Session L22: Focus Session: Fe-based Superconductors: -- Inelastic Neutron Scattering and Magnetic Excitations

Interstitial Iron Effects on Magnetic Excitations in Parent Phases Fe$_{1+x}$Te from Polarized and Inelastic Neutron Scattering

One of the simplest systems of the iron-based superconducting family, Fe$_{1+x}Ch$ (where $Ch$ = S, Se, or Te) presents ample opportunity to study the relationship between antiferromagnetism and superconductivity. Several studies have demonstrated how the makeup of the $Ch$ anions changes the electronic properties drastically, but the effect of excess interstitial iron, the $x$ in Fe$_{1+x}Ch$, is not as well understood. Our previous diffraction experiments on samples varying $x$ from 4 \% to 16 \% demonstrated how the magnetic ordering changes from collinear antiferromagnetic to helical incommensurate via a spin-density wave state at the special composition of $x \approx 12\%$. We present inelastic neutron scattering measurements of the phases Fe$_{1+x}$Te for two amounts of interstitial iron in the lattice, 5\% and 14 \%. We have combined data from cold neutron triple-axis, thermal neutron triple-axis, and spallation source time-of-flight to provide a full picture of the magnetic excitations in Fe$_{1+x}$Te for $x=14 \%$ from 0.5 meV to 150 meV. In addition, we present polarized inelastic studies on this particular composition to investigate the nature of the spin waves, $i.e.$ longitudinal $vs.$ transverse. The results are compared with those found in the phase with low amounts of interstitial iron ($\approx$ 5 \%), in order to understand the nature of the exchange interactions in this important parent compound.   

Temperature-enhanced nearly critical magnetic scattering in nearly superconducting FeTe$_{0.87}$S$_{0.13}$

Recent neutron scattering measurements [1] have revealed an unusual temperature-induced enhancement of dynamical magnetism in iron telluride, FeTe, the parent material of the chalcogenide family of iron-based superconductors. Here we report a study of magnetic excitations in the sulfur-doped FeTe$_{0.87}$S$_{0.13}$, where bulk measurements show presence of the filamentary superconductivity [2]. Our neutron measurements probe the bulk of the material, which is nearly critical (filamentary superconductivity emerges in a small volume fraction of the sample, which is supercritical). We observe a peculiar pattern of low-energy magnetic scattering, characteristic of critical magnetic fluctuations, and find that it is also anomalously enhanced with the increasing temperature up to $\sim 100$ K. \\[4pt] [1] I. A. Zaliznyak, \emph{et. al.}, Z. J. Xu, J. M. Tranquada, G. D. Gu, A. M. Tsvelik, M. B. Stone, Phys. Rev. Lett., in press (2011).\\[0pt] [2] Rongwei Hu, E. S. Bozin, J. B. Warren, C. Petrovic, Phys. Rev. B {\bf 80}, 214514 (2009).   

Friedel-like Oscillations from Interstitial Iron in Superconducting Fe$_{1+y}$Te$_{0.62}$Se$_{0.38}$

Using polarized and unpolarized neutron scattering we show that interstitial Fe in superconducting Fe$_{1+y}$Te$_{1-x}$Se$_x$ induces a magnetic Friedel-like oscillation that diffracts at ${\bf Q}_{\perp}=(\frac{1}{2}0)$ and involves $>$50 neighboring Fe sites. The interstitial $>2$~$\mu_B$ moment is surrounded by compensating ferromagnetic four spin clusters that may seed double stripe ordering in Fe$_{1+y}$Te. A semi-metallic 5-band model with $(\frac{1}{2}\frac{1}{2})$ Fermi surface nesting and four fold symmetric super-exchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.   

Copper-tuned magnetic order and excitations in iron-based superconductors Fe1+yTe1-xSex

We report neutron scattering results on the Cu-substitution effects in the iron-based superconductors, Fe$_{1+y}$Te$_{1-x}$Se$_x$. In the parent compound, it is found that Cu drives the low-temperature magnetic ground state from long-range commensurate antiferromagnetic order in Fe$_{1.06}$TeCu$_{0.04}$ to short-range incommensurate order in FeTeCu$_{0.1}$. In the former sample, the structural and magnetic ordering temperature is 40~K; in FeTeCu$_{0.1}$, the structural phase transition is not obvious and a transition to the spin-glass state is found at 22~K. Cu suppresses superconductivity in FeTe$_{0.5}$Se$_{0.5}$---$T_c$ is reduced to 7~K with a 2\% Cu doping, and no superconductivity is found in the 10\% Cu-doped sample. In the meantime, the intensity and energy of the resonance mode are suppressed in the 2\% Cu-doped sample, while there is no resonance in the non-superconducting sample. Besides, the low-temperature magnetic excitation spectra are distinct for these two samples, with the superconducting one having an ``hour-glass" shape and the other one having a ``waterfall" shape. Our results provide further insights on the interplay between magnetism and superconductivity in the iron-based superconductors.   

Neutron Scattering Study of Magnetic Excitation Spectrum on Fe1-x(Ni/Cu)xTe0.5Se0.5

We have performed a series of neutron scattering and magnetization measurements on Fe1-x(Ni/Cu)xTe0.5Se0.5 with different Ni/Cu compositions to study the interplay between magnetism and superconductivity. Substituting 2{\%} and 4{\%} of Ni for Fe reduces Tc from 15 K to 12 K and 8 K, while 10{\%} of Cu results in lost of superconductivity. Spin resonance with lower energy are found in all superconducting samples. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T$>>$Tc to a distinctly different U-shaped dispersion at low temperature in superconducting samples. This spectral reconstruction is apparent for temperature up to 3Tc. On the other hand, no static order around (0.5,0,0.5) was found in any of these samples.   

