Coherent phonon dynamics and electron-phonon coupling of (Li_0.8Fe_0.2)OHFeSe revealed with femtosecond transient optical spectroscopy

Femtosecond transient optical spectroscopy was used to study the coherent phonon dynamics and electron-phonon coupling in (Li_0.8Fe_0.2)OHFeSe. The temperature-dependent behavior of both fast and slow relaxation processes was investigated, providing insight into the origins of relaxation processes. The fast relaxation process reflects interaction of <i>e</i>-<i>ph</i> coupling. While the slow relaxation process is attributed to the phonon bottleneck effect. The coherent phonon oscillations observed at all measured temperatures are attributed to the <i>A</i>₁ g phonon mode, which corresponds to the in-phase vibration of Se anions along the <i>c</i>-axis. The redshift phenomenon in the coherent phonon is well explained by the phonon anharmonic effect. The <i>e</i>-<i>ph</i> coupling constant <i>λ</i> is estimated to be 0.23 using the Allen model.