Fermi surface collapse and energy scales in Ce2RhIn8

TL;DR

This study uses angle-resolved photoemission spectroscopy to reveal that Fermi surface changes in Ce2RhIn8 occur at much higher temperatures than previously thought, driven by electronic fluctuations related to crystal field levels, challenging conventional Kondo lattice models.

Contribution

It demonstrates that Fermi surface evolution in Ce2RhIn8 occurs at higher temperatures than hybridization and coherence, highlighting the role of crystal field excitations in Kondo physics.

Findings

Fermi surface changes occur at temperatures an order of magnitude higher than hybridization gap opening.

Fermi surface evolution is driven by crystal field level accessibility.

Conventional Kondo lattice models need revision to incorporate these energy scales.

Abstract

In some metals containing a sub-lattice of rare earth or actinide ions, free local $f$ spins at high temperatures dissolve into the sea of quantum conduction electrons at low temperatures, where they become mobile excitations. Once mobile, the spins acquire charge, forming electrons of heavy mass, known as heavy fermions. In turn, the incorporation of heavy charges into the conduction sea leads to an increase in the volume of the Fermi surface. This process, called Kondo scattering, is accompanied by a dramatic, temperature dependent transformation of the electronic interactions and masses. Since the Kondo phenomena is controlled by quantum fluctuations, here we ask, at which point does the Fermi surface change character? A priori, the answer is not clear, since near its onset, the Kondo effect cannot be described as a simple hybridization of electronic eigenstates. Conventional descriptions of this Kondo scattering process consider that hybridization, Fermi volume change, and $f$-electron mobility occur simultaneously. However, using angle resolved photoemission spectroscopy to measure the evolution of excitations, we find that the changes of the Fermi surface emerge at temperatures an order of magnitude higher than the opening of the hybridization gap, and two orders of magnitude higher than the onset of the coherent character of the $f$-electrons. We suggest that the large changes in Fermi volume, driven by electronic fluctuations, occur at temperatures where the various $\Gamma_x \to \Gamma_y$ crystal field-split $f$ levels become accessible to conduction states of the corresponding symmetries. The separation of these energy scales significantly modifies the conventional description of the Kondo lattice effect, which still lacks a full theoretical description.