标题： 笼状结构 CeBa$_7$Au$_6$Si$_{40}$ 中的晶体场激发 显示英文标题

标题： Crystal field excitations in rattling clathrate CeBa$_7$Au$_6$Si$_{40}$

摘要： 我们利用共振非弹性X射线散射（RIXS）和磁化率测量研究了笼状化合物CeBa$_7$Au$_6$Si$_{40}$（Ce-BAS）中铈的局域晶体场环境。Ce-BAS是一个罕见的例子，在这个系统中重费米子物理与低能振动声子模式共存，使其成为增强热电性能的候选材料。磁化率测量揭示了一种温度依赖性的局域磁矩，这种磁矩无法用静态晶体场模型来解释。为了拟合磁化率数据，需要在j=5/2和j=7/2的晶体场态之间有强烈的混合，这表明内部分裂很大，与RIXS光谱不符。相比之下，RIXS数据可以用一个几乎无j=5/2-j=7/2混合且基态与第一激发态晶体场态之间的能量差为12 meV的模型很好地描述。这两种数据集无法在静态框架内协调一致，这表明晶体场势存在动力学修正。我们认为这是由于Ce 4f电子与Ce振动相关的低能声子之间的耦合。这一解释与声子增强的Kondo效应和动力学Jahn-Teller畸变的理论预测一致。我们的结果强调了需要包含声子耦合的多轨道杂质模型来全面描述Ce-BAS的低能物理。 显示英文摘要

摘要： We investigate the local crystal-field environment of cerium in the clathrate compound CeBa$_7$Au$_6$Si$_{40}$ (Ce-BAS) using resonant inelastic x-ray scattering (RIXS) and magnetic susceptibility measurements. Ce-BAS is a rare example of a system where heavy-fermion physics coexists with low-energy rattling phonon modes, making it a candidate for enhanced thermoelectric performance. Magnetic susceptibility measurements reveal a temperature-dependent local moment that cannot be explained within a static crystal-field model. A fit to the susceptibility data requires strong mixing between the j = 5/2 and j = 7/2 crystal-field states, which implies large internal splittings inconsistent with RIXS spectra. In contrast, RIXS data are well described by a model with negligible j = 5/2 - j = 7/2 mixing and an energy separation of 12 meV between the ground and first excited crystal-field states. The inability to reconcile these two datasets within a static framework points to a dynamical modification of the crystal-field potential. We attribute this to coupling between the Ce 4f electrons and low-energy phonons associated with the Ce rattling motion. This interpretation is consistent with theoretical predictions of phonon-enhanced Kondo effects and dynamical Jahn-Teller distortions. Our results highlight the need for multi-orbital impurity models that include phonon coupling to fully describe the low-energy physics of Ce-BAS.