Magnetism, nematicity and superconductivity in 1111 oxypnictide single-crystals

• Applicants: Professor Dr. Markus Braden; Dr. Sabine Wurmehl
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461247437

The discovery of superconductivity in F-doped LaFeAsO polycrystals initiated the so-called iron-age of high-temperature superconductivity. These materials are frequently referred to as Fe-based superconductors (IBS). Three types of order manifest in the low-temperature phases of IBS: (i) orbital or electronic ordering (ii) orthorhombic distortion, and (iii) the antiferromagnetic spin density wave order. Precedent to long-range ordered states, a phase with broken rotational symmetry but with preserved time-reversal and translation symmetries emerges and is referred to as nematic state. Magnetism, nematicity and superconductivity are intimately interwoven but the question, which of these three orderings is central, remains matter of controversy. While there have been comprehensive studies on many families of IBS, the initial system of doped REFeAsO (1111) compounds with RE a rare earth has been very little studied due to the absence of suitable single crystals. This lack of understanding the 1111 system is highly regrettable, as these compounds exhibit the highest bulk superconducting transition temperatures, and as there are significant differences with the other classes such as a higher degree of orbital degeneracy and a reduced electronic dispersion perpendicular to the layers.A deep understanding of IBS in general and of 1111 materials in particular requires single crystals of appropriate size, quality and composition. Due to the major challenges in crystal growth, the 1111 family is currently the least understood among all IBS. Only recently, large and well faceted LaFeAsO single crystals with considerable c axis growth were grown in the group of PI Wurmehl at the IFW in Dresden using solid-state single-crystal growth. This approach is not commonly known and not fully understood, the growth is diffusion controlled and crystals are grown from a polycrystalline matrix via abnormal grain growth. This success in growing LaFeAsO crystals will set the stage for exciting experiments both in the field of chemistry and physics. We wish to profit of the opportunities the first availability of the 1111 crystals offers by combining the crystal growth efforts with scattering studies using X-ray and neutron radiation. Thereby we want to clarify the crystal structure and in particular to search for the structural distortions evidenced by NQR experiments. We will characterize magnetic order and short-range magnetic correlations as function of composition, temperature and uniaxial strain. Magnetic excitations are considered as a key element of the superconducting pairing and can be studied by inelastic neutron scattering when larger amounts of crystals become available. It seems most interesting to study the superconducting spin-resonance modes in materials with higher transition temperatures and to study the spin-space anisotropies in 1111 materials, which exhibit a different electronic structure.

• DFG Programme: Research Grants