Pressure-Induced Electron Transitions and Physical Anomalies in Pd-Ce Alloys of the f-Electron Systems

在本研究中，我们开发了使用传统金刚石砧压力盒 (DAC) 和 KEK 光子工厂 BL-7C 聚焦同步加速器光束的荧光 XANES 测量方法。通过该方法，在高达 10 GPa 的各种高压下测量了 PdCe 和 GaCe 合金的 Ce-L_<III> XANES 光谱。我们证实了这些合金相中4f电子的压力诱导跃迁（Ce价态的压力变化）。我们还进行了高压电阻率测量和高压X射线衍射实验，以研究Ce价态与反常电阻率（密集近藤效应）之间的关系，并得到以下结果：1.GaCe合金相中，从荧光L_<III>XANES光谱估计的Ce价态从3.02（环境压力）到3.14（9 GPa）变化。由于PdCe中的Ce价态具有类似的压力依赖性，因此外部压力促进合金相中4f^1*4f^0的电子跃迁。2.在GaCe合金相中，与PdCe相一样，高达约6GPa的外部压力强烈增强致密近藤效应。当压力超过6 GPa时，该效应逐渐减弱。3.GaCe的体积随压力减小的速率或可压缩性明显大于PdCe。4.对于明显出现的致密近藤效应，存在一个最合适的Ce-Ce原子距离或Ce价态。

In this study, we have developed the fluorescence XANES measurement method using a conventional diamond anvil pressure cell (DAC) and a focused synchrotron beam at BL-7C of the Photon Factory in KEK.By this method, Ce-L_<III> XANES spectra were measured for PdCe and GaCe alloys under various high pressures up to-10 GPa. We confirmed the pressure-induced-transition of 4f electrons (the pressurechanges of Ce valence) in these alloy phases. We also carrid out high pressure electrical resistivity measurements and high pressure X-ray diffraction experiments in order to examine the relationship between the Ce valence and the anomalous resistivity (the dense Kondo effect) , and the following results were obtained.1.The Ce valence estimated from the fluorescence L_<III> XANES spectra varies from 3.02 (at ambient pressure) to 3.14 (at 9 GPa) in GaCe alloy phases. Since the Ce valence in PdCe has the similar pressure dependence, the electron transition of 4f^1*4f^0 in alloy-phases is promoted by the external pressure.2.In the GaCe alloy phase, the dense Kondo effect is strongly enhanced by external pressure up to about 6 GPa as in the PdCe phase. The effect becomes weaker gradually with pressure above 6 GPa.3.The rate of decrease in volume with pressure, or the compressibility, of GaCe is remarkably larger than that of PdCe.4.There is a most suitable Ce-Ce atomic distance, or Ce valence, for the distinct appearance of dense Kondo effect.