Observation of Magnetic Order in a YBa2Cu3O6.6{\rm YBa_2Cu_3O_{6.6}} Superconductor

Polarized beam neutron scattering measurements on a highly perfect crystal of ${\rm YBa_2Cu_3O_{6.6}}$ show a distinct magnetic transition with an onset at about 235K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 $\mu_B$ for each sublattice and have a correlation length of at least 75 \AA. We found the critical exponent for the magnetic neutron intensity to be 2$\beta$ =0.37$\pm$ 0.12. This is the proper range for the class of transition that has no specific heat divergence possibly explaining why none is found at the pseudogap transition.Comment: 3 figure

X-Ray-Diffraction Study of Charge-Density-Waves and Oxygen-Ordering in YBa2Cu3O6+x Superconductor

We report a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO_2 bilayers, show a non-uniform increase below \~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering'' peaks as a manifestation of a charge density wave in the CuO_2 planes coupled to the oxygen-vacancy ordering.Comment: 4 pages, 4 figure

Magnetic Coherence as a Universal Feature of Cuprate Superconductors

Recent inelastic neutron scattering (INS) experiments on La$_{2-x}$Sr$_x$CuO$_4$ have established the existence of a {\it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $\chi''$, in the superconducting phase. We show, using the spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that the magnetic coherence effect establishes the ability of INS experiments to probe the electronic spectrum of the cuprates, in that the effect arises from the interplay of an incommensurate magnetic response, the form of the underlying Fermi surface, and the opening of the d-wave gap in the fermionic spectrum. In particular, we find that the magnetic coherence effect observed in INS experiments on La$_{2-x}$Sr$_x$CuO$_4$ requires that the Fermi surface be closed around $(\pi,\pi)$ up to optimal doping. We present several predictions for the form of the magnetic coherence effect in YBa$_2$Cu$_3$O$_{6+x}$ in which an incommensurate magnetic response has been observed in the superconducting state.Comment: 9 pages, 12 figures; extended version of Phys. Rev B, R6483 (2000

Macrophage migration inhibitory factor, infection, the brain, and corticosteroids

Bacterial meningitis is a complex disorder in which injury is caused, in part, by the causative organism and, in part, by the host's own inflammatory response. Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine and a neuro-endocrine mediator that might play a role in pneumococcal meningitis. Here, we discuss the role of MIF in infection, the brain, and corticosteroids and conclude that experimental meningitis studies have to determine whether MIF is a potential target for adjunctive therapy in pneumococcal meningitis

Spacetime Embedding Diagrams for Black Holes

We show that the 1+1 dimensional reduction (i.e., the radial plane) of the Kruskal black hole can be embedded in 2+1 Minkowski spacetime and discuss how features of this spacetime can be seen from the embedding diagram. The purpose of this work is educational: The associated embedding diagrams may be useful for explaining aspects of black holes to students who are familiar with special relativity, but not general relativity.Comment: 22 pages, 21 figures, RevTex. To be submitted to the American Journal of Physics. Experts will wish only to skim appendix A and to look at the pictures. Suggested Maple code is now compatible with MapleV4r

The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates

Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned stripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one order of magnitude near hole doping x=1/8. The NQR lineshape reveals three inequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.Comment: 4 pages, 3 figures, to appear in Physica C, Proceedings of the 6th International Conference on Materials and Mechanisms of Superconductivity, Houston, February 200

QPOs: Einstein's gravity non-linear resonances

There is strong evidence that the observed kHz Quasi Periodic Oscillations (QPOs) in the X-ray flux of neutron star and black hole sources in LMXRBs are linked to Einstein's General Relativity. Abramowicz&Klu\'zniak (2001) suggested a non-linear resonance model to explain the QPOs origin: here we summarize their idea and the development of a mathematical toy-model which begins to throw light on the nature of Einstein's gravity non-linear oscillations.Comment: Proceeding of the Einstein's Legacy, Munich 200