CeCo₂P₂: An antiferromagnetic topological heavy-fermion system with a PT -protected Kondo effect and nodal-line excitations

Based on high-throughput screening and experimental data, we find that CeCo₂P₂ is unique in heavy-fermion materials: It has a Kondo effect at a high temperature which is nonetheless below a Co antiferromagnetic ordering temperature. This begs the question: How is the Kondo singlet formed? All other magnetic Kondo materials do not first form magnetism on the atoms whose electrons are supposed to screen the local moments. We theoretically explain these observations and show the multifaceted uniqueness of CeCo₂P₂: a playground for Kondo, magnetism, flat band, and topological physics. At high temperatures, the itinerant Co c electrons of the system form nonatomic bands with a narrow bandwidth, leading to a high antiferromagnetic transition temperature. We show that the quantum geometry of the bands promotes in-plane ferromagnetism, while the weak dispersion along the z direction facilitates out-of-plane antiferromagnetism. At low temperatures, we uncover a phase that manifests the coexistence of Co antiferromagnetism and the Kondo effect, linked to the PT -protected Kramers’ doublets and the filling-enforced metallic nature of c electrons in the antiferromagnetic phase. Subsequently, the emergence of the Kondo effect, in cooperation with glide-mirror-z symmetry, creates nodal-line excitation near the Fermi energy. Our results emphasize the importance of lattice symmetry and quantum geometry, Kondo physics, and magnetism in the understanding of the correlation physics of this unique compound. We also test our theory on the structurally similar compound LaCo₂P₂ and show how we are able to understand its vastly different phase diagram.