Anomalous pressure dependence of optical reflectivity in the superconductorBa0.6K0.4<

Abstract

The pressure dependence of visible and near-infrared reflectivity of the superconducting compound Ba06K0.4Bi03 has been measured to 40 GPa at room temperature.The reflectivity decreases dramatically up to the maximum applied pressure, in marked contrast to copper oxide-based high-T, superconducting materials.The superconducting compound Bao sKo 4Bi03 (Ba-KBi-0) is a member of the family of superconductors based on the perovskite BaBi03. Despite a relatively low T, of 30 K, " Ba-K-Bi-0 has in common with the copper-based materials a higher T, than any previously known metallic superconductor, as well as a low density of states near the Fermi level. Ba-K-Bi-0 differs, however, from the other high-T, materials in that it does not contain copper, and has a cubic perovskite crystal structure, rather than the complicated layered perovskite structures characteristic of the copper-based materials. Thus, there are no one-or two-dimensional structures and no preferred directions that could give rise to anisotropy in properties such as critical currents, coherence lengths, or penetration depths.Soon after the discovery of high-T, superconductivity in copper-oxide-based materials, high-pressure measurements demonstrated that large compression shifted the superconducting transition to a higher temperature in tnany materials.Reflectivity in the infrared and visible region, which is broadly indicative of metallic character, also increases with pressure in the normal state for the copperbased materials. s We report here reflectivity in the near infrared and visible region for Ba-K-Bi-0 at high pressure and demonstrate that the reflectivity dramatically decreases with pressure up to 40 GPa. A sample of nominal composition BapsKo4Bi03 was prepared from BaO, KOt, and Biq03 powders. BaO and KO2 are both moisture sensitive, so the starting materials were ground together in a glove bag filled with N2. The sample was exposed to air only long enough (-2 min) to press it into a pellet and place it into the furnace. Firing was done in a two-stage process. The pellet was heated initially to 700'C in Aowing N~and quenched to room temperature. This was followed by annealing at 450'C in Aowing 02 with a slow cool back to room temperature.The quench minimizes phase separation and the slow cool insures proper oxygenation. Since the sample is quite air sensitive after the N2 firing, the gas Aow was switched without exposing the sample to air. This process was repeated twice more to obtain sample uniformity. The sample was then characterized with powder x-ray diffraction and dc magnetization.

Single spin asymmetry in inclusive pion production, Collins effect and the string model

“…For completeness we show in Fig. 7 the earlier data measured with 13.3 and 18.5 GeV polarized protons [9]. Here the agreement of the model and the data is also good.…”

“…The asymmetries vary from about 0 at x F ∼ 0.2 to about +0.4, +0.15 and −0.4, for π + , π 0 and π − respectively, at x F ∼ 0.7 − 0.8 and p ⊥ > 0.7GeV. The earlier 13.3 and 18.5GeV data [9] showed the asymmetry of π + reaching 0.1 at x F = 0.6 but that of π − consistent with zero. However, that measurement was done for π + and π − in different p ⊥ regimes.…”

“…Fig. 7 The single spin asymmetries of charged pions measured with 13.3 and 18.5GeV proton beams [9]. The dashed lines are the predictions of our model for 13.3GeV and the full ones are for 18.5GeV.…”

Single spin asymmetry for p↑p → πX in perturbative QCD

“…[1]- [4], [14]; in particular, they do not show sizeable values of A N for π − [16,17], show strong p T dependences for π 0 [15] and π + [17] and have contradictory results on x F dependences [16,17]. However, all these data come from experiments at much smaller energy (proton beams of 13.3, 18.5 or 24 GeV/c) than those of Refs.…”