Y.H. Kao

Effects of silver doping in the high‐Tc superconductor system Y‐Ba‐Cu‐O

Changes in various superconducting properties as a function of silver substitution in the bulk granular high‐Tc system Y‐Ba‐Cu‐O have been investigated. In the low doping limit, Ag impurities behave as an excellent oxygen stabilizer and the critical current density is enhanced by an order of magnitude while the Tc remains practically unchanged. When the Ag content is increased to above 20%, the Tc starts to drop and the effects of Ag clusters as well as impurity phases become more important. However, the compound is still a superconductor with a Tc around 20 K by full substitution of Ag for Cu. The problems concerning the location of Ag atoms and formation of Ag clusters are examined with measurements of x‐ray absorption fine structure using synchrotron radiation. Our results indicate that at least some Ag atoms occupy the Cu(1) and Cu(2) sites in the material. Magnetic field dependence shows that the critical current density of a bulk sintered high‐Tc superconductor below a characteristic value H’c is ma...

Observation of voltage locking and other interactions in coupled microbridge Josephson junctions

Abstract We have observed pronounced interactions between two closely spaced microbridge Josephson junctions. Major effects are locking of the two critical currents to a single value and the synchronization of the voltages across the two bridges to the same value.

Soft X-ray spectroscopy with a scintillation detector

Abstract Spectroscopy, especially EXAFS, in the soft X-ray region has been limited by many factors. Among them the lack of adequate detectors is an important one. Recently scintillation detectors have been shown to provide high signal to noise ratio and smooth spectral response in this energy range [1,2]. P31 outperforms many other scintillators in the energy range 300–1000 eV. In this work, the basic properties of P31 are studied: its response is linear in the range of 10 7 –10 10 photons/(s mm 2 ), its optical spectrum is independent of the incident X-ray energy. A gas phase and a solid state experiment employing these detectors are presented. The oxygen K-edge spectra thus obtained enabled EXAFS analysis in this energy range.

Interfacial roughness in InAs/GaAs heterostructures determined by soft x‐ray reflectivity

X‐ray reflectivity has been used to determine interfacial roughness of as‐grown 250‐A‐thick InAs overlayers deposited on GaAs substrates by molecular‐beam epitaxy under differing growth and substrate conditions. Results indicate that in each case the top surface was smooth, but that the buried interface separating these highly lattice‐mismatched materials exhibits root‐mean‐square roughness parameters in the range of 10–19 A. As‐stabilized growth produced the roughest InAs‐GaAs (buried) interface, while In‐stabilized growth on a slightly misoriented substrate produced the smoothest InAs‐GaAs interface.

Characterization of SiO2/Si heterostructures by soft x‐ray reflection

Grazing incidence x‐ray reflectivity has been used to characterize four as‐grown SiO2/Si(100) heterostructures with SiO2 overlayer thicknesses ranging from about 126 to 1100 A. Root‐mean‐square roughness at both the top surface and buried interface is determined, as is the (complex‐valued) x‐ray refractive index of the SiO2 heterostructure material for photons in the energy range 400–800 eV. Knowledge of the SiO2 and Si refractive indices allows the oxygen atomic scattering factor to be deduced at these energies. Evidence that one of the four thermally grown structures may consist of more than just a single homogeneous overlayer is also examined.

Size-dependent electrical conduction in bismuth

Abstract The variation of resistivity with sample thickness in Bi single crystals was studied. A new feature in this variation was found with thickness less than 1.0 mm at 4.2°K.

Transmission measurements of soft X-ray absorption fine structure in solids by a new scintillation technique

Abstract Transmission measurements of the extended X-ray absorption fine structure above the oxygen K-edge have been made by using a new scintillation detection technique. The absorption spectrum is in close resemblance with the total electron yield spectrum recorded at the same time. This new technique is of interest for studies of soft X-ray absorption spectroscopy for thin films of solid materials.

Anomalies in the resistivity of thin Bi films

Abstract For film thickness less than 400 A, the value of the electron wavelength in Bi, hysteresis and other anomalies in the resistivity versus temperature curves are observed.

TUNNELING OF SUPERCURRENT THROUGH BARRIERS CONNECTED IN SERIES.

Abstract Experimental studies of Josephson junctions connected in series were made and I–V characteristics for the series circuit were compared with those for the individual junctions.

Impedance of thin conducting films in a longitudinal magnetic field

Abstract Real and imaginary parts of the a.c. resistivity if thin conducting films in a longitudinal magnetic field are calculated. A method for obtaining the electronic mean free path, Fermi velocity and the effective mass is suggested.