YbPtBi is one of the heavy-fermion systems with largest Sommerfeld coefficient γ and is thus classified as a `super'-heavy fermion material. In this work, we resolve the long-debated question about the hierarchy of relevant energy scales, such as crystal-electric field (CEF) levels, Kondo and magnetic ordering temperature, in YbPtBi. Through measurements of the a.c. elastocaloric effect and generic symmetry arguments, we identify an \textit{elastic level splitting} that is uniquely associated with the symmetry-allowed splitting of a quartet CEF level. This quartet, which we identify to be the first excited state at Δ/kB≈1.6K above the doublet ground state at ambient pressure, is well below the Kondo temperature TK≈10K. Thus, our analysis provides strong support for models that predict that the heavy electron mass is a result of an enhanced degeneracy of the CEF ground state, i.e., a quasi-sextet in YbPtBi. At the same time, our study shows the potential of the a.c. elastocaloric effect to control and quantify strain-induced changes of the CEF schemes, opening a different route to disentangle the CEF energy scales from other relevant energy scales in correlated quantum materials.
@article{arxiv.2303.08770,
title = {Elastocaloric effect of the heavy-fermion system YbPtBi},
author = {Elena Gati and Burkhard Schmidt and Sergey L. Bud'ko and Andrew P. Mackenzie and Paul C. Canfield},
journal= {arXiv preprint arXiv:2303.08770},
year = {2023}
}