Investigação do efeito Meissner por dicroísmo de raios X.

Abstract

Superconductivity is defined by the ability of certain materials to conduct electric current with no resistance. This capacity produces interesting and potential useful effects. One of the most notable effects of superconductivity (SC) is the Meissner effect. It occurs due to screening currents near the surface that produce a magnetic field to cancel the field inside the material when this is cooled to lower than the critical temperature Tc. In that case an applied magnetic field lower than the critical field Hc would not be able to penetrate the interior of the superconductor. Although the Meissner effect is known since 1933, its physical mechanism is still not fully understood today. One possible explanation was recently postulated by Jorge Eduardo Hirsch [1], which in the absence of an applied magnetic field the orbital expansion for the angular momentum in the presence of an ionic electric field would generate through spin-orbit interaction a spin current near the surface of the superconductor. This spin-Meissner effect would therefore exist in all superconductors and be at the root of the Meissner effect. It was also recently postulated [2] that x-ray beams carrying orbital angular momentum (OAM) could induce strong dichroic effects. However, the production of such x-ray orbital polarization requires nanofocalized beam, which has thwarted attempts to experimentally probe this effect. Here we propose a dynamical method to probe the spin and the conventional Meissner effect in type I and type II superconductors using the spin-orbit interactions and the possibility to probe both orbital and angular momentum in an experiment analogous to X-ray magnetic circular dichroism (XMCD). In this work we used Ta (type I superconductor) and IrZr2 (type II superconductor) samples as prototypical compounds to validate this methodology. XMCD spectra collected in the superconducting state of Tametal and IrZr2 indicate a small but clear signal arising from the coupling between the orbital moment of the screening currents and the orbital moment of the photoelectron. Tantalum metal samples differing in their granularity and morphological shape where used. The maximum signal correlated to Meissner effect was observed considering the coherence length and the penetration depth of all the samples [3-5].