Recently discovered alongside its sister compounds ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, demonstrating bulk superconductivity in single crystals with a $T_c=2.5\mathrm{K}$. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a ${\mathbb{Z}}_2$ topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level ($E_F$), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wave vector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments, combined with ab initio calculations, show that it is the wave vector dependence of the electron-phonon coupling that drives the CDW formation in $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$ and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$ and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.

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.

NeXus is an effort by an international group of scientists to define a common data exchange and archival format for neutron, X-ray and muon experiments. NeXus is built on top of the scientific data format HDF5 and adds domain-specific rules for organizing data within HDF5 files, in addition to a dictionary of well defined domain-specific field names. The NeXus data format has two purposes. First, it defines a format that can serve as a container for all relevant data associated with a beamline. This is a very important use case. Second, it defines standards in the form of application definitions for the exchange of data between applications. NeXus provides structures for raw experimental data as well as for processed data.