Crystallographic, electronic, thermal, and magnetic properties of single-crystal SrCo²As²

In tetragonal SrCo₂As₂ single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T=5 K that are the same as in the isostructural AFe₂As₂ (A = Ca, Sr, Ba) parent compounds of high-T_c superconductors. This surprising discovery suggests that SrCo₂As₂ may also be a good parent compound for high-T_c superconductivity. Here structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity C_p, magnetic susceptibility χ, 75As NMR, and neutron diffraction measurements of SrCo₂As ₂ crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient α_c is negative from 7 to 300 K, whereas α_a (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 E_F. The band calculations exhibit an extremely sharp peak in the density of states D(E≈E_F) arising from a flat d_x2-y2 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 SrCo ₂As₂ has a gapless quantum spin-liquid ground state.