H. M. O’Bryan

Water interaction with the superconducting YBa2Cu3O7 phase

We show that the superconducting YBa2Cu3O7 phase is highly sensitive to water and water vapor. This is probably due to the presence of nonequilibrium Cu3+ ions in this compound. In particular, the YBa2Cu3O7 phase decomposes in water to CuO, Ba(OH)2 and Y2BaCuO5 and evolves oxygen. Samples with a reduced oxygen content, e.g., YBa2Cu3O6.0, also decompose in an aqueous ambient. The superconductivity of YBa2Cu3O7 samples is greatly degraded by the interaction with water and humid air. This effect should not preclude practical application of these materials since it should be possible to protect them with coatings of metal, glass, or plastic.

Large magnetoresistance in polycrystalline La–Y–Ca–Mn–O

A large magnetoresistance ratio in excess of 10 000% (at 140 K, H=6 T) has been obtained in a sintered perovskitelike material with a composition of La0.60Y0.07Ca0.33MnOx. The doping of La–Ca–Mn–O with Y resulted in a decrease in the lattice parameter by ∼0.2%, and improved the magnetoresistance by an order of magnitude. The fact that such a large magnetoresistance can be obtained in a polycrystalline material implies that an epitaxial film growth may not be necessary for device applications, with fewer restrictions in substrate selection and processing parameters. Low field measurements at 77 K gave a magnetoresistance ratio of ∼6.5% at H=500 Oe in the La–Y–Ca–Mn–O sample.

Thickness dependence of magnetoresistance in La–Ca–Mn–O epitaxial films

Colossal magnetoresistance in excess of 106% has been obtained (at 110 K, H=6 T) in epitaxially grown La–Ca–Mn–O thin films. The as‐deposited film exhibits a substantial magnetoresistance value of 39 000%, which is further improved by heat treatment. The magnetoresistance is found to be strongly dependent on film thickness, with the value reaching the maxima at ∼1000 A thickness, and then reduced by orders of magnitude when the film is made thicker than ∼2000 A. This behavior is interpreted in terms of lattice strain in the La–Ca–Mn–O films.

Enhanced flux pinning by phase decomposition in Y-Ba-Cu-O

Significantly improved flux pinning has been achieved in bulk YBa2Cu3O7−δ superconductor (‘‘123’’ compound) containing fine‐scale defects (<∼50 A thick). The measured Jc intragrain of ∼105 A/cm2 at 77 K, H=0.9 T is about ten times higher than the typical values for bulk Y‐Ba‐Cu‐O. The improved structure was produced by rapid decomposition at 920 °C of the YBa2Cu4O8 (‘‘124’’) precursor. This new and simple processing route could lead to a commercially viable processing technique for flux‐pinning enhancement in bulk Y‐Ba‐Cu‐O.

Cr‐doped GaAs/AlGaAs semi‐insulating multiple quantum well photorefractive devices

Semi‐insulating multiple quantum well photorefractive devices using GaAs/Al0.29Ga0.71As with an electric field applied perpendicular to the layers are demonstrated. Semi‐insulating behavior is obtained by doping with Cr(1016/cm3) during epitaxial growth of the material. Diffraction efficiencies as high as 3% with an applied voltage of 20 V and microsecond response times are obtained in a 2 μm thick device. These devices are of importance for implementation of fast and sensitive two‐dimensional optical information processing systems at wavelengths compatible with current diode lasers without the spatial‐bandwidth limitations of thick photorefractive materials.

Sharp angular sensitivity of pinning due to twin boundaries in Ba2YCu3O7

We have measured the pinning force due to twin boundaries by measuring the magnetic torque produced when a field is applied perpendicular to the c axis and rotated with respect to the ab axes. At 76 K, the torque increases roughly fourfold, when the field is within 2° of the parallel to the twin boundaries. This peak is absent at 27 K. Irradiation with 5×1016 cm−2 3.5 MeV protons increases the overall critical current and eliminates the peaks observed at 76 K.

Low‐resistivity contacts to bulk high Tc superconductors

Convenient methods for obtaining extremely low resistivity contacts to bulk high Tc superconductors ( ρc in the range of 10−11–10−12 Ω cm2) are described. Three different configurations of silver contact metal in Y‐Ba‐Cu‐O have been employed, i.e., embedded Ag wire, embedded Ag particles, and selectively patterned Ag clad on superconductor wire. In all three cases, the low‐resistivity metallic contacts are formed in situ during the sintering or melt processing of the superconductor, thus eliminating the need for separate steps of contact preparation such as vacuum deposition of contact metal and additional heat treatment. The distribution and morphology of the silver contacts will be discussed. The measured contact resistivities in the present work are the lowest reported for the high Tc superconductors, and these methods may serve as a useful basis for important contact technologies needed for bulk superconductor applications.

In0.53Ga0.47As metal‐semiconductor‐metal photodiodes with transparent cadmium tin oxide Schottky contacts

A metal‐semiconductor‐metal (MSM) In0.53Ga0.47As photodiode using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes was investigated. The transparent contact prevents shadowing of the active layer by the electrodes, thus allowing greater collection of incident light. The barrier height (φBn) of CTO on i‐In0.52Al0.48As was determined to be 0.47 eV, while the Ti/Au barrier height was 0.595 eV. The reduced barrier height for CTO is caused by tunneling through the sputter‐damaged cap layer. Responsivity for 1.3 μm incident light was 0.49 and 0.28 A/W, respectively, for the CTO and Ti/Au MSM photodiodes. No antireflection (AR) coating was utilized over the bare semiconductor surface. The CTO MSM photodiode shows a factor of almost two improvement in responsivity over conventional Ti/Au MSM photodiodes.

Improved press forging of Ba2YCu3Ox superconductor

An improved press forging technique has yielded ceramic Ba2YCu3Ox with a very high degree of orientation and mechanical integrity. The as‐forged samples are dense and tetragonal, and have an oxygen content of ≊6.3. By a partial oxidation of the samples, a superconducting skin 20–100 μm thick is formed on the outer surface. Transport measurements on this skin show a Jc >3000 A/cm2 with an improved field dependence as compared with randomly oriented samples. The restriction of oxidation to the thin outer skin has reduced the microcracking which accompanies the oxidation of tetragonal Ba2YCu3Ox.

Transparent and opaque Schottky contacts on undoped In0.52Al0.48As grown by molecular beam epitaxy

The Schottky barrier height was measured for five different materials on undoped In0.52Al0.48As grown by molecular beam epitaxy (MBE). Of the materials tested, two were transparent conductors, indium‐tin‐oxide (ITO), and cadmium tin oxide (CTO) and for comparison, three were opaque metals (Au, Ti, and Pt). The barrier heights were measured using I–V measurements. Due to the high series resistance created by the undoped In0.52Al0.48As, the Norde method [J. Appl. Phys. 50, 5052 (1979)] was used to plot the I–V characteristics and extract the Schottky barrier height. The Schottky barrier heights were determined to be 0.639, 0.637, 0.688, 0.640, and 0.623 eV for ITO, CTO, Au, Ti, and Pt, respectively. Previously published results for Schottky barriers on In0.52Al0.48As are compared with our measurements.