Two coherent phonon modes at the metal-insulator transition of the spinel CuIr2S4 compound using femtosecond pump-probe spectroscopy

We report an observation of coherent phonons in thio spinel CuIr2S4 by using femtosecond pump-probe spectroscopy. We found two pronounced and long-lived phonon modes (2.5 and 4.2 THz) superimposed on the electronic response which corresponds to optical excitation from Ir 5d t(2g) to e(g). At the metal-insulator transition (230 K), the lifetime of the 2.5 THz phonon mode increases abruptly, whereas the other phonon parameters, including the 4.2 THz phonon mode, show negligible changes. Moreover, we observed the lifetime extension of the 4.2 THz phonon mode attributed to the photoinduced phase transition which is enabled below 100 K. These anomalous phonon behaviors are strongly correlated with a complicated nature of electron-phonon interaction in CuIr2S4.