Vortex Softening: Origin of the second peak effect in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

Transverse ac permeability measurements in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta }$ single crystals at low fields and temperatures in a vortex configuration free of external forces show that the decrease of the critical current as measured by magnetization loops at the second peak effect is an artifact due to creep. On the other hand, the increase of critical current at the second peak is due to a genuine softening of the tilting elastic properties of vortices in the individual pinning regime that precedes the transition to a disorder state.Comment: 4 pages, 5 figures, RevTex, two column versio

Complexation of oppositely charged polyelectrolytes: effect of ion pair formation

Complexation in symmetric solutions of oppositely charged polyelectrolytes is studied theoretically. We include polyion crosslinking due to formation of thermoreversible ionic pairs. The electrostatic free energy is calculated within the Random Phase Approximation taking into account the structure of thermoreversible polyion clusters. The degree of ion association is obtained self-consistently from a modified law of mass action, which includes long-range electrostatic contributions. We analyze the relative importance of the three complexation driving forces: long-range electrostatics, ion association and van der Waals attraction. The conditions on the parameters of the system that ensure stability of the complex with addition of salt are determined

Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra

We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We report on narrow radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km/s. The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent 3D MHD simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical RMS velocities of about 1.2 km/s.Comment: Accepted for publication in A&A (06-March-2013). Minor corrections made in this version

Indirect constraints to branon dark matter

If the present dark matter in the Universe annihilates into Standard Model particles, it must contribute to the gamma ray fluxes detected on the Earth. Here we briefly review the present constraints for the detection of gamma ray photons produced in the annihilation of branon dark matter. We show that observations of dwarf spheroidal galaxies and the galactic center by EGRET, Fermi-LAT or MAGIC are below the sensitivity limits for branon detection. However,future experiments such as CTA could be able to detect gamma-ray photons from annihilating branons of masses above 150 GeV.Comment: 4 pages, 3 figures. Contribution to the Proceedings of Spanish Relativity Meeting ERE2011, Madrid 29 August - 2 September 201