Session L39: Focus Session: Iron Based Superconductors: Magnetism and Structure

Pair-breaking in iron-pnictides

The puzzling features of the slopes of the upper critical field at the critical temperature, dHc2/dT ~ T$_c$, and of the specific heat jump Delta-C ~ T$_c^3$ of iron-pnictides are interpreted as caused by a strong pair-breaking. The low temperature behavior of the penetration depth will be discussed within the same framework.   

Magnetic Properties of Low-Energy Effective Model for Iron-Based Superconductor LaFeAsO

Recently discovered iron-based superconductors [1] have attracted much interest, not only in their high superconducting critical temperatures but also in variety of their magnetic properties. In particular, typical iron-based superconductor LaFeAsO has small magnetic ordered moment and its origin is still under hot debates. To clarify the origin of small magnetic ordered moment, we study the effective low-energy model for LaFeAsO derived from the downfolding scheme based on the first-principles calculations [2] by using variational Monte Carlo (VMC) method [3]. By combining with quantum-number projection method, our VMC method offers an efficient way of computation for strongly correlated electron systems [3]. We show that quantum fluctuations beyond the mean-field approximations reduce the magnetic moment dramatically, and this low-energy model can quantitatively reproduce the small ordered moment of LaFeAsO. Furthermore, we find that iron-based superconductors are located in largely orbital fluctuating region, which is sandwiched by the antiferromagnetic Mott insulators and weak correlated metals. [1] Y. Kamihara \textit{et al}.: J. Am. Chem. Soc. \textbf{130} (2008) 3296. [2] K. Nakamura \textit{et al}.; J. Phys. Soc. Jpn. \textbf{77} (2008) 093711. [3] D. Tahara and M. Imada: J. Phys. Soc. Jpn. \textbf{77} (2008) 093703;D. Tahara and M. Imada: J. Phys. Soc. Jpn. \textbf{77} (2008) 114701.   

Magnetic excitation in the Ni-overdoped BaFe$_{2}$As$_{2}$

We use neutron scattering to systematically investigate the evolution of spin excitations in Ni-doped BaFe2As2.~ Previous works on underdoped and optimally doped materials have found evidence for spin gap and neutron spin resonance.~ We use flux method to grow overdoped material approaching to the nonsuperconducting regime.~ We have carried out inelastic neutron scattering measurements on spin and lattice excitations of the overdoped BaFe$_{1.8}$Ni$_{0.2}$As$_{2}$ ($T_{c} \quad \sim $5 K) and the outcome of these investigations will be report.   

Doping Dependence of Resonant spin excitations in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

The observation of a resonant spin excitation at $\omega $ = 14meV in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ [Nature \textbf{456}, 930 (2008)] provided the first phase-sensitive evidence of extended $s_{\pm}$ symmetry in the iron arsenide superconductors. We will discuss subsequent measurements of the doping dependence of the dynamic magnetic susceptibility in Ba$_{1- x}$K$_{x}$Fe$_{2}$As$_{2}$ from $x$ = 0.2 to 0.9 using inelastic neutron scattering from polycrystalline samples. The resonance is observed below T$_{\mathrm{c}}$ at all values of $x$ centered on the $\Gamma$-M point at $Q \approx 1.2${\AA}$^{-1}$, but it progressively broadens and weakens in the overdoped regime. We will discuss the scaling of the resonance energy with T$_{\mathrm{c}}$ and compare the Q- dependence with theoretical estimates based on the evolution of the Fermi surface with hole doping.   

Localized and itinerant magnetic excitations in CaFe$_{2}$As$_{2}$

Elementary magnetic excitations in the striped phase of CaFe$_{2}$As$_{2}$ are studied with linear-response density-functional theory. Itinerant, Stoner-like elementary excitations are found to coexist with the usual antiferromagnetic spin waves observed in neutron experiments. When the Fe moment $M$ exceed 1.1$\mu_{B}$, spin waves are dominant; while as $M$ decreases below 1$\mu_{B}$ there is a rather sharp transition to itinerant behavior, with spin waves being damped by Stoner excitations of several types. An unusual low energy excitation was found, whose origin can be traced to excitations within a narrow Fe $d$ band of \emph{xy} and \emph{yz} symmetry. This band lies above the Fermi level when $M$ is 1.1$\mu_{B}$, and passes through it as $M$ decreases. The more conventional spin waves are associated with transitions from a different band of $d$ states, at lower energy. This establishes that independent localized and itinerant elementary excitations coexist in CaFe$_{2}$As$_{2}$, and are present even below the N{\'e}el temperature.   

