P. H. Hor

Superconductivity at 52.5 k in the lanthanum-barium-copper-oxide system.

A superconducting transition with an onset temperature of 52.5 K has been observed under hydrostatic pressure in compounds with nominal compositions given by (La0.9Ba0.1)2 CuO4-Y. Possible causes for the high-temperature superconductivity are discussed.

Growth of large, defect-free pure C60 single crystals

Millimeter‐sized single crystals of C60 were grown by sublimation of C60 powder in a vacuum for 6–24 h. The crystals had excellent facets, were free of C70 or solvent, and showed face‐centered cubic symmetry with a very small mosaic spread down to 0.01°.

Calorimetric evidence of antiferromagnetism in GdBa2Cu3O6+δ

Abstract Heat capacity measurements between 1.8 and 10K have been made on the superconductor GdBa 2 Cu 3 O 6+δ with δ ⋍ 0.7 and T c ⋍ 94K. A well defined magnetic transition prevails at 2.22K. The entropy involved agrees well with the complete antiferromagnetic ordering of Gd 3+ ions.

Practical adaptation in bulk superconducting magnetic bearing applications

Lifting capacities greater than 41 N/cm2 (60 psi) at 77 K have been achieved using a combination of permanent magnets and high quality melt‐textured YBa2Cu3O7−δ (YBCO). The key concept of this hybrid superconducting magnetic bearing (HSMB) is the use of strong magnetic repulsion and attraction from permanent magnets to support high loads in conjunction with flux pinning in a type II superconductor to counteract instabilities in a system consisting of magnets only. To illustrate this concept, radial and axial forces between magnet/superconductor, magnet/magnet, and magnet/superconductor/magnet, were measured and compared for the thrust and journal bearing configurations on a bearing prototype.

Pressure effect on the superconducting transition temperature of Tl2Can-1Ba2CunO2n+4-δ

Abstract The pressure effect on the superconducting transition temperature (d T c /d P ) has been examined for the hole-high temperature superconductors (HTSCs) Tl 2 Ca n −1 Ba 2 Cu n O 2 n +4 for n=1, 2 and 3. A negative d T c /d P was observed in this family of HTSCs with n =1, in strong contrast to other hole-HTSCs, raising a question concerning the charge asymmetry in HTSCs previously proposed. By examining all existing α(≡ d T c / T c d P )-results, it was found that hole HTSCs and electron-HTSCs fall into two distinct regions in the α − T c plot. A universal α − T c behavior also emerges for the hole-HTSCs. The implications of these observations are discussed.

Comparison between the magnetic and transport critical current densities in high critical current density melt‐textured yttrium barium copper‐oxide

Using our recently developed pulsed critical current density (Jc) measuring system, the Jc of the high Jc melt‐textured YBa2Cu3O7−δ (Y123) bulk samples has been determined. I‐V curves with a voltage resolution of 0.5 μV were obtained, and transport Jc’s along the a‐b plane (Jc∥ a‐b) as high as 7.2×1 04 A/cm2 were extracted. These results are comparable to the values obtained magnetically. On the other hand, transport Jc along the c axis (Jc∥c) were found to be two orders of magnitude smaller, even though the magnetic Jc∥c is only about five times smaller than Jc∥a‐b. We suggest that for the high‐temperature superconducting materials which are highly anisotropic, caution should be taken when using the nontransport magnetic methods to determine Jc.

Local chemistry and vibrational behavior of the rare-earth in the high TC superconductor EuBa2Cu3O7

Temperature dependent 151Eu Mossbauer spectroscopy experiments on the high Tc≧90K superconductor EuBa2Cu3O7 in the range of 4.2K < T < 300K display evidence of Eu3+ site only. No evidence of Eu2+ sites is observed. Variation of isomer-shift σ(T) and recoil-free-fraction f(T) shows that the mean square displacement (lnf) and mean square velocity (σ) of the Eu3+ site are both characterized by a Debye temperature θD=280(5)K. No discernible evidence of a phonon mediated structural transition as a precursor to Tc is observed in the temperature range examined.

A basic step toward understanding skin surface temperature distributions caused by internal heat sources

This study uses numerical solutions of a bio-heat transfer equation to investigate the relationship between skin surface temperature distributions and internal heat sources under various physiological and environmental conditions. It is found that although a surface temperature distribution depends on all heat source parameters, the properly normalized distribution is primarily affected only by the depth of the heat source. This study provides a physical basis for determining the depth and type of an internal heat source from a thermogram acquired in various environmental conditions and an understanding of the basic relationship between skin surface temperature distributions and internal heat sources.

Structural stability and doping in R2CuO4 (R = rare-earth)

Abstract Several new compounds have been synthesized. By analyzing the new compounds and reviewing data from existing compounds, we conclude that the atomic radius of R in R 2 CuO 4 dictates the stability of and the type of doping permitted in the various homomorphies of the compounds. Our efforts have yet to induce a direct electron-to-hole HTS transition or to achieve 90 K-electrons HTS.

Grain growth in monolithic YBa2Cu3O7 with high Jc

Abstract Several parameters affecting the grain growth in monolithic YBa2Cu3O7 using the melt-texturing and continuous processing technique have been identified and investigated. Steps have been found to promote grain-growth, to improve the grain-orientation, to control microcracks, to lower the processing temperature, to increase the processing rates, and to improve the overall chemical uniformity. The results will be presented and discussed.