  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 rho, angle-resolved photoemission spectroscopy (ARPES), heat
capacity Cp, magnetic susceptibility chi, 75As 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 alpha_c is negative from 7 to 300 K, whereas
alpha_a is positive over this T range. The rho(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 E_F. The band
calculations exhibit an extremely sharp peak in the density of states D(E_F)
arising from a flat d_{x^2 - y^2} band. A comparison of the Sommerfeld
coefficient of the electronic specific heat with chi(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 chi(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 propose that metallic SrCo2As2 has a
gapless quantum spin-liquid ground state.
