Inelastic neutron scattering measurements on single crystals of superconducting ${\mathrm{BaFe}}_{1.84}{\mathrm{Co}}_{0.16}{\mathrm{As}}_2$ reveal a magnetic excitation located at wave vectors ($1/21/2L$) in tetragonal notation. On cooling below $T_C$, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) $k_B{T}_C$. This is in good agreement with the canonical value of 5 $k_B{T}_C$ 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 the results of a systematic investigation of the phase diagram of the iron-based superconductor Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ from $x$ $=$ 0 to $x$ $=$ 1.0 using high-resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of $\Delta x$ $\lesssim$ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ crystallizes in a tetragonal structure with the space group symmetry of $I$4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with the space group Fmmm and a striped antiferromagnet (AF) with the space group $F_{\mathrm{c}}mm$′$m$′. The transition temperature falls from a maximum of 139 K in the undoped compound to 0 K at $x$ $=$ 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of $x$ $=$ 0.130(3), and the superconducting transition temperature grows linearly with $x$ until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum $T_{\mathrm{c}}$ of 38 K at $x$ $=$ 0.4 that falls to 3 K at $x$ $=$ 1.0. We discuss reasons for the suppression of the spin density wave order and the electron–hole asymmetry in the phase diagram.

High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe₂As₂ parent compound modifies the low-energy spin excitations and their correlation with superconductivity (100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above T(c) can account for the superconducting condensation energy. These results suggest that high-T(c) superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in $\mathrm{Pr}{\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$, in which the ground state in the cubic crystal field potential of $T_h$ symmetry is a ${\Gamma }_1$ singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. $\mathrm{Pr}{\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair breaking and increase $T_c$.