In tetragonal ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature $T=5$ K [Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A$ $=$ Ca, Sr, Ba) parent compounds of high-$T_{\mathrm{c}}$ superconductors. This surprising discovery suggests that ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ may also be a good parent compound for high-$T_{\mathrm{c}}$ superconductivity. Here structural and thermal expansion, electrical resistivity $\rho$, angle-resolved photoemission spectroscopy (ARPES), heat capacity $C_{\mathrm{p}}$, magnetic susceptibility $\chi$, ${}^{75}$As NMR, and neutron diffraction measurements of ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The $c$-axis thermal expansion coefficient ${\alpha }_c$ is negative from 7 to 300 K, whereas ${\alpha }_a$ (the $a$-axis thermal expansion coefficient) is positive over this $T$ range. The $\rho \left(T\right)$ shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy $E_{\mathrm{F}}$. The band calculations exhibit an extremely sharp peak in the density of states $\mathcal{D}(E\approx E_{\mathrm{F}})$ arising from a flat $d_{x^2-y^2}$ band, where the $x$ and $y$ axes are along the $a$ and $b$ axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with $\chi (T\to 0)$ suggests the presence of strong ferromagnetic itinerant spin correlations, which on the basis of the Stoner criterion predicts that ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ should be an itinerant ferromagnet, in conflict with the magnetization data. The $\chi \left(T\right)$ does have a large magnitude, but also exhibits a broad maximum at $\approx 115$ K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ has a gapless quantum spin-liquid ground state.

Neutron and x-ray diffraction measurements are presented for powders and single crystals of ${\mathrm{CaCo}}_2{\mathrm{As}}_2$. The crystal structure is a collapsed-tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition ${\mathrm{CaCo}}_{1.86\left(2\right)}{\mathrm{As}}_2$. The thermal expansion coefficients for both the $a$ and $c$ axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature $T_{\mathrm{N}}=52$(1) K with the ordered moments directed along the tetragonal $c$ axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis.

The crystallographic and physical properties of polycrystalline and single-crystal samples of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ and K-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Co}}_2{\mathrm{As}}_2$ $(x=0.06,0.22)$ are investigated by x-ray and neutron powder diffraction, magnetic susceptibility $\chi$, magnetization, heat capacity $C_{\mathrm{p}},{}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR), and electrical resistivity $\rho$ measurements versus temperature $T$. The crystals were grown using both Sn flux and CoAs self-flux, where the Sn-grown crystals contain 1.6–2.0 mol% Sn. All samples crystallize in the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure (space group $I4/mmm)$. For ${\mathrm{BaCo}}_2{\mathrm{As}}_2$, powder neutron diffraction data show that the $c$-axis lattice parameter exhibits anomalous negative thermal expansion from 10 K to 300 K, whereas the $a$ axis lattice parameter and the unit cell volume show normal positive thermal expansion over this $T$ range. No transitions in ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ were found in this $T$ range from any of the measurements. Below 40–50 K, we find $\rho \propto T^2$, indicating a Fermi liquid ground state. A large density of states at the Fermi energy $\mathcal{D}\left(E_{\mathrm{F}}\right)\approx 18$ states/(eV f.u.) for both spin directions is found from low-$T$ $C_{\mathrm{p}}\left(T\right)$ measurements, whereas the band-structure calculations give $\mathcal{D}\left(E_{\mathrm{F}}\right)=8.23$ states/(eV f.u.). The enhancement of the former value above the latter is inferred to arise from electron-electron correlations and the electron-phonon interaction. The derived intrinsic $\chi \left(T\right)$ monotonically increases with decreasing $T$, with anisotropy ${\chi }_{ab}>{\chi }_c$. The ${}^{75}\text{As-NMR}$ shift data versus $T$ have the same $T$ dependence as the derived $\chi \left(T\right)$ data, demonstrating that the derived $\chi \left(T\right)$ data are intrinsic. The observed ${}^{75}\text{As}$ nuclear spin dynamics rule out the presence of Néel-type antiferromagnetic electronic spin fluctuations, but are consistent with the presence of ferromagnetic and/or stripe-type antiferromagnetic spin fluctuations. The crystals of ${\mathrm{Ba}}_{0.78}{\mathrm{K}}_{0.22}{\mathrm{Co}}_2{\mathrm{As}}_2$ were grown in Sn flux and show properties very similar to those of undoped ${\mathrm{BaCo}}_2{\mathrm{As}}_2$. On the other hand, the crystals from two batches of ${\mathrm{Ba}}_{0.94}{\mathrm{K}}_{0.06}{\mathrm{Co}}_2{\mathrm{As}}_2$ grown in CoAs self-flux show evidence of weak ferromagnetism at $T\lesssim 10$ K with small ordered moments at 1.8 K of $\approx 0.007$ and $0.03{\mu }_{\mathrm{B}}$ per formula unit, respectively.

