We use neutron scattering to study spin excitations in single crystals of LiFe0.88Co0.12As, which is located near the boundary of the superconducting phase of LiFe1−xCoxAs and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe0.88Co0.12As with a combined density functional theory (DFT) and dynamical mean field theory (DMFT) calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the dxy orbitals, while high-energy spin excitations arise from the dyz and dxz orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe1−xCoxAs are consistent with electron-hole Fermi surface nesting condition for the dxy orbital, the reduced superconductivity in LiFe0.88Co0.12As suggests that Fermi surface nesting conditions for the dyz and dxz orbitals are also important for superconductivity in iron pnictides.

We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe2As2, which has a tetragonal-to-orthorhombic structural distortion at Ts and antiferromagnetic (AF) phase transition at TN with ordered moments along the orthorhombic aaxis below Ts ≈ TN ≈ 136 K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with Ma = M b = Mc, where Ma, M b , and Mc are spin excitations polarized along the a, b, and c-axis directions of the orthorhombic lattice, respectively. On cooling towards TN , significant spin excitation anisotropy with Ma > M b ≈ Mc develops below 3 meV with a diverging Ma at TN . The in-plane spin excitation anisotropy in the tetragonal phase of BaFe2As2 is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.

The heteroepitaxial growth of C60 on GeS(001) has been studied using low-energy electron diffraction, selected-area electron diffraction, high-resolution electron microscopy, x-ray diffraction, and x-ray and ultraviolet-photoelectron spectroscopy (UPS). The simultaneous observation of diffraction spots characteristic of the substrate and the C60(111)overlayer allows us to specify the geometry of the epitaxy. The shape of the intensity curves of the C 1s and Ge 3d photoemission lines strongly suggests a layer-bylayer-type growth, confirmed by the observation in the synchrotron x-ray diffraction spectrum of finitesize oscillations on the (111)Bragg reflection peak of a thin C60(111)film. From a theoretical simulation of the C ls and Ge 3d line-intensity curves, the mean free path of a C 1s and Ge 3d photoelectron in solid C60 is estimated to about 15. 4 and 17 A. , respectively. The plot of the film thickness versus deposition time shows evidence for a small difference in sticking coefBcient between the first monolayer and the upper ones. A detailed analysis of the C 1s line shapes for normal and grazing emission suggests the existence of inequivalent carbon sites at the interface. The first valence-band feature of the substrate presents a downward band bending of about 200 meV with increasing C60 coverage. From the shift of the cutoff in the UPS spectra we deduce a work function increase of about 100 meV upon monolayer adsorption. The characteristic spectral features of C60 observed in the UPS spectra for bulk fullerite are slightly broadened and shifted to lower binding energies at submonolayer coverages and show no direct evidence for significant hybridization, indicating that the C60-substrate interaction is mainly dominated by van der Waals bonding. All these observations can be explained by a positive effective dipole of about 8 X 10 ' C m induced on the C60 molecule upon adsorption onto the GeS substrate.

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Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe1−xNixAs, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature Ts in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie–Weiss temperature dependence and become suppressed below the superconducting transition temperature Tc, suggesting that they result from the large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry breaking above Ts, and attest to the presence of significant nematic fluctuations near optimal superconductivity.