标题： 具有可能的中间电子态的电荷密度波拓扑半金属 EuAl4 中增强的声子-声子相互作用和减弱的电子-声子耦合 显示英文标题

标题： Enhanced Phonon-Phonon Interactions and Weakened Electron-Phonon Coupling in Charge-Density-Wave Topological Semimetal EuAl4 with a Possible Intermediate Electronic State

摘要： 电荷密度波（CDW）的起源是一个长期存在的开放问题。 此外，很少有人研究过声子-声子相互作用（PPI）在CDW转变过程中的演化。 另外，关于CDW转变后电子-声子耦合（EPC）是被削弱还是增强，仍然存在争议。 此外，CDW为揭示有趣的中间电子态提供了肥沃的土壤。 在这里，我们报告了对拓扑半金属EuAl4中PPI和EPC的拉曼光谱研究，该材料在温度Tc约145 K以下表现出CDW相。两个声子模式A1g和B1g的自由电荷载流子密度(nc)和温度依赖性Fano不对称因子(1/|q|)表明，在Tc以下，由于nc的减少，EPC可能变得减弱。 有趣的是，在50到145 K的温度范围内，导致1/|q|显著偏离nc线性依赖关系的急剧增长，以及在约50 K时1/|q|和nc温度演化的肩状特征，暗示了EuAl4中可能存在一种中间电子态，其EPC明显大于CDW基态。 此外，在Tc以下，表示CDW调制的Al1原子集体振动的B1g声子模式的半高宽(full width at half maximum)的更快下降表明，B1g声子模式在CDW相变后的PPI显著增加，这与EPC的减弱形成对比，因此可能主要源于EuAl4晶格非谐性的增强。 我们的结果不仅突出了增强的PPI和减弱的EPC在完全理解CDW相形成中的重要意义，还开启了对EuAl4中新中间电子态的探索。 显示英文摘要

摘要： The origin of charge density wave (CDW) is a long-term open issue. Furthermore, the evolution of phonon-phonon interactions (PPI) across CDW transitions has rarely been investigated. Besides, whether electron-phonon coupling (EPC) would be weakened or enhanced after CDW transitions is still under debate. Additionally, CDW provides a fertile ground for uncovering intriguing intermediate electronic states. Here, we report a Raman spectroscopy study of the PPI and EPC in topological semimetal EuAl4 exhibiting a CDW phase below temperature Tc ~ 145 K. The free-charge-carrier-density (nc) and temperature dependences of the Fano asymmetric factors (1/|q|) of the two phonon modes A1g and B1g indicates that below Tc, the EPC becomes weakened probably due to the reduction of the nc. Interestingly, in the temperature range from 50 to 145 K, the steep growth of the 1/|q| leading to the significant deviation from the linear dependence on the nc, together with the shoulder-like features in the temperature evolutions of the 1/|q| and the nc around 50 K, implies the possible existence of an intermediate electronic state with the EPC distinctly larger than the CDW ground state in EuAl4. Furthermore, below Tc, the faster decrease in the full width at half maxima of the B1g phonon mode representing the collective vibrations of the CDW-modulated Al1 atoms suggests that a remarkable growth of the PPI for the B1g phonon mode after the CDW phase transition, which is in contrast to the weakening of the EPC and thus may mainly arise from the strengthening of lattice anharmonicity in EuAl4. Our results not only highlight the significance of the enhanced PPI and the weakened EPC in completely understanding the formation of the CDW phase but also initiate the exploration of novel intermediate electronic states in EuAl4.