Search

Dirac semimetal PdTe2 single-crystal temperature-dependent ultrafast carrier and phonon dynamics were studied using ultrafast optical pump-probe spectroscopy. Quantitative analysis revealed a fast relaxation process (τf) occurring at a subpicosecond time scale originating from electron–phonon thermalization. This rapid relaxation was followed by a slower relaxation process (τs) on a time scale of ∼ 7-9.5 ps which originated from phonon-assisted electron–hole recombination. Two significant vibrational modes resolved at all measured temperatures. These modes corresponded to in-plane (Eg), and out-of-plane (A1g), Te atoms motion. Test results suggested that pure dephasing played an important role in the relaxation processes. Analysis of the electron–phonon coupling constant suggested that the A1gmode contributes greatly to the superconductivity, and high-frequency phonons are also involved forming of Cooper pairs. Our observations should improve the understanding of complex superconductivity of PdTe2.

We present the study of the ultrafast carrier dynamics in bulk GaAs at 6 K, by time-resolved optical reflectivity in a wide range of excitation densities. We observe an exciton-phonon interaction at low excitation densities, in terms of a mono-exponential relaxation. Further increase of excitation density above a critical value nc, stimulates a bi-exponential relaxation in which the fast one is added owing to an exciton-exciton scattering channel. Our results reveal nc∼2×1024photons/m3, about an order of magnitude higher than the Mott density previously determined by photoluminescence and terahertz spectroscopy measurements and theoretical calculations. Keywords: Exciton, Semiconductor, Ultrafast pump-probe spectroscopy

We present systematic studies of the transient dynamics of GaAs by ultrafast time-resolved reflectivity. In photoexcited non-equilibrium states, we found a sign reverse in reflectivity change Δ R / R , from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a free carrier metallic state, while the latter is attributed to an exciton insulating state, in which the transient electronic properties is mostly dominated by excitons, resulting in a transient metal–insulator transition (MIT). Two transition temperatures ( T _1 and T _2 ) are well identified by analyzing the intensity change of the transient reflectivity. We found that photoexcited MIT starts emerging at T _1 as high as ∼ 230 K, in terms of a dip feature at 0.4 ps, and becomes stabilized below T _2 that is up to ∼ 180 K, associated with a negative constant after 40 ps. Our results address a phase diagram that provides a framework for the inducing of MIT through temperature and photoexcitation, and may shed light on the understanding of light-semiconductor interaction and exciton physics.