Tobias Zier

Fractional Diffusion in Silicon

Microscopic processes leading to ultrafast laser-induced melting of silicon are investigated by large-scale ab initio molecular dynamics simulations. Before becoming a liquid, the atoms are shown to be fractionally diffusive, which is a property that has so far been observed in crowded fluids consisting of large molecules. Here, it is found to occur in an elemental semiconductor.

Signatures of nonthermal melting

Intense ultrashort laser pulses can melt crystals in less than a picosecond but, in spite of over thirty years of active research, for many materials it is not known to what extent thermal and nonthermal microscopic processes cause this ultrafast phenomenon. Here, we perform ab-initio molecular-dynamics simulations of silicon on a laser-excited potential-energy surface, exclusively revealing nonthermal signatures of laser-induced melting. From our simulated atomic trajectories, we compute the decay of five structure factors and the time-dependent structure function. We demonstrate how these quantities provide criteria to distinguish predominantly nonthermal from thermal melting.

Coherent and Incoherent Structural Dynamics in Laser-Excited Antimony

We investigate the excitation of phonons in photoexcited antimony and demonstrate that the entire electron-lattice interactions, in particular coherent and incoherent electron-phonon coupling, can be probed simultaneously. Using femtosecond electron diffraction (FED) with high temporal resolution, we observe the coherent excitation of the fully symmetric \Ag\ optical phonon mode via the shift of the minimum of the atomic potential energy surface. Ab initio molecular dynamics simulations on laser excited potential energy surfaces are performed to quantify the change in lattice potential and the associated real-space amplitude of the coherent atomic oscillations. Good agreement is obtained between the parameter-free calculations and the experiment. In addition, our experimental configuration allows observing the energy transfer from electrons to phonons via incoherent electron-lattice scattering events. The electron-phonon coupling is determined as a function of electronic temperature from our DFT calculations and the data by applying different models for the energy-transfer.

Ab initio molecular dynamics simulations of femtosecond-laser-induced anti-Peierls transition in antimony

Antimony is an interesting elemental crystal because, in its ground state, it is stabilized by a Peierls distortion. Here we perform density-functional-theory molecular dynamics simulations of this intriguing material before and after femtosecond-laser excitation using a simulation box with N = 864 atoms and periodic boundary conditions, where the atoms are treated in the Γ-point approximation and the electrons are integrated over 8 k points. After an appropriate initialization of the atoms in the harmonic approximation we thermalize our system during 20 picoseconds. Then an intense femtosecond-laser excitation is simulated by instantaneously raising the electronic temperature to 8000 Kelvin. Our results show a laser-induced anti-Peierls transition.

Simulations of Highly-Excited Silicon

The Code for Highly-excIted Valence Electron Systems (CHIVES) performs fast first-principles molecular-dynamics simulations of silicon under the extreme conditions induced by an intense ultrashort laser pulse. We illustrate this by the example of thermal phonon squeezing.

Laser-induced nonthermal melting in Si

In Si an ultrashort laser pulse excitation induces a nonthermal state with ensuing bond softening which leads to nonthermal melting. Our simulations allow us to explain the concerted decay of several x-ray diffraction peaks.