重费米子系统YbPtBi的弹热效应
Elastocaloric effect of the heavy-fermion system YbPtBi
摘要：
YbPtBi是索末菲系数$γ$最大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，我们解决了长期争论的问题有关的层次结构的相关能量尺度，如晶体电场（CEF）水平，近藤和磁有序温度，在YbPtBi。通过对交流阻抗的测量，弹性热效应和一般的对称性参数，我们确定了一个\textit{elastic level splitting}，这是唯一的四重CEF水平的分裂允许的分裂。这四重态，我们确定为在$Δ/k_\text B\approx1.6\,\rm K$的第一个激发态以上的双重基态在环境压力下，远低于近藤温度$T_\text K\approx10\,\rm K$。因此，我们的分析为预测重电子质量是CEF基态简并增强的结果的模型提供了强有力的支持，即，YbPtBi中的准六重态。与此同时，我们的研究显示了交流的潜力。弹性热效应来控制和量化CEF方案的应变引起的变化，打开了一条不同的路线来解开CEF能量尺度与相关量子材料中的其他相关能量尺度。
英文摘要：
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 $Δ/k_\text B\approx1.6\,\rm K$ above the doublet ground state at ambient pressure, is well below the Kondo temperature $T_\text K\approx10\,\rm K$. 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.
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