Session B0: Focus Session: Fe-based Superconductors – Neutrons and X-rays

Nematic spin correlations in the tetragonal state of uniaixial-strained BaFe$_{2-x}$Ni$_x$As$_2$

Understanding the microscopic origins of electronic phases in high-transition temperature (high-$T_c$) superconductors is important for elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe$_{2-x}T_x$As$_2$ (where $T$ is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.   

Coexistence of diffuse liquid-like scattering and emergent coherent mode in Fe$_{1+y}$Te

Neutron scattering in the chalcogenide parent material FeTe reveals diffuse dynamical magnetic response, suggesting dynamical correlation length of only 1-2 lattice repeats. The wave-vector structure of magnetic fluctuations can be best described by a liquid-like state with local order in the form of weakly correlated, co-aligned four-spin plaquettes. Additionally, this dynamical magnetism is extremely robust. It survives to temperatures well above the Neel ordering T $\sim$ 60 K, and even exhibits unusual temperature-induced enhancement. More surprisingly, no coherent excitation was seen to emerge when system is cooled into magnetically ordered state. Nevertheless, spin-wave theory has been applied to interpret the measured spectra, but the agreement of its predictions with the experiment was found to be marginal at best. Here we report the discovery of a coherent magnetic excitation in the magnetically ordered state at low temperature, which coexists with the diffuse, liquid-like magnetic response. This resolves puzzle of surprising absence of coherent excitations associated with magnetic order in FeTe, where emergence of sharp electronic resonance near the Fermi energy has been observed by ARPES. This work was supported by the US DOE under Contract DE-AC02-98CH10886.   

Decoupling neutron resonances from superconducting transition temperatures in NaFe$_{1-x}$Co$_x$As

We report a comprehensive study of the evolution of neutron resonance modes in wide doping range of NaFe$_{1-x}$Co$_x$As. Double resonances, a sharp followed by a broad one, are observed in certain underdoped region. Surprisingly, the sharp resonance energy is nearly unchanged with variation of doping concentrations and therefore $T_c$. In overdoped region without competing phases, single resonance presents and is gradually broadening with further increasing doping. Nevertheless, the resonances peak at the same energy while $T_c$ falls half from slightly overdoped one ($T_c$=18 K) to heavily overdoped one ($T_c$ = 9 K). The decoupling between $E_r$ and $T_c$ indicates unappreciated coherence which had not been predicted by any theoretical models. Our results manifest the distinct multiorbital character in FeSC, guiding a new direction in the pursuit of a generic connection in diversified unconventional superconductors.   

Elastic and Inelastic neutron scattering results in heavily Cu doped NaFe$_{1-x}$Cu$_x$As with x approaching 0.5

Compared to Co and Ni doping, doping Cu in NaFe$_{1-x}$Cu$_x$As causes the system to become insulating at high dopings. We found in heavily Cu doped NaFe1-xCuxAs, low energy spin excitations are strongly enhanced similar to Cu doped Fe(Se,Te). We have further found for x approaching 0.5, Fe and Cu tend to order and results in a novel magnetic state, explaining the enhanced low energy spin excitations in heavily Cu doped NaFe$_{1-x}$Cu$_x$As.   

Inelastic neutron scattering study on Co-doped LiFeAs

We performed inelastic neutron scattering on Co-doped LiFeAs material. In 12{\%} Co doped LiFeAs, where Tc is dramatically suppressed, the low energy spin excitation is commensurate at (pi,0) point which is different from pure LiFeAs case. Based on the fact that in this material the perfect nesting exists between outer hole pocket and electron pocket and is dominated by dxy orbital, we argue that the superconductivity is actually associated with electron scattering from dxz/dyz orbital and the dxy orbital barely contributes to the superconducting pairing.   

Phase diagram of BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

As a unique system of high temperature Iron-based superconductors, recent experimental results indicate that there is a quantum critical point (QCP) around the optimal level in BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$. We use neutron diffraction, high resolution X-ray scattering and NMR techniques to map out the detailed phase diagram. It is found that the long-range antiferromagnetic (AF) order survives up to the optimal doping level within the instrument resolution. Our results suggest that the evolution of the AF order upon doping in BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ is different from that in the electron-doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ or Ba(Fe$_{1-x}$Ni$_{x})_{2}$As$_{2}$.   

Magneto-elastic coupling in detwinned Sr(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ by inelastic x-ray scattering

We present phonon dispersion measurements from detwinned single crystals of \linebreak Sr(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with different doping levels(x=0 and 0.08) using inelastic x-ray scattering with 1.5 meV resolution at SPring-8. The crystals were detwinned by application of in-plane uniaxial stress. This allowed us to measure single domain phonon structure in the magnetically ordered state. We clearly identified the change in the phonon dispersion induced by the onset of magnetic order, with phonon energies depending on orientation of the phonon wave vector relative to the Fe moment. We compare our results to $\it ab$-$\it initio$ pseudo potential calculations: Magnetic calculations give better agreement than non-magnetic calculations, but in most parts of the Brillouin zone investigated, they overestimate the magnitude of the mode splitting and in fact often fail to predict even the correct sign of the splitting.   

