Jq Li

Incommensurate modulation in minute crystals revealed by combining high-resolution electron microscopy and electron diffraction

Abstract Image processing combining high-resolution electron microscopy and electron diffraction is for the first time applied to the determination of incommensurate modulated structures. An image of a minute crystal of the high- T c superconductor Bi 2 Sr 2 Cacu 2 O x is averaged and then transformed to an image of the average structure by a deconvolution technique based on the principle of maximum entropy. The image resolution is then enhanced to about 1 A by the direct-method phase extension. All the unoverlapped atoms and their modulation are clearly seen in the final image.

Effects of Al doping on the superconducting and structural properties of MgB2

We have investigated the structural and superconducting properties of a series of Al doped MgB2 samples. The results show that the superconductivity of MgB2 is destroyed when Al is alloyed into MgB2 as reported previously. A structural instability was observed from the data of X-ray diffraction. Further study by TEM shows that the Al substitution leads to an ordering occupation on Mg sites. This corresponds to a superstructure with a repeat period of 2c along the c-axis. The superstructure was identified to be a form of MgAlB4 structure which intergrows with MgB2. Measurements on the low doping level samples showed that the upper critical field B-c2, irreversible field B-irr and the critical current density J(c) of these samples were reduced as the All content was increased. Analysis of the upper critical field data suggests that the density of states near the Fermi surface N(E-F) is reduced by Al doping. This is consistent with the decrease in the superconducting transition temperature T-c. The field dependence of pinning force is also discussed for these samples

Structural features of the incommensurate modulation in the Pb-doped Bi-2223 high-Tc phase revealed by direct-method electron diffraction analysis

Structural modulation waves in the Pb-doped Bi-2223 (Bi2Sr2Ca2Cu3Ox) phase are observed for the first time on the potential distribution function projected along the a axis. The newly developed multidimensional direct methods were used. Unlike the previous techniques, there is no need to assume any modulation model before the modulation waves can be seen directly from the resulting Fourier map. The symmetry of the title compound belongs to the four-dimensional space group P111Bbmb with three-dimensional unit cell parameters a=5.49 AA, b=5.41 AA, c=37.1 AA, alpha = beta = gamma =90 degrees and the modulation wavevector q=0.117b*. Occupational and positional modulations are obvious for most atoms. Disordered oxygen atoms arranged perpendicular to the b axis bridging the Cu(2)-Ca-Cu(1)-Ca-Cu(2) layers were observed.

Structural and superconducting properties of La2CuO4+δ chemically oxidized by NaClO

Abstract A series of La 2 CuO 4+ δ samples chemically oxidized in NaClO aqueous solution at room temperature with different oxidation time of 2–120 h have been prepared. The X-ray powder diffraction analyses indicate that all the oxidized samples are orthorhombic and single phase, and show an enhanced orthorhombic distortion with the oxidation time, compared with that of the as-prepared sample. The magnetic susceptibility measurements indicate that all the oxidized samples are bulk superconductors with an onset transition temperature at 45 K. The excess oxygen contents of the oxidized samples determined by iodometric titration increase with the oxidation time, which versus the oxidation time obeys the law of the second order exponential decay. The grain size observed by scanning electron microscopy has no obvious changes for the sample before and after oxidation. All the results indicate that NaClO is indeed an efficient oxidizing agent for La 2 CuO 4 .

Incommensurate modulation of the Bi-2223 superconducting phase observed by direct-method electron diffraction analysis

Structure details of the incommensurate modulation in the Pb-doped Bi-2223 high Tc superconducting phase have been revealed by a multidimensional direct-method analysis of the electron diffraction pattern of the (100) reciprocal plane. Occupational and positional modulations have been found for most atoms. Disordered oxygen atoms arranged perpendicular to the b axis bridging the Cu(2)-Ca-Cu(1)-Ca-Cu(2) layers were observed. This implies the existence of large amount of O-vacancies on the Cu(1)-O plane.

A STUDY OF THE SINGLE-CRYSTALS WITH NOMINAL COMPOSITION OF BI2SR2CACU2OY BY STEM AND SINGLE-CRYSTAL X-RAY-DIFFRACTION

Abstract The structure and the composition of single crystals with the nominal composition Bi 2 Sr 2 CaCu 2 O y have been studied by EDX and single crystal X-ray diffraction. The A.C. magnetic susceptibility measurements indicated that the superconducting transition temperature of these single crystals were ranging from 30K to 90K. The EDX elemental analysis showed that in general the actual composition of the single crystals deviated from the ideal composition Bi 2 Sr 2 CaCu 2 O y . Partly substitution of Ca by Bi was frequently observed and, with the decreasing of the Ca content, the T c of the single crystals decreased progressively from 90K to 30K. Single crystal x-ray diffraction indicated that the unit cell parameters of the single crystal, a, b and c, increase with the decrease of the T c in the range of 65–90K.

Short-range-oxygen order and superconducting phase separation in La2CuO4+x

Structural features and physical properties in La2CuO4+x materials have been systematically investigated. A short-range-order (SRO) state has been observed at low temperatures for 0.05<x<0.1. Anomalies in physical properties, in particular a superconducting phase segregation, are found to be in connection with the presence of this SRO state. We herein explain this SRO state in terms of intercalated-oxygen ordering, which could yield a local excess-oxygen content of x congruent to1/16 and a local hole concentration of n congruent to1/8.