A close look at correlated electrons in heavy-fermion metal through ARPES

Showing astonishing properties like magnetism, superconductivity, Kondo and heavy-fermion (HF) behavior, rare-earth intermetallic compounds have been at the forefront of modern solid state physics for many years. Most of these properties are related to a delicate interplay between the partially filled 4f-shell and conduction electrons. Studying HF system YbRh₂Si₂ we made the observation of crystal-electric field (CEF) splittings of a 4f state by means of k-resolved photoemission. Their interaction with extended valence bands can force the localized CEF-split 4f states to become dispersive and induce Fermi level crossings in specific parts of the k-space. This can change the ground-state symmetry as well as the occupancy, number, energy separation, energy order and degeneration of the CEF-split magnetic 4f states k-dependently, i.e. very different from the widely believed scenario based on non-interacting atomic-like 4f orbitals. We got direct access to the Fermi surface of this system and: (i) detected its strong f-character, (ii) disentangled its topology and features reflecting f-d coupling at the surface and bulk of the material, (iii) explored evolution of the iso-energy surfaces closely below the Fermi energy that indeed change dramatically at the meV range.