Description:

RbEuFe4As4 is a recently discovered superconductor in which superconductivity and magnetic order coexist at unusually high temperatures with Tc ~ 37 K and a magnetic ordering temperature of Eu-magnetic moments at 15 K.  Materials parameters of RbEuFe4As4, such as high values of Tc and the upper critical field, weak single-ion magnetic anisotropy of Eu, the unusual behavior of the RKKY interaction for open Fermi surfaces, and weak vortex pinning, are such that aspects of the interplay of magnetism and superconductivity arise which are usually no observable. Here, I will present an overview of the properties RbEuFe4As4 in comparison to other families of Fe-based superconductors, and then highlight two aspects: self-induced critical states of the vortex system and the helical magnetic structure of the Eu-moments.  Magnetization measurements and magneto-optical imaging reveal highly non-uniform distributions of superconducting vortices even on cooling in constant applied magnetic fields, which are induced by the strong temperature dependence of susceptibility of the Eu-moments.  Resonant X-ray scattering and neutron diffraction reveal that Eu-moments order into ferromagnetic in-plane polarized sheets which twist by 90 deg from layer to layer yielding a commensurate helix with a period of four c-axis lattice constants.  We propose a model in which this magnetic structure arises from the competition between ferromagnetic exchange interactions and strong superconductivity.

Dr. Ulrich Welp, Materials Science Division, Argonne National Laboratory

Host: Venkat Chandrasekhar

Keywords: Physics, Astronomy, Condensed Matter