Skyrmions around Kerr black holes and spinning BHs with Skyrme hair

We study solutions of the Einstein-Skyrme model. Firstly we consider test field Skyrmions on the Kerr background. These configurations -- hereafter dubbed Skerrmions -- can be in equilibrium with a Kerr black hole (BH) by virtue of a synchronisation condition. We consider two sectors for Skerrmions. In the sector with non-zero baryon charge, Skerrmions are akin to the known Skyrme solutions on the Schwarzschild background. These `topological' configurations reduce to flat spacetime Skyrmions in a vanishing BH mass limit; moreoever, they never become "small" perturbations on the Kerr background: the non-linearities of the Skyrme model are crucial for all such Skerrmions. In the non-topological sector, on the other hand, Skerrmions have no analogue on the Schwarzschild background. Non-topological Skerrmions carry not baryon charge and bifurcate from a subset of Kerr solutions defining an existence line. Therein the appropriate truncation of the Skyrme model yield a linear scalar field theory containing a complex plus a real field, both massive and decoupled, and the Skerrmions reduce to the known stationary scalar clouds around Kerr BHs. Moreover, non-topological Skerrmions trivialise in the vanishing BH mass limit. We then discuss the backreaction of these Skerrmions, that yield rotating BHs with synchronised Skyrme hair, which continously connect to the Kerr solution (self-gravitating Skyrmions) in the non-topological (topological) sector. In particular, the non-topological hairy BHs provide a non-linear realisation, within the Skyrme model, of the synchronous stationary scalar clouds around Kerr.Comment: 23 pages, 7 figures; to appear in JHE

«UNE SÉRIE DE HASARDS SOIGNEUSEMENT PRÉPARÉS». NOTES SUR «LE CHIENDENT» DE RAYMOND QUENEAU

Tackling the first novel of Raymond Queneau, in this essay we analyze the relations between hazard and necessity, arbitrary and motivation, at the level of artistic fiction. We try to reveal the ways of the meaning creation, and the existential and philosophical implications resulting from the peculiar vision upon the world that is designed within the narrative of the French author

Reduction formalism for Dirac fermions on de Sitter spacetime

The reduction formulas for Dirac fermions are derived, using the exact solutions of free Dirac equation on de Sitter spacetime. In the framework of the perturbation theory one studies the Green functions and derive the scatering amplitude in the first orders of perturbation theory.Comment: 12 pages, no figure

Remarks on the spherical waves of the Dirac field on de Sitter spacetime

The Shishkin's solutions of the Dirac equation in spherical moving frames of the de Sitter spacetime are investigated pointing out the set of commuting operators whose eigenvalues determine the integration constants. It is shown that these depend on the usual angular quantum numbers and, in addition, on the value of the scalar momentum. With these elements a new result is obtained finding the system of solutions normalized (in generalized sense) in the scale of scalar momentum.Comment: 7 pages, no figure

Spin-transfer torques in anti-ferromagnetic metals from first principles

In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting anti-ferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in anti-ferromagnetic non-collinear spin valves and domain walls. We report a first principles study on the electronic transport properties of anti-ferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in anti-ferromagnets are very nonlocal. The torques acting far away from the center of an anti-ferromagnetic domain wall should facilitate current-induced domain wall motion.Comment: The paper has substantially been rewritten, 4 pages, 5 figure

Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30nm to 250nm were achieved and characterized using transmission electron diffraction and X-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast X-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time