Chalcogen-height dependent magnetic interactions in iron chalcogenide superconductors

We have investigated the magnetic properties of iron chalcogenide superconductors using the first-principles pseudopotential method. It is found that the stability of magnetic phases is very sensitive to the height of chalcogen atoms from the Fe plane: while FeTe with optimized Te height exhibits the double stripe magnetic ordering, the single stripe ordering becomes the ground state phase when Te height is lowered below a critical value by, e.g., Se doping. This behavior is understood by opposite Te-height dependences of the strength of superexchange interactions among nearest and next nearest neighbor iron atoms, and a longer-range magnetic interaction mediated by itinerant electrons. We also demonstrate a linear temperature dependence of the macroscopic magnetic susceptibility in the single stripe phase in contrast to a constant behavior in the double stripe phase. Compared with known experimental observations, our results suggest a comprehensive and unified view in the magnetism and the superconductivity in iron-based superconducting materials. This work was supported by NRF of Korea (Grant No. 2009- 0081204) and KISTI Supercomputing Center (Project No. KSC-2008-S02-0004).   

Local distortions with the substitution of Se for Te in FeSe$_{0.5}$Te$_{0.5}$

The isovalent substitution of Te for Se in the superconductor FeSe$_{1-x}$Te$_{x}$ increases T$_{c}$ in comparison to $\alpha$-FeSe but, on average, decreases the chalcogen-Fe bond angle. However, we find that the local symmetry is lower than the one presumed on average, \textit{P4/nmm}, because the Se and Te ions do not share the same site, and have two distinct textit{z}-coordinates exhibiting two types of bond angles and bond lengths with Fe. Simultaneously, ab-initio calculations based on spin density function theory yielded an optimized structure with distinct \textit{z} -coordinates for Se and Te, in agreement with the experiment.   

Effect of magnetism on the electron-phonon coupling in iron-based superconductors

Phonons have been excluded as possible mediators for superconductivity in Fe pnictides based on early non-magnetic DFT calculations. However, it has later been argued that the lattice properties of Fe pnictides are better described by AFM calculations, with large moments. In this work, we calculate the effect of antiferromagnetic order on the phonon dispersion and electron-phonon coupling constant of BaFe$_{2}$As$_{2}$, using Density Functional Perturbation Theory. We show that the inclusion of magnetism increases the average electron-phonon matrix element by 50 $\%$ with respect to non-magnetic calculations. This is still not enough to explain the observed $T_{c}$ in terms of e-ph coupling alone. We also show that the large magnetoelastic effects observed in FeBSC are not due to the electron-phonon coupling, but to the change in Fe-As bonding induced by magnetism.   

Neutron Diffraction Studies of Structural and Magnetic Phase Transitions in Ru Doped [1111] Fe Pnictides

We report neutron diffraction studies of pnictide [1111] compounds PrFe(1-x)Ru(x)AsO. The undoped parent compound PrFeAsO undergo transitions from a tetragonal to an orthorhombic crystal structure, as well as transitions to a state of magnetic order. Doping on different sites can induce various ground states, including superconductivity. Here we examine the effect of substitutionally doping Ru for Fe. In particular we report neutron diffraction measurements for PrFe (1-x)Ru(x)AsO with x=0, 0.33 and 0.75. Previous bulk and x-ray measurements (M. A. McGuire et al., J. Sol. St. Chem. 182, 2326 (2009)) have shown that doping with Ru moves the structural transition to lower temperature and can also suppress the magnetic order. The neutron diffraction confirms this and shows no sign of magnetic order down to 4K for x=0.33 or 0.75.   

