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

在本研究中，我们在KEK光子工厂的BL-7 C实验室中，利用传统的金刚石压砧（DAC）和聚焦同步辐射束，发展了荧光XANES测量方法，<III>测量了PdCe和GaCe合金在不同压力下的Ce-L_ XANES光谱，最高压力可达-10GPa。我们证实了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的价态与异常电阻率（致密近藤效应）之间的关系，并得到以下结果：1.由荧光L_ XANES谱估算的<III>Ce在GaCe合金相中的价态变化范围为3.02（常压）~ 3.14（9 GPa）。由于PdCe中Ce的价态具有与PdCe相似的压力依赖性，外压促进了合金相的4f^1*4f^0电子跃迁。2.在GaCe合金相中，与PdCe相一样，当外压达到约6 GPa时，致密的近藤效应也被强烈增强。随着压力超过6 GPa，这种效应逐渐减弱。3. GaCe的体积随压力的减小率或压缩率明显大于PdCe。4.存在一个最合适的Ce-Ce原子距离或Ce价态，以明显出现致密近藤效应。

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.