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 elastic level splitting that is unambiguously associated with the symmetry-allowed splitting of a quartet CEF level. This quartet, which we identify to be the first excited state at Δ/k_B ≈ 1.6 K above the doublet ground state at ambient pressure, is well below the proposed Kondo temperature T_K ≈ 10 K. Consequently, this analysis of the energy scheme can provide support 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.

YbPtBi 是具有最大索末菲系数γ的重费米子系统之一，因此被归类为“超”重费米子材料。在这项工作中，我们解决了有关 YbPtBi 中相关能量尺度层次结构的长期争议问题，例如 YbPtBi 中的晶体电场 (CEF) 水平、Kondo 和磁有序温度。通过测量反弹性热效应和通用对称性参数，我们确定了一种弹性能级分裂，它与四重 CEF 能级的对称性允许分裂明确相关。我们确定这个四重态是 环境压力下双重态基态之上Δ/ k _{B ≈ 1.6 K 处的第一个激发态，远低于提出的近藤温度}T _K  ≈ 10 K。因此，对能量方案的分析可以提供支持模型，预测重电子质量是 CEF 基态简并性增强的结果，即 YbPtBi 中的准六重态。与此同时，我们的研究表明了反弹性热效应在控制和量化 CEF 方案中应变引起的变化方面的潜力，开辟了一条不同的途径，将 CEF 能级与相关量子材料中的其他相关能级分开。