Pressure effects on the superconducting properties of YBa_2Cu_4O_8

Measurements of the magnetization under high hydrostatic pressure (up to 10.2 kbar) in YBa_2Cu_4O_8 were carried out. From the scaling analysis of the magnetization data the pressure induced shifts of the transition temperature T_c, the volume V and the anisotropy \gamma have been obtained. It was shown that the pressure induced relative shift of T_c mirrors essentially that of the anisotropy. This observation uncovers a novel generic property of anisotropic type II superconductors, that inexistent in the isotropic case.Comment: 4 pages, 3 figure

The 110 watt per kilogram lightweight solar array system

This ultralightweight solar array system is applied to three generic mission types: (1) interplanetary, (2) geosynchronous, and (3) manned space station. The requirements of each of these missions, as they pertain to the solar array, are presented. Existing lightweight solar array system concepts are reviewed, along with conclusions regarding the applicability of this technology to the feasibility of the ultralightweight solar array system. Several system concepts are included for further evaluation. The existing technology base, as it pertains to solar cells, solar cell covers, interconnects and substrates, and deployable booms, is reviewed. The attitude control of spacecraft with large flexible solar arrays is also discussed

Magnetic field induced 3D to 1D crossover in Sr0:9La0:1CuO2

The effect of the magnetic field on the critical behavior of Sr0:9La0:1CuO2 is explored in terms of reversible magnetization data. As the correlation length transverse to the magnetic field Hi,applied along the i-axis, cannot grow beyond the limiting magnetic length LHi, related to the average distance between vortex lines, one expects a magnetic field induced finite size effect. Invoking the scaling theory of critical phenomena we provide clear evidence for this effect. It implies that in type II superconductors there is a 3D to 1D crossover line Hpi(T). Consequently, below Tc and above Hpi(T) uperconductivity is confined to cylinders with diameter LHi(1D). Accordingly, there is no continuous phase transition in the (H,T)-plane along the Hc2-lines as predicted by the mean-field treatment.Comment: 4 pages, 5 figure

Generalized Robba rings

We prove that any projective coadmissible module over the locally analytic distribution algebra of a compact $p$-adic Lie group is finitely generated. In particular, the category of coadmissible modules does not have enough projectives. In the Appendix a "generalized Robba ring" for uniform pro-$p$ groups is constructed which naturally contains the locally analytic distribution algebra as a subring. The construction uses the theory of generalized microlocalization of quasi-abelian normed algebras that is also developed there. We equip this generalized Robba ring with a self-dual locally convex topology extending the topology on the distribution algebra. This is used to show some results on coadmissible modules.Comment: with an appendix by Peter Schneider; revised; new titl

A Note on Asymptotic Freedom at High Temperatures

This short note considers, within the external field approach outlined in hep-ph/0202026, the role of the lowest lying gluon Landau mode in QCD in the high temperature limit. Its influence on a temperature- and field-dependent running coupling constant is examined. The thermal imaginary part of the mode is temperature-independent in our approach and exactly cancels the well-known zero temperature imaginary part, thus rendering the Savvidy vacuum stable. Combining the real part of the mode with the contributions from the higher lying Landau modes and the vacuum contribution, a field-independent coupling alpha_s(T) is obtained. It can be interpreted as the ordinary zero temperature running coupling constant with average thermal momenta \approx 2pi T for gluons and \approx pi T for quarks.Comment: 4 pages; minor changes, version to appear in Phys. Rev.

NLTE spectroscopic analysis of the 3^3He anomaly in subluminous B-type stars

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the $^3$He stars, which exhibit a remarkable enrichment of $^3$He with respect to $^4$He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known $^3$He BHBs and nine known $^3$He sdBs to determine their isotopic helium abundances and $^4$He/$^3$He abundance ratios. We redetermined their atmospheric parameters and analyzed selected neutral helium lines, including $\lambda$4922 $\mathring{A}$ and $\lambda$6678 $\mathring{A}$, which are very sensitive to $^4$He/$^3$He. Most of the $^3$He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48$^\circ$ 2721 is reclassified as a BHB star because of its low temperature ($T_{\mathrm{eff}}=$ 20700 K). Whereas $^4$He is almost absent ($^4$He/$^3$He$<$ 0.25) in most of the known $^3$He stars, other sample stars show abundance ratios up to $^4$He/$^3$He$=$2.51. A search for $^3$He stars in the ESO SPY survey led to the discovery of two new $^3$He sdB stars (HE 0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48$^\circ$ 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic

Finite-size and pressure effects in YBa_2Cu_4O_8 probed by magnetic field penetration depth measurements

We explore the combined pressure and finite-size effects on the in-plane penetration depth \lambda_{ab} in YBa_2Cu_4O_8. Even though this cuprate is stoichiometric the finite-size scaling analysis of \lambda_{ab}^{-2}(T) uncovers the granular nature and reveals domains with nanoscale size L_{c} along the c-axis. L_{c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence of a phase coherent macroscopic superconducting state in cuprate superconductors.Comment: 7 pages, 6 figure