Enhanced Phonon–PhononInteractions and WeakenedElectron–Phonon Coupling in Charge Density Wave TopologicalSemimetal EuAl₄ with a Possible Intermediate ElectronicState

The origin of the charge density wave (CDW) is a long-term open issue. Furthermore, the evolution of phonon–phonon interactions (PPIs) across CDW transitions has rarely been investigated. Whether electron–phonon coupling (EPC) would be weakened or enhanced after CDW transitions is still under debate. Additionally, CDW provides 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 <i>T</i>_c ≈ 145 K. The free-charge-carrier-density (<i>n</i>_c) and temperature dependences of the Fano asymmetric factors (1/<i>|q|</i>) of the two phonon modes A_1g and B_1g indicate that below <i>T</i>_c, the EPC becomes weakened probably due to the reduction of the <i>n</i>_c. Interestingly, in the temperature range of 50–145 K, the steep growth of the 1/|<i>q</i>| leading to the significant deviation from the linear dependence on the <i>n</i>_c, together with the shoulder-like features in the temperature evolutions of the 1/|<i>q</i>| and the <i>n</i>_c 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 <i>T</i>_c, the faster decrease in the full width at half maximum of the B_1g phonon mode representing the collective vibrations of the CDW-modulated Al¹ atoms suggests remarkable growth of the PPI for the B_1g 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.