Existence of a flat band in highly correlated metal BaCo₂P₂

We report on the observation of a flat band situated at the Fermi level E F along with the structural, electrical transport, and magnetic properties of BaCo₂P₂ that crystallizes in the ThCr₂Si₂-type body-center tetragonal structure. This compound has the largest inter-layer pnictide (Pn) distance d Pn − Pn as well as the largest c / a ratio among all the known ACo 2 P n 2 (A = alkaline earth metal) compounds, where a and c are the tetragonal lattice parameters. Hence, the magnetic and electronic properties of this compound are expected to have a quasi-two-dimensional character. Despite the evidence of the presence of sizable magnetic interactions, magnetic susceptibility χ ( T ) of BaCo₂P₂ does not show magnetic ordering down to 1.8 K. The material shows good metallic conduction with a large residual resistivity ratio ρ 300 K / ρ 1.8 K ≈ 70 and a Fermi liquid behavior at low temperature. Kadowaki-Woods ratio R KW of BaCo₂P₂ suggests the presence of sizable electronic correlations within this system. Additionally, a large many-body enhancement of 2.3 of the experimental density of states D γ ( E F ) over the band-structure D band ( E F ) is inferred to arise from sizable electron-electron and/or electron-phonon interactions leading to a substantial deviation from the free-electron behavior.