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Weyl fermions that emerge at band crossings in momentum space caused by the spin–orbit interaction act as magnetic monopoles of the Berry curvature and contribute to a variety of novel transport phenomena such as anomalous Hall effect and magnetoresistance. However, their roles in other physical properties remain mostly unexplored. Here, we provide evidence by neutron Brillouin scattering that the spin dynamics of the metallic ferromagnet SrRuO3 in the very low energy range of milli-electron volts is closely relevant to Weyl fermions near Fermi energy. Although the observed spin wave dispersion is well described by the quadratic momentum dependence, the temperature dependence of the spin wave gap shows a nonmonotonous behaviour, which can be related to that of the anomalous Hall conductivity. This shows that the spin dynamics directly reflects the crucial role of Weyl fermions in the metallic ferromagnet.

The crystal structure, superconducting properties, and electronic structure of a novel superconducting 122-type antimonide, BaPt 2 Sb 2 , have been investigated by measurements of powder X-ray diffraction patterns, electrical resistivity, ac magnetic susceptibility, specific heat as well as ab-initio calculations. This material crystallizes in a new-type of monoclinic variant of the CaBe 2 Ge 2 -type structure, in which Pt 2 Sb 2 layers consisting of PtSb 4 tetrahedra and Sb 2 Pt 2 layers consisting of SbPt 4 tetrahedra are stacked alternatively and Ba atoms are located between the layers.Measurements of electrical resistivity, ac magnetic susceptibility and specific heat revealed that BaPt 2 Sb 2 is a superconducting material with a T C of 1.8 K. The electronic heat capacity coefficient γ n and Debye temperature θ D were 8.6(2) mJ/mol K 2 and 146(4) K, where the figures in parentheses represent the standard deviation. The upper critical field µ 0 H C2 (0) and the Ginzburg-Landau coherent length ξ(0) were determined to be 0.27 T and 35 nm. Calculations showed that it has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs, leading to anisotropic transport properties. The d-states of the Pt atoms in the Pt 2 Sb 2 layers mainly contribute to N(E F ). A comparison between experimental and calculated results indicates that BaPt 2 Sb 2 is a superconducting material with moderate coupling.

The spin dynamics in single crystal, electron-doped Ba͑Fe 1−x Co x ͒ 2 As 2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound ͑x = 0.06͒ that share a remarkable similarity with those in the paramagnetic state of the parent compound ͑x =0͒. In the overdoped superconducting compound ͑x = 0.14͒, magnetic excitations show a gaplike behavior, possibly related to a topological change in the hole Fermi surface ͑Lifshitz transition͒ while the imaginary part of the spin susceptibility Љ prominently resembles that of the overdoped cuprates. For the heavily overdoped, nonsuperconducting compound ͑x = 0.24͒ the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.

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