Magnetic excitations in Fe1.01Te0.7Se0.3

Recently, there have been intense studies on the magnetism in FeTe1-xSex, which resulted in contradicting observations making the nature of its magnetism controversial. While the Fermi surface nesting picture can well predict the position of magnetic resonance in superconducting region, it clearly fails to predict the magnetic ordering wave vector in the parent compound. To investigate the magnetism in this iron chalcogenide series, we synthesized Fe1.01Te0.7Se0.3. At this doping, it resides very close to the superconducting doping region, but is in spin glass phase. By using the Time-of-Flight neutron scattering, we obtained magnetic dispersions in this material at energies up to 257meV. We find characteristic lines of diffuse scattering in Q-space, which provide the evidence for highly frustrated interactions. These lines of degeneracy persist up to about 10meV, and start to disperse above it. Based on the shape of this quasi-degenerate manifold in Q-space, we propose a description of spin excitations using the J1-J2-J3 Heisenberg model on square lattice model near the limit of maximum frustration with weak extrinsic perturbation.   

Temperature Dependence of Magnetic Correlations in Fe-based Parents

Key results from time of flight (TOF) neutron scattering measurements of BaFe$_{2}$As$_{2}$ and Fe$_{2}$Te$_{2}$ will be reviewed. These measurements were carried out over temperatures directly above and below T$_{N}$ as well as base temperature measurements deep inside the order state and intermediate temperatures between T$_{N}$ and room temperature where loosely correlated spin fluctuations persist. The large (TOF) data sets provide enough coverage of (Q, $\omega$) space to integrate out the correlated fluctuating moment, dispersion softening, and correlation length of the excitations as a function of temperature. Taken together, these results provide a systematic accounting of how the magnetic excitations in these parents evolve as they transition from long range ordered magnets to paramagnets.   

Effect of the in-plane magnetic field on the neutron spin resonance in optimally doped FeSe0.4Te0.6 and BaFe1.9Ni0.1As2 superconductors

We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe0.4Te0.6 (Tc = 14 K) and BaFe1.9Ni0.1As2 (Tc = 20 K). While the magnetic field up to 14.5 T does not change the energy of the resonance, it partially suppresses Tc and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore it is still unclear if the resonance is the long-sought singlet-triplet excitation directly coupled to the superconducting electron Cooper pairs.   

Itinerant Spin Excitations in SrFe$_2$As$_2$ Measured by Inelastic Neutron Scattering

We have performed inelastic neutron scattering (INS) measurements of the magnetic excitations in SrFe$_2$As$_2$, an iron pnictide parent compound [1]. The data extend up to energies of $\sim 260$\,meV. We find that the spectrum calculated from a local-moment J$_1$-J$_2$ model fails to describe our data in several key respects. In particular, the data can only be fitted by using different exchange parameters at high and low energies, for which there is no obvious physical justification. In both cases the nearest-neighbor exchange parameters, J$_{1\rm{a}}$ and J$_{1\rm{b}}$, are very different. Also, on warming above the magnetic/structural ordering transition, one would expect J$_{1\rm{a}}$ $\rightarrow$ J$_{1\rm{b}}$ and hence a soft mode at $\mathbf{Q}=(0.5,0.5)$, due to frustration in the local-moment model. However we find that the spectrum is largely unaltered. We find that the qualitative features of the INS spectra that cannot be described by the J$_1$-J$_2$ model are readily explained by calculations from a 5-band itinerant mean-field model. This also implies that it is not necessary to invoke additional broken symmetry, such as electronic nematic or orbital order, to explain the lack of a soft mode. [1] R. A. Ewings et al, Phys. Rev. B \textbf{83}, 214519 (2011)   

Magnetic resonance from the interplay of frustration and superconductivity

Motivated by iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wave vector {\bf Q}$_1$ = $(\pi,0)$ and $s^{+?}$ superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector {\bf Q}$_2$ = $(0,\pi)$, where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near {\bf Q}$_1$, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.   

Spin excitations in isovalently doped Ba(Fe1-xRux)2As2

Iron based superconductors exhibit remarkably rich phase diagrams: superconductivity can be obtained through carrier doping, application of external pressure, or isovalent doping. Here we present the elastic and inelastic neutron scattering measurements on the magnetic correlations in isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$. For the underdoped sample the static order moment are partially suppressed and the low energy spin correlation length increases upon entering the superconducting state. The spin excitation intensity is reduced with increasing Ru doping toward the over doped regime. We also studied the transfer of the magnetic spectral weight across $T_c$ in this system.   

Polarized Neutron Scattering Study of Over-doped BaFe$_{2-x }$Ni$_{x}$As$_{2}$

Spin excitations in over-doped superconducting BaFe$_{2-x }$Ni$_{x}$As$_{2}$ (x=0.15) were investigated using polarized neutron scattering. In a previous experiment on the optimally (x=0.1) doped sample, polarization analysis reveals that the magnetic response is highly anisotropic, at energies as high as 10 meV. Since this optimally doped sample is close to the boundary where antiferromagnetism disappears, there are still relatively strong antiferromagnetic correlations presented in the normal state. We will report our results on the over-doped sample, where the correlation is considerably weakened but still superconducting.   

Neutron scattering studies on LiFeAs and BaFe$_{2}$As$_{2}$

The anisotropy of the magnetic excitations in BaFe$_{2}$As$_{2}$ was studied by polarization analysis. We find the in-plane polarized transverse magnon to lie at higher energy than the out-of-plane polarized one indicating very strong in-plane single-ion anisotropy. Superconducting LiFeAs exhibits the suppression of local susceptibility expected for spin-singlet pairing. Inelastic correlations appear in LiFeAs at the incommensurate wave vectors, they respond to the opening of the superconducting gap by a transfer of spectral weight.