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

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 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 indicate that below Tc, the EPC becomes weakened probably due to the reduction of the nc. Interestingly, in the temperature range of 50–145 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 maximum of the B1g phonon mode representing the collective vibrations of the CDW-modulated Al1 atoms suggests 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.