In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility χ, 75 As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient αc is negative from 7 to 300 K, whereas αa (the a-axis thermal expansion coefficient) is positive over this T range. The ρ(T ) 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 EF. The band calculations exhibit an extremely sharp peak in the density of states D(E ≈ EF) 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 χ(T → 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T ) does have a large magnitude, but also exhibits a broad maximum at ≈ 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 SrCo2As2 has a gapless quantum spin-liquid ground state.

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo 2 As 2 . The crystal structure is a collapsed-tetragonal ThCr 2 Si 2 -type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo 1.86(2) 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 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.

Crystallography and Physical Properties of BaCo 2 As 2 , Ba 0.94 K 0.06 Co 2 As 2 and Ba 0.78 K 0.22 Co 2 As 2