On the far IR transmission spectra of high-Tc superconductors

After several years, the Far IR (FIR) results from several laboratories seem to converge. The very FIR transmission data at different temperatures in the normal phase, combined with dc resistivity measurements, lead to a constant plasma frequency in that phase, and a linear variation of collision frequency v_c versus temperature T. Approximately the same plasma frequency can be used to explain the FIR transmission spectra at 7 K from 20 to 200 cm^?1. These remarks obtained from purely FIR data are in good accordance with the recent conclusions of Fiory et al obtained by a quite different method, i.e. electrostatic measurements.     

Tunneling spectroscopy of high-Tc superconductors

We have performed tunneling spectroscopy experiments on different kinds of high-temperature superconductors (HTS) and different types of junctions. Firstly, spectra were taken on surfaces of YBa₂Cu₃O_7-δ (YBCO) thin films and Nd_1.85Ce_0.15CuO_4-y (NCCO) single crystals by low-temperature scanning tunneling microscopy/spectroscopy (LTSTM/S) in a superconductor-insulator-normal metal (SIN) configuration. Second, the tunneling spectra of grain-boundary junctions (GBJs) fabricated on bicrystal substrates were measured for the materials YBCO, La_1.85Sr_0.15CuO_4-δ (LSCO), Bi₂Sr₂CaCu₂O_8+δ (BSCCO) and NCCO in a SIS configuration. The characteristics of the spectra in both systems could be related to the symmetry of the order parameter in the superconducting electrode(s). In particular, a zero bias conductance peak (ZBCP) was observed only in systems with a d-wave symmetry, but was completely absent in the s-wave system NCCO.     

A new approach for the growth of high-Tc superconductors

In this paper we describe a modified liquid phase separation (double crucible) method for growth of superconducting Bi-Sr-Ca-Cu-O single crystals. We used alumina crucibles. The morphology and composition of as-grown crystals were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The stacking thin-plate single crystals of maximum 5 × 6 × 0.1 mm³ were obtained.     

On the use of high-Tc superconductors for Perot-Fabry mirrors coatings

In a very rough experiment using two MgO plates, coated with a 200 Å thick YBaCuO film to increase their reflectivity, and facing each other their YBaCuO films to make a Perot-Fabry (PF) interferometer, we have observed a 43% maximum transmission and a finesse around 33 at v=29 cm^?1, in good accordance with a new phenomenological model for YBaCuO. Much higher performances can be expected by optimizing the YBaCuO film thickness.     

Processing and fabrication of high-Tc superconductors for electric power applications

Recent developments in the powder-in-tube fabrication of (Bi,Pb)₂Sr₂Ca₂Cu₃O_x tapes include: identification of high-current-transport regions of the superconductor core; optimization of conductor design and processing to take advantage of these high-current regions; optimization of superconductor powders and heat treatments; and incorporation of flux-pinning defects into the superconductor grains. These developments are briefly discussed and their implications are assessed.     

Factors dominating the transport critical current behavior in high-Tc superconductors

The previous experimental studies on transport J_c and grain boundary (micro)structure of YBa₂Cu₃O_x have been summarized. A novel interpretation is proposed for the transport current behavior in high-T_c superconductors. Based on extensive transport, magnetization, and electron microscopy experimental data reported previously, we show that the transport J_c(H) characteristics in high-T_c superconductors are dominated by the portion of strongly coupled regions at the grain boundaries. As a result of inhomogeneity of the grain boundaries, the coupling strength varies and causes a distribution of J_go (inter-granular J_c) related to the fluctuations of the superconducting order parameter at the boundary area. We also discuss formation mechanisms of strongly coupled regions in bulk materials, and we present inductive methods that can be used to determine the coupling strength of a material.     

Magnetic flux diffusion in high-Tc superconductors: step-responseand AC-susceptibility

Theoretical calculations for the response of high-Tc ceramic superconductors to external magnetic field-step and ac excitations were performed. Despite our simplifying assumptions, the analytical solutions we obtain provide a clear insight into the flux penetration phenomenon in these materials. We discuss the characteristic time of flux penetration, the ac-susceptibility, and their dependence on field amplitude and frequency, Our results are in good agreement with experiments     

Simple method for investigating electromagnetic properties of high-Tc superconductors at two-millimeter wavelengths

A method for investigating the electromagnetic properties of high-T_c superconductors (HTS) in a 2-mm wavelength band has been proposed. The method is based on the use of a cylindrical tube open dielectric resonator (ODR) loaded with a HTS sample. The Josephson harmonic generation at microwave frequencies can be investigated by means of the method. A small cross section of the ODR makes it possible to investigate surface variations of HTS sample electromagnetic properties and their temperature dependencies.     

