报告摘要：
　　Recent developments in time-dependent Density functional theory (TDDFT)-molecular dynamics (MD) make it possible to explore excited carrier dynamics and its coupling with lattice dynamics at femtosecond time scale. This talk presents several recent examples to illustrate the significance of such a development [1]. First, we consider energy transfer of photoexcited carriers in hydrogenated graphene. It was found that H desorption from a graphene sparsely populated by the H is difficult due to an inefficient transfer of the excitation energy into the kinetic energy of the H. In contrast, H can be easily desorbed from a fully hydrogenated graphane. This result is at variance with ground-state MD simulation, which predicts H in the sparse case would be much less stable than in fully hydrogenated case [2]. Second, while electron-phonon coupling usually hampers carrier diffusion either by scattering or by the formation of mass-enhanced polarons, it can also enable the diffusion of the excited carriers. Taking the non-polar InAs (110) surface as an example, it was found that phonon mediated electronic coupling could cause initially localized surface excitations, both holes and electrons, to propagate into bulk with a velocity as high as 106 cm/s, despite their nominally infinite effective mass . Third, phase transition at a temperature significantly below melting point has been observed by femtosecond laser experiment in phase change Ge-Sb-Te alloys. The phenomenon has been attributed to a lattice weakening effect by carrier excitation [3]. Here, we show that a totally unexpected effect, i.e., carrier multiplication by the relaxation of high-energy carriers, at the early stage of the excitation (~1 ps) is important by setting the trajectory for the athermal phase transition thereafter. This finding may offer a new strategy in designing faster non-volatile phase change memory devices.

Reference:
[1] S. Meng and E. Kaxiras, J. Chem. Phys. 129, 054110 (2008).
[2] J. Bang, et al., PNAS 110, 908 (2013).
[3] X.-B. Li, et al., Phys. Rev. Lett. 107, 015501 (2011).