confirmed by observation of the ac Josephson effect, and by magnetic field and temperature studies of the critical currents. The Br-etch process has produced 10×10 μm2 devices with critical current densities greater than 400 A/cm2 and resistivities as low as 4×10−7 Ω cm2.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the fabrication and low-frequency testing of the first edge-geometry NbN/MgO/NbN superconducting tunnel junctions. The use of an edge geometry allows very small junction areas to be obtained, while the all-NbN electrodes permit operation at 8–10 K with a potential maximum operating frequency above 1 THz. Edge definition in the base NbN film was accomplished utilizing Ar ion milling with an Al2O3 milling mask, followed by a lower energy ion cleaning step. This process has produced all-refractory-material tunnel junctions with areas as small as 0.1 μm2, resistance-area products less than 21 Ω μm2, and subgap to normal state resistance ratios larger than 18.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A nonaqueous solution of Br in absolute ethanol (EtOH) has recently been reported [R. P. Vasquez, B. D. Hunt, and M. C. Foote, Appl. Phys. Lett. 53, 2692 (1988)] to be effective at removing nonsuperconducting surface species from YBa2 Cu3 O7−x films, leaving the surface close to the ideal stoichiometry. This same etchant is shown here to be an effective bulk etchant in chemical depth profiling through 1-μm-thick films. The Cu remains in the 2+oxidation state and the stoichiometry, as determined by x-ray photoelectron spectroscopy, is close to ideal and nearly constant throughout the profile, indicating the absence of any large preferential etching effects. The reaction of YBa2 Cu3 O7−x films with HF/EtOH, HCl/ EtOH, and I/EtOH solutions is also reported.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Wet chemical techniques are described for treatment of YBa2Cu3O7−x surfaces, resulting in the formation of native compounds with little or no reactivity to water. Promising native compounds include CuI, BaSO4, CuS, Cu2S, and the oxalates, all of which are either insoluble or have very low solubility in water. Treatment with dilute HI results in the formation of anative iodide film which is 80–90% CuI with small amounts of YI3 and BaI2. Treatment with dilute H2SO4 results in the formation of a film which is 95% BaSO4 and 5% Y2(SO4)3. Cu2S is formed on the surface with a dilute Na2S solution. An oxalate film with equal amounts of Y2(C2O4)3 and BaC2O4 results from treatment with dilute oxalic acid. X-ray photoelectron spectra show no significant changes when the sulfide, sulfate, or oxalate films are dipped in water, while the iodide film shows evidence of Cu(OH)2 formation.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

High quality YBa2Cu3O7−x/normal-metal/YBa2Cu3O7−x edge-geometry weak links have been fabricated using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25–100 A(ring) thick barrier layers exhibit current-voltage characteristics consistent with the resistively shunted junction model, with strong microwave and magnetic field response at temperatures up to 85 K. The critical currents vary exponentially with barrier thickness, and the resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area, indicating good barrier uniformity, with an effective normal metal coherence length of 20 A(ring).

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Epitaxial, high-Tc superconductor/normal-metal/superconductor (SNS) edge-geometry weak links and superconducting quantum interference devices (SQUIDs) have been fabricated with integrated YBa2Cu3O7 (YBCO) groundplanes and SrTiO3 insulators, using a process which incorporates six epitaxial layers. The SNS edge junctions were produced using off-axis sputtered films and Co-doped-YBCO normal metal interlayers. These devices show excellent performance with typical critical current-resistance (IcRn) products of 500–800 μV for 100–150 A(ring) thick normal metal layers at 65 K, and 1-σ critical current density ( Jc) spreads as small as 12%. SNS SQUIDs incorporating groundplanes exhibit voltage modulation of up to 130 μV at 65 K and 40 μV at 77 K. SQUID inductance measurements indicate microstrip inductance values of 1 pH per square at 65 K. © 1996 American Institute of Physics.

Electronic Resource

1745-6584

Blackwell Publishing Journal Backfiles 1879-2005

Energy, Environment Protection, Nuclear Power Engineering

Geosciences

Electronic Resource

1089-7550

AIP Digital Archive

Physics

X-ray photoelectron spectroscopy (XPS) has been used to investigate the reaction of YBa2Cu3O7−x films with solutions of HF, HCl, Br2, HBr, I2, and HI in absolute ethanol (EtOH). The XPS core level and x-ray excited Auger spectra from untreated and halogen-treated surfaces are used to identify surface species by comparison with XPS data from the literature and with XPS spectra from more than 20 Y, Ba, and Cu halides, oxides, hydroxides, and carbonates measured in this work. XPS measurements on a number of these materials are being reported for the first time. Treatment of films with HF/EtOH results in the formation of an oxyfluoride with Y:Ba:Cu relative concentrations of 1:4:3. Additional features in the XPS spectra from HF-treated films are also consistent with the formation of CuF, a compound which does not exist in bulk form. Treatment of films with HCl/EtOH results primarily in the formation of BaCl2 (∼75%), with smaller amounts of YCl3, CuCl, and CuCl2. Treatment of films with Br2/EtOH or HBr/EtOH results in the formation of YBr3, BaBr2, and CuBr with relative concentrations 1:4:3. YBa2Cu3O7−x films were found to have no discernible reaction with I2/EtOH solutions, but treatment of films with HI/EtOH results in the formation of CuI on the surface.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have measured the optical constants of epitaxial films of CoSi2 and NiSi2, grown by molecular beam epitaxy on Si(111), in the energy range of 0.9–4.0 eV. The behavior of the optical constants is characteristic of metals: Drudelike in the low energy region and deviating from Drude behavior as interband transitions set in. Interband transitions are found to have already set in at 1 eV. The absorption varies significantly with energy, which has implications for photoresponse studies of internal photoemission in these material systems.

Electronic Resource