Interlayer Coupling of a Two-Dimensional Kondo Lattice with a Ferromagnetic Surface in the Antiferromagnet CeCo₂P₂

Abstract

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo 2 P 2 . In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentumresolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Long-lived spin waves in a metallic antiferromagnet

“…Our electronic structure calculations (Fig. 1 d) as well as recent ARPES data 28 give no indication of flat bands in the bulk of CeCo 2 P 2 . The very low Sommerfeld coefficient found in specific heat measurements 28 further excludes the presence of heavy quasiparticles close to E F .…”

“…We found that the intermetallic system CeCo 2 P 2 28 is a rare exception from this general behaviour. In CeCo 2 P 2 , the Co moments have a stacked antiferromagnetic (AFM) order along the c axis with each layer being ferromagnetically (FM) ordered along c below T N ≈ 440 K 30 .…”

“…The extraordinarily high ordering temperature and the large Co moments of 0.94 μ B in comparison to similar systems make this material very attractive for practical applications. Most importantly, first-principle calculations for this material suggest a reduced density of states around E F 28 , 31 , while a recent high-throughput calculations even propose CeCo 2 P 2 to be an enforced semi-metal with topological properties 32 . Therefore, CeCo 2 P 2 seems to be a promising candidate to exhibit long-living THz magnons.…”

“…In contrast to most Ce intermetallics, there are no Ce 4 f contributions to the specific heat which can be interpreted as Ce behaving tetravalently 28 , 30 . The tetravalent behaviour is also evidenced by a reduced unit cell volume leading to a short distance between the phosphorus atoms d P−P and thus to a strongly three-dimensional character.…”

“…Both, the ordering temperature T C ≈ 135 K and the Co moments μ Co ≈ 0.45 μ B are significantly lower than in CeCo 2 P 2 33 – 35 . In addition, de Haas-van Alphen measurements 36 as well as a large Sommerfeld coefficient 28 indicate a large DOS at E F . We would therefore expect a much weaker magnon dispersion and a strong electron-magnon damping.…”