The short-range order (SRO) and medium-range order of electrostatically levitated $\mathrm{Z}{\mathrm{r}}_{80}\mathrm{P}{\mathrm{t}}_{20}$ and $\mathrm{Z}{\mathrm{r}}_{77}\mathrm{R}{\mathrm{h}}_{23}$ liquids are presented based on a combination of high-energy x-ray diffraction and time-of-flight neutron diffraction studies. The atomic structures of the $\mathrm{Z}{\mathrm{r}}_{80}\mathrm{P}{\mathrm{t}}_{20}$ liquids were determined as a function of temperature from constrained reverse Monte Carlo simulations using x-ray and elastic neutron scattering measurements and two partial pair-distribution functions obtained from molecular dynamics (MD) simulations. Analysis of both the Faber-Ziman and Bhatia-Thornton partial structure factors shows that the $\mathrm{Z}{\mathrm{r}}_{80}\mathrm{P}{\mathrm{t}}_{20}$ and $\mathrm{Z}{\mathrm{r}}_{77}\mathrm{R}{\mathrm{h}}_{23}$ liquids have similar topological SRO. Interestingly, the chemical SRO appears to be much more strongly tied to the topological order in the $\mathrm{Z}{\mathrm{r}}_{77}\mathrm{R}{\mathrm{h}}_{23}$ liquid than in the $\mathrm{Z}{\mathrm{r}}_{80}\mathrm{P}{\mathrm{t}}_{20}$. These results demonstrate that the combination of experimental scattering measurements with MD results provides a powerful approach for obtaining details of chemical and topological ordering in metallic glasses and liquids.

Neutron diffraction studies of metallic liquids provide valuable information about inherent topological and chemical ordering on multiple length scales as well as insight into dynamical processes at the level of a few atoms. However, there exist very few facilities in the world that allow such studies to be made of reactive metallic liquids in a containerless environment, and these are designed for use at reactor-based neutron sources. We present an electrostatic levitation facility, NESL (for Neutron ElectroStatic Levitator), which takes advantage of the enhanced capabilities and increased neutron flux available at spallation neutron sources (SNSs). NESL enables high quality elastic and inelastic neutron scattering experiments to be made of reactive metallic and other liquids in the equilibrium and supercooled temperature regime. The apparatus is comprised of a high vacuum chamber, external and internal neutron collimation optics, and a sample exchange mechanism that allows up to 30 samples to be processed between chamber openings. Two heating lasers allow excellent sample temperature homogeneity, even for samples approaching 500 mg, and an automated temperature control system allows isothermal measurements to be conducted for times approaching 2 h in the liquid state, with variations in the average sample temperature of less than 0.5%. To demonstrate the capabilities of the facility for elastic scattering studies of liquids, a high quality total structure factor for Zr₆₄Ni₃₆ measured slightly above the liquidus temperature is presented from experiments conducted on the nanoscale-ordered materials diffractometer (NOMAD) beam line at the SNS after only 30 min of acquisition time for a small sample (∼100 mg).