A theoretically designed van der Waals ferromagnet Fe ₄ GeTe ₂ is synthesized and shows the nearly room temperature ferromagnetism.

We report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi2, based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)+ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of $\sim${}8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions.

Low-energy electronic structures in $A$MnBi${}_2$ ($A$=alkaline earths) are investigated using a first-principles calculation and a tight binding method. An anisotropic Dirac dispersion is induced by the checkerboard arrangement of $A$ atoms above and below the Bi square net in $A$MnBi${}_2$. SrMnBi${}_2$ and CaMnBi${}_2$ have a different kind of Dirac dispersion due to the different stacking of nearby $A$ layers, where each Sr (Ca) of one side appears at the coincident (staggered) $xy$ position of the same element at the other side. Using the tight binding analysis, we reveal the chirality of the anisotropic Dirac electrons as well as the sizable spin-orbit coupling effect in the Bi square net. We suggest that the Bi square net provides a platform for the interplay between anisotropic Dirac electrons and the neighboring environment such as magnetism and structural changes.

Significance: The temperature ( T )-dependent evolution of the Kondo lattice electronic structure is a long-standing topic of theoretical and experimental investigation, still lacking a truly microscopic theory that agrees with a full experimental characterization. Here multiple characteristic T scales of the interaction of localized f moments with conduction electrons in the Kondo lattice CeCo I n 5 are identified and investigated using angle-resolved photoemission measurements that substantiate dynamical mean-field theory which newly includes the full realism of crystalline electric-field (CEF) f splittings. Thereby errors in the itinerant versus localized f -state classification from standard density functional theory are corrected, microscopic insight into the broad T -range crossover of f -hybridization effects is gained, and a prediction of CEF degeneracy crossover below the lattice coherence T is made.