Existence of a flat band in highly correlated metal BaCo₂P₂

We report on the observation of a flat band situated at the Fermi levelEFalong with the structural, electrical transport, and magnetic properties of BaCo₂P₂that crystallizes in the ThCr₂Si₂-type body-center tetragonal structure. This compound has the largest inter-layer pnictide (Pn) distancedPn-Pnas well as the largestc/aratio among all the knownACo2Pn2(A= alkaline earth metal) compounds, whereaandcare 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 susceptibilityχ(T)of BaCo₂P₂does not show magnetic ordering down to 1.8 K. The material shows good metallic conduction with a large residual resistivity ratioρ300K/ρ1.8K≈70and a Fermi liquid behavior at low temperature. Kadowaki-Woods ratioRKWof BaCo₂P₂suggests the presence of sizable electronic correlations within this system. Additionally, a large many-body enhancement of 2.3 of the experimental density of statesDγ(EF)over the band-structureDband(EF)is inferred to arise from sizable electron-electron and/or electron-phonon interactions leading to a substantial deviation from the free-electron behavior.