Temperature dependence of the resonance and low energy spin excitations in superconducting FeTe0.6_{0.6}Se0.4_{0.4}

We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe$_{0.6}$Se$_{0.4}$ ($T_c=14$ K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi surfaces nesting wave vector $Q=(0.5,0.5)$, $\chi^{\prime\prime}(Q,\omega)$, has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing $\hbar\omega$ for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above $T_c$, the energy of the mode is weakly temperature dependent and vanishes concurrently above $T_c$. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe$_{2-x}$(Co,Ni)$_x$As$_2$, the mode in FeTe$_{0.6}$Se$_{0.4}$ is not directly associated with the superconducting electronic gap.Comment: 7 pages, 6 figure

Three-dimensional Resonance in superconducting BaFe1.9_{1.9}Ni0.1_{0.1}As2_2

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ above and below its superconducting transition temperature $T_c=20$ K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector $(1,0,0)$, the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at $(1,0,0)$ and $(1,0,1)$. If the resonance energy is directly associated with the superconducting gap energy $\Delta$, then $\Delta$ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.Comment: 5 pages, 4 figure

Anisotropic Neutron Spin Resonance in Superconducting BaFe1.9_{1.9}Ni0.1_{0.1}As2_2

We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm1$ components of the triplet are involved

Short-range cluster spin glass near optimal superconductivity in BaFe2−x_{2-x}Nix_{x}As2_{2}

High-temperature superconductivity in iron pnictides occurs when electrons are doped into their antiferromagnetic (AF) parent compounds. In addition to inducing superconductivity, electron-doping also changes the static commensurate AF order in the undoped parent compounds into short-range incommensurate AF order near optimal superconductivity. Here we use neutron scattering to demonstrate that the incommensurate AF order in BaFe$_{2-x}$Ni$_{x}$As$_{2}$ is not a spin-density-wave arising from the itinerant electrons in nested Fermi surfaces, but consistent with a cluster spin glass in the matrix of the superconducting phase. Therefore, optimal superconductivity in iron pnictides coexists and competes with a mesoscopically separated cluster spin glass phase, much different from the homogeneous coexisting AF and superconducting phases in the underdoped regime.Comment: 4 figure

Spin Waves in Detwinned BaFe2_2As2_2

Understanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as $A$Fe$_2$As$_2$ ($A=$ Ba, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone (BZ), measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of $\sim$100\% detwinned BaFe$_2$As$_2$. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model with important contributions from electronic correlations.Comment: 6 pages, 4 figures, with supplemental materia

Frustrated magnetic interactions and quenched spin fluctuations in CrAs

The discovery of pressure-induced superconductivity in helimagnets (CrAs, MnP) has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity. However, the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear. Here, we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure. Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV, which can be effectively described by the Heisenberg model with nearest neighbor exchange interactions. Most surprisingly, the spin excitations are largely quenched above the Neel temperature, in contrast to cuprates and iron pnictides where the spectral weight is mostly preserved in the paramagnetic state. Our results suggest that the helimagnetic order is driven by strongly frustrated exchange interactions, and that CrAs is at the verge of itinerant and correlation-induced localized states, which is therefore highly pressure-tunable and favorable for superconductivity.Comment: 6 pages, 4 figure