Fermi surface collapse, gap, coherence: an ARPES study of the hybridization in Ce$_2$RhIn$_8$

The crossover of localized magnetic moments at high temperatures into itinerant states of heavy mass at low temperatures in some metals containing \emph{f} electrons, first addressed by Kondo, is a fundamental problem in condensed matter physics involving a temperature dependent hybridization between \emph{f} levels and conduction electrons (\emph{ce}). Here we present an extensive angular resolved photoemission spectroscopy study performed in Ce$_2$RhIn$_8$\ as a function of temperature. Our experiments reveal the presence of three energy scales, differing by an order of magnitude from each other: first at room temperature, where the \emph{f} levels are localized, we observe a small Fermi surface (FS), which undergoes dramatic topological changes toward a large FS near the minimum in the resistivity around 200K; the opening of a spectral gap below 30K without a change in topology of the Fermi surface; and finally, below 5K, composite quasiparticles form, as the resistivity suddenly decreases. The expectation that hybridization, spectral gap, and \emph{f} electron coherence go hand in hand should be expanded to include the possibility of separate energy scales for each of these phenomena.