Ultrafast carrier and coherent phonon dynamics in van der Waals ferromagnet CrI3

Abstract

Two-dimensional van der Waals ferromagnetic materials are attractive for their potential in advanced optoelectronic applications. Herein, femtosecond transient optical spectroscopy is used to study the dynamics of photoexcited carriers and coherent phonons in single-crystal CrI₃ at 10–300 K. The optical response in time domain consists of a sub-picosecond relaxation process due to electron–phonon thermalization and damped oscillations due to excitation of two coherent A_1g phonons. The carrier decay characteristics closely follow the two-temperature model. The frequency of the A¹_1g phonons decreases with increasing temperature, which is described by the conventional anharmonic phonon model including lattice thermal expansion and phonon–phonon coupling. The phonon lifetime shows an anomalous temperature dependence with two abrupt changes occurring at 60 and 220 K, near the magnetic and electronic structural transitions of CrI₃. The former arises from the weakening of spin–phonon coupling due to the renormalization of a narrow gap in the spin waves near the Curie temperature (61 K), while the latter can be attributed to the increased vacancy concentrations in the high-temperature monoclinic phase. Our findings provide valuable insight into the nonequilibrium carrier and lattice properties in the van der Waals ferromagnetic materials.