Dependence of critical current density on microstructure and processing of high-Tc superconductors

Microstructural origins for reduced weak-link behavior in high-J_c melt-processed YBCO, spray pyrolyzed thick films of Tl-1223, metallic precursor Y-124 polycrystalline powder-in-tube (PIT) wires and PIT Bi-2212/2223 are discussed. Since the materials studied are the highest J_c, polycrystalline, high-T_c superconductors fabricated worldwide, the results provide important guidelines for further improvements in superconducting properties, thereby enabling practical applications of these materials. It is found that strongly linked current flow within domains of melt-processed 123 occurs effectively through a single crystal path. In c-axis oriented, polycrystalline Tl-1223 thick films, local in-plane texture has been found to play a crucial role in the reduced weak-link behavior. Formation of “colonies” of grains with a common c-axis and modest in-plane misorientation was observed. Furthermore, a colony boundary in general has a varying misorientation along the boundary. Large regions comprised primarily of low angle boundaries were observed. Percolative transport through a network of such small angle boundaries appears to provide the nonweak-linked current path. Although powder-in-tube BSCCO 2212 and 2223 also appear to have a “colony” microstructure, there are some important differences. Colonies in BSCCO consist of stacks of grains with similar c-axis orientation in contrast to colonies in Tl-1223 films where few grains are stacked on top of one another. Furthermore, most grains within a colony in BSCCO have the same lateral dimensions as that of the colony, resulting largely largely in “twist” boundaries. Further microstructural characterization of high-J_c PIT 2212 and 2223 is currently underway. In the case of Y-124 wires, weak macroscopic in-plane texture is found. Additional measurements are underway to determine if a sharper, local in-plane texture also exists. It is found that in three of the four types of superconductors studied, reduced weak-link behavior can be ascribed to some degree of biaxial alignment between grains, either on a “local” or a “global” scale.     

Role of silver addition on mechanical and superconducting properties of high-Tc superconductors

The effect of silver additions on the mechanical and superconducting properties of sintered bulk YBa₂Cu₃O_δ (YBCO), Bi₂Sr_1.7CaCu₂O_δ (BSCCO-2212), and Bi_1.8Pb_0.4Sr_2.2Ca₂Cu₃O_δ (BSCCO-2223) has been evaluated. Strength and fracture toughness of YBCO and BSCCO bars increased with increasing Ag content up to 30 vol.% Ag. Addition of 30 vol. % Ag to YBCO increased strength from 87 to 136 MPa and fracture toughness from 1.82 to 3.9 MPa√m. Addition of 30 vol.% Ag to 2212 and 2223 increased strength from 58 to 107 and 41 to 90 MPa, respectively. Corresponding increases in fracture toughness were from 1.89 to 2.79 and 1.09 to 1.94 MPa√m, respectively. These improvements in strength and fracture toughness are believed to be due to the presence of Ag particles that may induce compressive stresses in the superconducting matrix and resist crack propagation by pinning the propagating cracks. The values of strength and fracture toughness of BSCCO-30 vol.% Ag specimens are comparable to those of monolithic BSCCO obtained by sinter forging, hot pressing, and hot isostatic pressing. On the other hand, the hardness of YBCO and BSCCO decreased with increasing Ag contents because of the lower hardness of Ag. Addition of Ag showed no adverse effects on superconducting properties (J_c and T_c) of YBCO or BSCCO superconductors.     

用于红外探测的铱硅化物的X射线光电子能谱

用Ｘ射线光电子能谱方法测量了铱硅化物的芯能级谱。得到了与化学键有关的化学多和芯能级对称性变化方面的信息，提出了在ＩｒＳｉ／Ｓｉ肖特基势垒形成机理与界面外ＩｒＳｉ和Ｓｉ原子的化学键密切相关本文结果有益于解用铱硅化物肖特基势垒制备红外探测器。     

Application of SEM-modified X-ray microscope to entomology and histology, and effects of X-ray coherence in imaging

A projection X-ray microscope has been constructed by modifying a scanning electron microscope (HITACHI S-2500CX). Characteristic internal structures and their changes in an aphid, a fly, an ant, a water bear and a beetle are observed first, non-destructively, by X-ray microscope. Stereo-pair X-ray micrographs of Golgi-stained sections of porcine liver show a network of bile canaliculi and stellate cells. The very clear micrographs presented indicate the usefulness of the microscope in studies of entomology and histology. In some micrographs, very thin whiskers or hairs are visible, which suggests that a phase-contrast effect is present in the imaging. X-ray micrographs of an ant hair taken at various distances between the X-ray source and the specimen, D(s), indicate that the image enhancement due to the phase contrast increases with increasing D(s). Degree of the lateral coherence of X-rays of wave length 0.1 nm is theoretically estimated near the hair, with the result that the coherence increases from about zero (almost complete incoherence) at D(s) = 0.1 mm to about unity (almost complete coherence) at D(s) = 6 mm, in agreement with the observation.     

Temperature effects in GaAs OEMS spectra

Wavelength modulated infra-red radiation has been used to modulate the components of electrical impedance of GaAs MIS and Schottky barrier device structures. The temperature dependence of the energy and magnitude of the spectral features has been studied with sub as well as super band-gap radiation; a sub band-gap feature which was seen in VPE above 250 K, but not at all in MBE material. It had an energy of 0.80 eV that was constant in the temperature range 250 to 300 K. This could have been due to an intra-centre transition within a defect or a transition to the conduction band. Insulator-GaAs interface states did not produce an observable OEMS response. One of the super band-gap features had an optical cross-section which increased monotonically with energy and appeared at 1.56 eV at 300 K. This value is in good agreement with the compositionally extrapolated DX centre energy in the AlGaAs alloy system for GaAs. Its energy with respect to theГ-VBM did not change between 200 and 300 K. Another similarly behaved feature was observed at 1.70 eV and has been tentatively assigned to phonon assisted transitions between theГ-VBM and the L-CBM. It is however surprising that its energy did not change with temperature. The constant energy separation between the states that have been assigned to the DX centre and the L-CBM suggests that they are related.     

