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In the framework of real time real space time-dependent density functional theory we have studied the electron-ion dynamics of a hydrogen-terminated silicon surface H-Si(111) subjected to intense laser irradiation. Two surface fragments of different sizes have been used in the simulations. When the intensity and duration of the laser exceed certain levels (which depend on the wavelength) we observe the desorption of the hydrogen atoms, while the underlying silicon layer remains essentially undamaged. Upon further increase of the laser intensity, the chemical bonds between silicon atoms break as well. The results of the simulations suggest that with an appropriate choice of laser parameters it should be possible to remove the hydrogen layer from the H-Si(111) surface in a matter of a few tens of femtoseconds. We have also observed that at high laser field intensities (2-4 V/Å in this work) the desorption occurs even when the laser frequency is smaller than the optical gap of the silicon surface fragments. Therefore, nonlinear phenomena must play an essential role in such desorption processes. [ABSTRACT FROM AUTHOR]

The dynamics of hydrogen desorption from H-terminated silicon surface clusters has been simulated in the framework of real space time-dependent density functional theory complemented with molecular dynamics for ions. It has been demonstrated that by choosing an appropriate frequency and intensity of the laser it is possible to remove the hydrogen layer from the surface without destroying the structure of underlying silicon. At the laser field intensities used in the current study (0.5-2.0 V/Å) the desorption process is notably nonlinear. [ABSTRACT FROM AUTHOR]

The spatial current distribution in H-terminated zigzag graphene nanoribbons (ZGNRs) under electrical bias is investigated using time-dependent density-functional theory solved on a real-space grid. A projected complex absorbing potential is used to minimize the effect of reflection at simulation cell boundary. The calculations show that the current flows mainly along the edge atoms in the hydrogen terminated pristine ZGNRs. When a vacancy is introduced to the ZGNRs, loop currents emerge at the ribbon edge due to electrons hopping between carbon atoms of the same sublattice. The loop currents hinder the flow of the edge current, explaining the poor electric conductance observed in recent experiments. [ABSTRACT FROM AUTHOR]