A. Krol

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...

Investigation of interfacial roughness of InxGa1−xAs epitaxial layers on GaAs and InP substrates by soft x‐ray reflectivity

The grazing incidence x‐ray reflectivity is a nondestructive and sensitive technique for probing the depth profile of electron density in layered structures. This method has been utilized in the soft x‐ray regime to determine the roughness of interfaces, and the epilayer thickness in InxGa1−xAs/InP and InxGa1−xAs/GaAs heterostructures, for x=0.57 and x=0.60, grown by molecular beam epitaxy. By fitting the experimental results to our model, assuming uncorrelated interfacial roughness, we conclude that the top surface roughness does not depend on the type of the substrate or presence of stress in the epilayer, and is always smaller than interfacial roughness. The main factors which control the interfacial roughness are the quality of substrate and/or growth conditions rather than strain or lattice mismatch.

Local Mn structures in III-V diluted magnetic semiconductor (In,Mn)As

We describe the local Mn structures obtained by the X-ray absorption fine structure (XAFS) measurements for MBE-grown (In,Mn)As epilayers with high Mn compositions (Mn>0.1). Both homogeneous and inhomogeneous epilayers were investigated. For the inhomogeneous layers grown at T s =280 o C, the entire shell structures are similar to those of NiAs-type bulk MnAs. For the homogeneous layers grown at T s =210 o C, the first whell around Mn was found to consist of disordered structure of six As atoms. This shell is then surrounded by twelve In atoms from the host InAs

A low pressure, parallel plate avalanche chamber for detection of soft x-ray fluorescence

A description is given of the design considerations and performance of a low pressure, parallel plate avalanche chamber (PPAC), for use in detecting soft x-ray fluorescence. Detection efficiency and energy resolution are discussed and the application of the PPAC to detection of fluorescence emission from a high {Tc} superconductor is outlined. 8 refs., 7 figs.

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.

Soft x-ray investigation of the effect of growth conditions on InAs/GaAs heterostructures

Abstract The effect of growth conditions on interfacial roughness in InAs/GaAs hetcrostructures prepared by molecular beam epitaxy has been investigated using synchrotron radiation. Measurement of grazing incidence X-ray reflectivity (GIXR) and total electron yield (TEY) in the soft X-ray wavelengths are used for nondestructive characterization of interfacial roughness in these semiconductor heterojunctions. Our results indicate that a smoothing process takes place during epitaxial growth, and that the measured interfacial roughness is lowest for In-stabilized deposition on a GaAs substrate at 2° off (100), and highest with As-stabilized overgrowth on GaAs(100). The surface and interfacial roughness are practically independent of the layer thickness in the range investigated (70–600 A).

Local Environment Around Barium Atoms in the Superconductor Y-Ba-Cu-O

The local structure around Ba atoms in the compound superconductor Y-Ba-Cu-O has been studied by measuring the extended x-ray absorption fine structure (EXAFS) near the Ba K edge. This technique is used to investigate the changes in the short range order structure when the system is doped with impurities.

O 1s x‐ray absorption spectroscopy of Tl2Ba2Ca3O10 high‐TC superconductors

X‐ray absorption around oxygen K‐edge of Tl2Ba2Ca2Cu3O10 high‐TC superconductors was measured by means of a bulk sensitive x‐ray fluorescence yield detection method. Three distinct pre‐edge peaks are revealed. They are ascribed to core‐level excitations of oxygen 1s electrons to empty states at Fermi level which have predominantly oxygen 2p character. These oxygen holes are located in CuO2, BaO and TlO planes. The strong dependence of TC on oxygen holes concenration on O(1) site in CuO2 layer is found.

Local environment about O atoms in Pr‐Y‐Ba‐Cu‐O films studied by O K‐edge XAFS

The local environment about oxygen atoms in PrxY1−xBa2Cu3O7 films obtained by laser ablation has been investigated by means of the x‐ray absorption fine structure (XAFS) spectroscopy at the O K‐edge. It has been found that the Pr atoms do not subsitute for the Cu atoms nor form interstitial defects in the CuO2 planes and that the presence of Pr dopant does not disturb the local environment of oxygen.