Renormalized behavior and proximity of BaCo₂As₂ to a magnetic quantum critical point

We report synthesis and single-crystal measurements of magnetic, transport, and thermal properties of single crystalline BaCo2 As2 as well as first-principles calculations of the electronic structure and magnetic behavior. These results show that BaCo2 As2 is a highly renormalized paramagnet in proximity to a quantum critical point, presumably of ferromagnetic character and that BaFeNiAs2 behaves similarly. These results are discussed in relation to the properties of Ba (Fe,Co) 2 As2 and Ba (Fe,Ni) 2 As2, which are superconducting for low Co and Ni concentrations.