Session B1: Magnetism in Fe Pnictides and Chalcogenides

Examination of the Role of Electronic Correlations in the Pnictides

In the new and rapidly developing field of Fe-pnictide superconductivity, the question of what constitutes the basic ingredients for high transition temperatures remains largely unanswered. Parallels have been drawn to the cuprate high-temperature superconductors, which contain partially filled d-electron bands whose spins in the parent phase are aligned antiferromagnetically like the pnictides, and high-temperature superconductivity emerges when magnetism can be suppressed. Common to many ideas is that superconductivity itself may be emergent from the two competing phases, driven by an underlying quantum critical point. A key question that needs to be addressed to understand this framework is whether or not the Fe pnictides are strongly correlated like the cuprates. In this talk I will review some experimental indications that address this issue, with a particular focus on photoemission and x-ray based spectroscopic investigations. Comparison of the data with a variety of calculated spectroscopies indicate that the electronic correlations are not as strong as the cuprates, but also reveal aspects where correlations might be of central importance.   

Localized vs. Itinerant Magnetism in Fe-based Superconductors

Since the discovery of a high temperature superconducting transition in ferropnictides approximately two years ago, the highly magnetic character of these compounds and the close relationship between superconductivity and magnetism has been widely recognized and intensely studied. Initially, debate about the nature of the magnetism was split into two camps: localized moments (as in cuprates) and pure itineracy (a spin-peierls type transition). We argue that the magnetism in pnictides and in the related chalcogenides is between these two extremes, consisting of Hund's rule (or Stoner) derived moments on the Fe atoms. Using density functional theory (DFT) calculations, we show that the ordering mechanism is not Fermi surface driven and is also unlikely to be of superexchange origin. We explain, from a computational perspective, how the magnetic and structural transitions are related and compare calculated doping and pressure dependent quantities to experiment. We discuss which quantities are well reproduced and explainable using DFT and what remaining questions need to be answered before magnetism, superconductivity and their relationship can be considered as understood. We argue that spin fluctuations are the driving force behind the superconductivity and that magnetic order is a competing, and therefore detrimental, phase. We propose a particular spin fluctuation scheme that could explain why the structural transition appears either in conjunction with or before the magnetic transition.   

Interplay between phonons and magnetism in 122 ferropnictides

The talk will focus on results of inelastic neutron and x-ray scattering measurements and density functional theory (DFT) calculations of phonon dispersions in doped and undoped CaFe$_{2}$As$_{2}$ (Ca122) and BaFe$_{2}$As$_{2}$ (Ba122). DFT predicts that the frequencies of some phonons depend strongly on the Fe moment and the relative orientation between the Fe moment and the phonon propagation vector. Thus a comparison of experimental phonon frequencies and the DFT calculations can serve as a probe of magnetism. The calculation with Fe moment fixed to zero gives correct frequencies of \textit{most} phonons in both compounds if the calculation is constrained to the experimental crystal structure. However, in Ba122 some phonons are softer in the experiment than calculated. The agreement is improved if a large Fe magnetic moment is included into the calculation. However, this Fe moment would result in phonon peak splitting that is not observed. In addition, we found very little doping or temperature dependence of the phonons, aside from some small shifts in Ca122 across the magnetic transition. The talk will focus on how these already published results relate to out understanding of the physics of the pnictides and the coupling between phonons and magnetic fluctuations. I will also present many new unpublished results, will discuss novel recently observed magnetic modes, and will reexamine phonon peak assignements of our previous neutron scattering measurements.   

Magnetism in Fe Pnictides and Chalcogenides Probed by Neutron Scattering

There has recently been considerable discussion on relationship between magnetism and superconductivity in the recently discovered Fe based superconductors. The original materials were discovered when Fe was doped into LaFeAsO and higher transition temperatures obtained with substitutions for La. Even simpler superconductors could be made with the Fe(TeSe) system and neutron scattering has uncovered quite unusual behavior in these materials. At low temperatures static magnetism was found at the (1/2, 0, 1/2) lattice position for FeTe, but this disappeared as Te was replaced by Se in which case steep incommensurate excitations were found on either side of the (1/2, 1/2, 0) position. As the Se was further increased the incommensurate excitations became closer to the (1/2, 1/2, 0) and the material becomes superconducting with a resonance at exactly (1/2,1/2, 0). Neutron scattering will be used to show how the magnetism and superconductivity interact in the FeSeTe and other materials of interest.   

Spin Dynamics and Local Inhomogeneity in Doped AFe$_{2}$As$_{2}$ (A = Ca, Ba)

We report $^{75}$As Nuclear Magnetic Resonance (NMR) data in CaFe$_{2}$As$_{2}$, Ba(Fe,Co)$_{2}$As$_{2}$ and Ba(Fe,Ni)$_{2}$As$_{2}$. The static hyperfine field in the antiferromagnetic state is a direct probe of the sublattice magnetization, which decreases with Co or Ni doping. The spectra reveal a broad distribution of local internal fields, consistent with spatial inhomogeneity associated with the dopants. The spin lattice relaxation rate is highly sensitive to the presence of dopants and/or impurities, and reveals the presence of enhanced antiferromagnetic fluctuations with increasing doping. In the antiferromagnetic state, we find that the relaxation is driven by low energy magnon excitations. CaFe$_{2}$As$_{2}$ exhibits an unexpected peak in the spin lattice relaxation rate at 10K that may be driven by glassy dynamics associated with interplanar coupling between different magnetic domains.