The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for
x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.
We have performed elastic and inelastic neutron experiments on single crystal samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz=pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements in
dicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 +/- 0.03 muB/Cu. This value is significantly smaller than the single ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two dimensional Heisenberg model with a nearest neighbor magnetic exchange constant J2d = 0.934 +/-0.0025 meV. The inter-layer interaction Jperp in this compound is less than 1.5% of J2d. The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by ~10.3 +/- 1.4 %. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations which include corrections for quantum fluctuations to linear spin wave theory.
The layered 5d transition metal oxide Sr2IrO4 has been shown to host a novel Jeff=1/2 Mott spin orbit insulating state with antiferromagnetic ordering, leading to comparisons with the layered cuprates. Here we study the effect of substituting Mn for
Ir in single crystals of Sr2Ir0.9Mn0.1O4 through an investigation involving bulk measurements and resonant x-ray and neutron scattering. We observe a new long range magnetic structure emerge upon doping through a reordering of the spins from the basal plane to the c-axis with a reduced ordering temperature compared to Sr2IrO4. The strong enhancement of the magnetic x-ray scattering intensity at the L3 edge relative to the L2 edge indicates that the Jeff=1/2 state is robust and capable of hosting a variety of ground states.
Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this
wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC. This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.
We report inelastic and elastic neutron scattering, magnetic susceptibility, and heat capacity measurements of polycrystalline sodium ruthenate (Na$_3$RuO$_4$). Previous work suggests this material consists of isolated tetramers of $S=3/2$ Ru$^{5+}$
ions in a so-called lozenge configuration. Using a Heisenberg antiferromagnet Hamiltonian, we analytically determine the energy eigenstates for general spin $S$. From this model, the neutron scattering cross-sections for excitations associated with spin-3/2 spin-tetramer configurations is determined. Comparison of magnetic susceptibility and inelastic neutron scattering results shows that the proposed lozenge model is not distinctly supported, but provides evidence that the system may be better described as a pair of non-interacting inequivalent dimers, textit{i.e} double dimers. However, the existence of long-range magnetic order below $T_c approx 28$ K immediately questions such a description. Although no evidence of the lozenge model is observed, future studies on single crystals may further clarify the appropriate magnetic Hamiltonian.
