Fermi surface evolution and crystal-field excitations in heavy-fermion compounds probed by time-domain terahertz spectroscopy*

In heavy-fermion (HF) compounds, the existence of heavy quasiparticles (QPs) is signaled by an enlarged Fermi volume. The energy scale for heavy QP formation is believed to be the Kondo lattice temperature. Hence, the origin of a large Fermi volume observed at temperatures much higher than the Kondo lattice temperature has recently been a controversial issue. We measure the quasiparticle weight in the HF compound CeCu_6-xAu_x (x=0, 0.1) by time-resolved THz spectroscopy for temperatures from 2 K up to 300 K. This method distinguishes contributions from the heavy Kondo band and from the crystal-electric-field satellite bands by different THz response delay times [1,2]. We find that the formation of heavy bands is controlled by an exponentially enhanced, high-energy Kondo scale once the crystal-electric-field states become thermally occupied. We corroborate these observations by temperature-dependent, high-resolution dynamical mean-field calculations for the multi-orbital Anderson lattice model and discuss the relevance for quantum critical scenarios.

[1] C. Wetli, S. Pal, J. Kroha et al., Nature Phys., DOI: 10.1038/s41567-018-0228-3 (2018).
[2] S. Pal, C. Wetli, F. Zamani, O. Stockert, H. v. Löhneysen, M. Fiebig, J. Kroha, arXiv:1810.07412 (2018).