Seeking Superconductivity "Glue": Magnetic Excitations of Iron-Arsenic Quantified by French & German Researchers

Image credit: Max Planck/CEA

For the first time, researchers in Germany (Max-Planck-Institute, FRM-II) and France (Laboratoire Léon Brillouin, CEA-CNRS) have quantified the magnetic excitations of Iron-Arsenic material in the superconducting state. The results should help test the hypothesis of a "glue" binding the magnetic electrons in that particular state of matter. These results have been published in Nature Physics (March 2010).

Superconductivity is a peculiar state of matter that allows some metals to conduct electricity without resistance. For most materials, it is observed below a "critical" temperature, varying from 1 to 20 Kelvin (between -272 and -253 ° C). While the electrons which carry the same charge, are known to reject the theories of superconductivity based on the existence of an attractive force - the "glue¹"- leading to the movement of electron pairs inside the material. For several decades, scientists wondered about the possibility of a glue based on superconducting magnetic fluctuations generated by the electrons themselves.

This research has experienced a sharp rebound in 2008 with the discovery of superconductivity at high critical temperatures (up to 50 Kelvin or -223 ° C) in compounds based on iron called "iron pnictides². The observation was surprising because the magnetic iron (iron combined with other elements) is considered rather as an antagonist of superconductivity. 

More recently, researchers at the Max Planck Institute and the Laboratoire Léon Brillouin measured the magnetic fluctuations in a family of these compounds as a function of temperature, using a technique of neutron scattering ³. In bringing the compound above and below the critical temperature, they studied its properties when it is in metallic phase (conduction of conventional electricity) and superconducting phase.In the superconducting state, the electron pairing leads to a profound restructuring of the spectrum of magnetic fluctuations. This variation of the spectrum only exhibits below the energy needed to "break" the pairs of electrons.

What distinguishes this work from those made by other teams is that they give a very complete and quantitative spectrum of excitations directly usable for testing the relevance of theoretical models for these materials. For the first time with these compounds, the researchers are able to quantify in absolute magnetic fluctuations, for several temperatures. This will allow scholars to compare precisely with their simulations and to better assess the role of "glue" magnetic superconductivity of these materials.

Notes:

(1) The superconducting glue: For many superconductors, the "glue" responsible for the formation of pairs of electrons known as "Cooper" is formed from the interaction of electrons with the vibrations of atoms in the material. Electrons have a negative electrical charge and are a kind of small magnetic moment called spin.Magnetic waves can propagate in metals in the form of small oscillations of the spins. The electrons then move in a bath of "magnetic fluctuations. A temperature superconducting two electrons could pair because of their interaction with magnetic waves.

(2) pnictides:  This term refers to compounds containing elements of the column nitrogen (fifteenth) of Mendeleev table (N, P, As, Sb ...). The iron pnictides are compounds FeAs, FeN, etc..

(3) Neutron inelastic scattering: The researchers used the technique of inelastic neutron "sample by bombarding a crystalline iron-arsenic by neutron reactor Orphée (Saclay) and FRM-II (Garching, Germany). The neutron is an uncharged particle that penetrates easily into the materials. And, like the electron, the neutron has a spin, which makes it sensitive to the magnetic properties of the material it traverses. It then behaves like a wave, whose propagation and frequency will be modified by the magnetic waves present in the material traversed. The technique of inelastic neutron scattering allows to know what the spins of electrons are in space and time.This is the only technique that allows access to the "spectrum of magnetic fluctuations.

Reference: Nature Physics Vol. 6, pp. 178-181, 2010 - website Nature Physics