Magnetic phase diagram of Ca(Co$_{1-x}$Fe$_{x}$)$_{2}$As$_{2}$ ($x \le $ 0.1) single crystals

Both CaCo$_{2}$As$_{2}$ and CaFe$_{2}$As$_{2}$ possess the tetragonal ThCr$_{2}$Si$_{2}$ crystal structure and are paramagnetic at room temperature and ambient pressure. Upon cooling, CaCo$_{2}$As$_{2}$ shows A-type antiferromagnetic (AFM) order below a N\'{e}el temperature of $T_{N\, }\approx $ 50-70 K, while CaFe$_{2}$As$_{2}$ transitions to an orthorhombic lattice with stripe-type AFM order below $T_{N\, }\approx $ 170 K. Here, we present results from neutron diffraction experiments on a series of Ca(Co$_{1-x}$Fe$_{x}$)$_{2}$As$_{2}$ single crystals ($x =$ 0 to 0.1) studying the evolution of CaCo$_{2}$As$_{2}$'s low-temperature state upon substituting Fe for Co. We find that A-type magnetic order persists for all of the values of $x$ studied with suppressed $T_{N}$ and ordered moment for increasing Co concentration and present the magnetic phase diagram for the Co rich region.\\[4pt] Work at Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC02-07CH11358. Research conducted at ORNL's High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US DOE.   

Study of lattice distortion in Sr(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals employing high-energy x-ray diffraction

For the iron arsenide family of superconductors, the interplay between structure, magnetism, and superconductivity is a major theme of research. Among $A$Fe$_{2}$As$_{2}$ ($A =$ Ca, Sr, Ba), a difference lies in the strength of magnetoelastic coupling: it is strongest in CaFe$_{2}$As$_{2}$ as indicated by strongly coupled first order phase transitions (structural and magnetic) and modest in BaFe$_{2}$As$_{2}$ in which the two phase transitions split with Co-substitution. Moreover, similar to the structural transition, the magnetic transition becomes second order with higher Co-concentration. SrFe$_{2}$As$_{2}$ shows intermediate behavior. Here we present a temperature-dependent study of the lattice distortion from tetragonal to orthorhombic in Sr(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals through diffraction measurements using x-ray radiation of two energies: 8.047 keV and 100 keV. The lower energy probes a few micrometers down from the surface of the sample whereas the higher energy characterizes the bulk. Details of the lattice distortion obtained with these two probes will be discussed.\\[4pt] The work at Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract DE-AC02-07CH11358. This research used resources of Advanced Photon Source, a US DOE, Office of Science User Facility.   

Tetragonal-to-orthorhombic lattice distortion and domains in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals

Similarly to other Fe-based pnictide superconductors, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals show a tetragonal-to-orthorhombic lattice distortion that is suppressed with increasing Co concentration. However, in contrast to other Fe-based pnictide superconductors, an unprecedented 45$^\circ$ rotation of the domain patterns in polarized-light microscopic studies has been observed for Co concentrations $x\,\approx\,0.2$ at temperatures around the onset of antiferromagnetic order and lattice distortion. We present a high-resolution high-energy x-ray diffraction study demonstrating that the observed change in domain arrangement is related to the coexistence of the tetragonal and orthorhombic phases in a well-defined geometric configuration allowed by a special relationship between the lattice parameters of both phases in these compounds. \\[4pt] The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, Department of Materials Sciences and Engineering under contract DE-AC02-07CH11358. This research used resources of the Advanced Photon Source, a US DOE, Office of Science User Facility.   

Competing magnetic and superconducting order and the role of vortices in iron-based superconductors

We discuss recent neutron and muSR measurements of the magnetic and superconducting (SC) properties of Co-doped Ba-122 as a function of temperature and external magnetic field [1]. Below the T$_{\mathrm{c}}$, the magnetic and SC order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the SC state [1]. We perform a microscopic modeling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the SC state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments [1]. Lastly, we also study the low energy bound states in the vortex core of LiFeAs, where the quasiparticle states in the vortex core can provide useful information about the gap structure [2]. \\[4pt] [1] J. Larsen, G. Stieber, S. L. Holm, K. Lefmann, C. Niedermayer, T. Wolf, preprint 2014\\[0pt] [2] B. Mencia Uranga, B. M. Andersen, preprint 2014   

Itinerant ferromagnetism in the As $4p$ conduction band of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ identified by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ show that the ferromagnetism (FM) below $T_{\rm{C}}\approx100$~K arises in the As $4p$ conduction band. No XMCD signal is observed at the Mn x-ray absorption edges, however, a clear XMCD signal is found below $T_{\rm{C}}$ at the As $K$ edge which increases with decreasing temperature. The XMCD signal is absent with the beam directed parallel to the crystalline \emph{\textbf{c}} axis, indicating that the ordered orbital moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant FM is associated with the As $4p$ conduction band and that distinct local-moment antiferromagnetism and itinerant FM coexist at low temperature.\\ Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences, Division of Materials Sciences \& Engineering, under Contract No. DE-AC02-07CH11358. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.   

Collapsed tetragonal phase in SrCo$_{2}$As$_{2}$ under high pressure

SrCo$_{2}$As$_{2}$ possesses the same tetragonal ThCr$_{2}$Si$_{2}$ structure as the $A$Fe$_{2}$As$_{2}$ ($A \quad =$ Ca, Sr, Ba) family of high-temperature superconductors but does not manifest magnetic order or superconductivity down to a temperature of 1.8 K. Nevertheless, inelastic neutron scattering data show the presence of magnetic fluctuations peaked at a wavevector of (1/2 1/2 1), which corresponds to the stripe antiferromagnetic propagation vector found for $A$Fe$_{2}$As$_{2}$. Here, we present evidence from high-energy x-ray diffraction experiments which show that SrCo$_{2}$As$_{2}$ undergoes a transition to a collapsed-tetragonal phase characterized by a 10{\%} reduction of the $c$-lattice parameter for an applied pressure of 5 GPa at 7 K. This fascinating result opens another path for studying the role of magnetic ordering, spin fluctuations, and magnetoelastic coupling in the development of superconductivity in the Fe-pnictides and related materials. -- Work at Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC02-07CH11358. This research used resources of the Advanced Photon Source, a US DOE, Office of Science User Facility.