Universal Magnetic and Structural Behaviors in the Iron Pnictides

In this talk, we will present analysis of the static, antiferromagnetic order in the parent and lightly doped iron pnictides suggesting that the magnetic order parameters in these systems collapse onto two universal curves. The relevant, empirical parameter controlling the detailed phase behavior of the magnetic phase transitions in these systems will be presented. Additionally, we will remark upon the seemingly general feature in the iron pnictides of an identical evolution (as a function of reduced temperature) of both the magnetic and structural phase transitions. The implications of these empirical observations for the universal magnetic and structural phase behavior in the iron pnictides will be discussed.   

Anomalous suppression of the orthorhombic distortion in superconducting Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

The interplay between superconductivity, magnetism and structure has become a major theme of research in the iron arsenide families of superconductors. Here we report high-resolution x-ray diffraction measurements that reveal an unusually strong response of the lattice to superconductivity in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals. Below $T_{C}$ the orthorhombic distortion of the lattice is suppressed and, for Co-doping near x $\sim $ 0.063, the orthorhombic structure evolves smoothly to a reentrant tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity. -- The work at the Ames Laboratory was supported by the US DOE, Office of Science, under contract No. DE-AC02-07CH11358.   

Local structure of cobalt-doped iron pnictide superconductors

Most superconducting iron-pnictides show no long-range AFM order, but strong spin excitations and core electron spectroscopy suggest that Fe ions may be locally spin polarized, with dynamic spin fluctuations. Further, measured magnetic moments are smaller than predicted. We report local structure studies of superconducting Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ (x=0.08,0.1). Negative thermal expansion was observed from Rietveld refinements with an inflection point at T$\sim $50K at x=0.08 (Tc = 22K). Pair density function analysis shows two distinct As-As bond lengths, indicating either distortion in the FeAs$_{4}$ tetrahedra, or two distinct environments for Fe. The presence of both the FeAs$_{4}$ tetrahedral distortion and the negative thermal expansion could indicate the increase in the local Fe magnetic moment below 50K. Further, this hypothesis would suggest that the maximum Tc of 55K may be bound by the change in the local magnetic state around 50K.   

Anisotropic 2D Spin Fluctuations in Superconducting Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$

We report inelastic neutron scattering from superconducting Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$. For energies up to 150 meV, magnetic excitations are found close to the 2D antiferromagnetic (AFM) wavevector $\mathbf{Q}_{AFM} = $ (0.5 0.5) in the tetragonal symmetry of the parent compound BaFe$_2$As$_2$. We observe an anisotropy of the in-plane spin fluctuations, allowing an estimate of the effective AFM spin exchange interactions: $J_2/J_1 = 1.0 (3)$ ($J_1$: NN; $J_2$: NNN). The 8 meV spin resonance displays a similar in-plane anisotropy. Above 100 meV, the anisotropy evolves into two separated maxima displaced transverse to $\mathbf{Q}_{AFM}$. While the low energy results can be interpreted in terms of competing exchange interactions, the more complex high energy/wavevector dependence requires an itinerant model.   

Proximity to Hidden Magnetic Order in Parent Compounds of Iron-Based High-Tc Superconductors

Motivated by the low ordered magnetic moment observed in the spin-density-wave (SDW) state of undoped iron-pnictide systems, we study the low-energy spectrum of the J1-J2 model over a square lattice of iron atoms with either two or four spin-1/2 orbitals per site in the presence of Hund's-Rule coupling. This is achieved within the linear spin-wave approximation and by exact diagonalization of 32 spin-1/2 moments. In the case of spin-1 iron atoms, both methods find hidden magnetic order with either ferromagnetic or Neel order per orbital at strong enough off-diagonal frustration. We successfully fit the spin-wave spectrum obtained recently from inelastic neutron scattering data taken on CaFe2As2 to the linear spin-wave spectrum at the quantum phase transition out of hidden order, into SDW order, with a Hund's-Rule exchange coupling energy of 68 meV. The fit notably accounts for the absence of a dip in the spectrum at the wave number that corresponds to Neel order. In the case of spin-2 iron atoms, exact diagonalization of the above J1-J2 model over the tilted 8-site lattice also obtains a quantum phase transition where true magnetic order disappears, but at much weaker values of the Hund's-Rule coupling energy.