类钠锗离子软X射线光谱的理论计算

本文将原子结构的HFR方法与关于能量参数的最小二乘优化方法相结合,详细计算了类钠锗离子1s~22S~22p~6nl组态(n=3～6,l=0～5)电偶极跃迁的软x射线光谱(波长<30nm)。本文的波长计算值在误差范围内与实验十分符合,振子强度与Wiese等人的结果一致。     

Hard radiation effects in X-ray lithography

Results of X-ray exposure of polymethyl methacrylate through various materials are described. These indicate that the hard radiation effect on exposed resist may be quite large.     

Nonnegative matrix factorization for rapid recovery of constituent spectra in magnetic resonance chemical shift imaging of the brain

We present an algorithm for blindly recovering constituent source spectra from magnetic resonance (MR) chemical shift imaging (CSI) of the human brain. The algorithm, which we call constrained nonnegative matrix factorization (cNMF), does not enforce independence or sparsity, instead only requiring the source and mixing matrices to be nonnegative. It is based on the nonnegative matrix factorization (NMF) algorithm, extending it to include a constraint on the positivity of the amplitudes of the recovered spectra. This constraint enables recovery of physically meaningful spectra even in the presence of noise that causes a significant number of the observation amplitudes to be negative. We demonstrate and characterize the algorithm's performance using 31P volumetric brain data, comparing the results with two different blind source separation methods: Bayesian spectral decomposition (BSD) and nonnegative sparse coding (NNSC). We then incorporate the cNMF algorithm into a hierarchical decomposition framework, showing that it can be used to recover tissue-specific spectra given a processing hierarchy that proceeds coarse-to-fine. We demonstrate the hierarchical procedure on 1H brain data and conclude that the computational efficiency of the algorithm makes it well-suited for use in diagnostic work-up.     

Reducing the effects of scattered photons in X-ray projection imaging

An iterative algorithm is introduced which can compute and subtract the scatter bias. It is assumed that the scatter-free image is spatially bounded and anything outside the boundary is the result of recorded scattered photons. This information and the fact that the scatter distribution does not have appreciable high-frequency content are utilized.     

An assessment of synthesis effects on the high-Tc superconducting transition: With application to thin-film bolometer devices

The characteristics of the transition to superconductivity in RBa₂Cu₃O_(7−δ), where R includes: Y, Gd, Eu and Dy, have been investigated, for applications to transition-edge thin-film bolometers. Devices were fabricated from thin, epitaxial films, which were formed by pulsed-laser deposition (PLD). Bulk ceramic samples (pellets) were synthesized for use as the ablation targets. A study of the (DC) resistive transition width (ΔT_c) of the bulk samples, reveals that the oxidation annealing cycle plays a dominant role in determining ΔT_c. Peak temperatures of the oxygenation must be very carefully controlled. A slightly too high processing temperature causes changes in the grainsurface morphology and an increase in ΔT_c. Rare earth (R) substitution has a significant effect. Residual phase impurity, off stoichiometry, lower density, pressing-induced orientation and smaller pellet sizes, were all found to be less deleterious to ΔT_c. Proper synthesis improves superconductivity, narrowing ΔT_c and affecting grain-boundary (GB) morphology, as revealed by XRD, SEM and AC magnetic-susceptibility analyses of the bulk ceramics. The occurrence of a substantial susceptibility-loss (χ″) peak correlates well with (undesired) larger ΔT_c transitions. In bulk ceramics this may be accompanied by a toe feature in the resistive transition, i.e. additional width to the lower-half of the transition. Thin, epitaxial films, made by PLD from these same bulk ceramics, indicate that certain types of non-optimal features of the bulk transition are not present in the fabricated films. In particular, the toe feature is removed due to the complete restructuring that occurs during the epitaxial film-growth process by PLD. For films to have narrow transitions, we find that it is necessary for the ablation target to have a sharp onset to its transition. The relationship to infrared bolometer detectors is discussed and examples are reported.     

Thermal effects of Brillouin gain spectra in single-mode fibers

Brillouin gain spectra in two 250-m-long single-mode fibers with GeO₂-doped core/pure-silica cladding (fiber A) and pure-silica core/F-doped cladding (fiber B) were measured at temperatures ranging from -40 to +60°C at a wavelength of 1.32 μm. The temperature coefficients of Brillouin frequency shift were found to be 1.17 and 1.33 MHz/°C for fibers A and B, respectively. Temperature coefficients of Brillouin gain bandwidth were found to be -0.12 and -0.10 MHz/°C. These measurements provide useful information for applications of stimulated Brillouin scattering