Self-Learning Method for Construction of Analytical Interatomic Potentials to Describe Laser-Excited Materials.

Large-scale simulations using interatomic potentials provide deep insight into the processes occurring in solids subject to external perturbations. The atomistic description of laser-induced ultrafast nonthermal phenomena, however, constitutes a particularly difficult case and has so far not been possible on experimentally accessible length scales and timescales because of two main reasons: (i) ab initio simulations are restricted to a very small number of atoms and ultrashort times and (ii) simulations relying on electronic temperature- (T_{e}) dependent interatomic potentials do not reach the necessary ab initio accuracy. Here we develop a self-learning method for constructing T_{e}-dependent interatomic potentials which permit ultralarge-scale atomistic simulations of systems suddenly brought to extreme nonthermal states with density-functional theory (DFT) accuracy.

Quasimomentum-Space Image for Ultrafast Melting of Silicon.

By exciting electron-hole pairs that survive for picoseconds strong femtosecond lasers may transiently influence the bonding properties of semiconductors, causing structure changes, in particular, ultrafast melting. In order to determine the energy flow during this process in silicon we performed ab initio molecular dynamics simulations and an analysis in quasimomentum space. We found that energy flows very differently as a function of increasing excitation density, namely, mainly through long wavelength, L-point, or X-point lattice vibrations, respectively.

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.

Electrons per atom ratio determination and Hume-Rothery electron concentration rule for P-based polar compounds studied by FLAPW-fourier calculations.

The extent to which reliable electrons per atom ratio, e/a, are determined and the validity of the Hume-Rothery stabilization mechanism are ensured upon increasing ionicity are studied by applying first-principles full potential linearized augmented plane wave (FLAPW)-Fourier band calculations to as many as 59 binary compounds formed by adding elements from periods 2-6 to phosphorus in group 15 of the Periodic Table. Van Arkel-Ketelaar triangle maps were constructed both by using the Allen electronegativity data and by using an energy difference between the center-of-gravity energies of FLAPW-derived s and p partial densities of states (DOSs) for the equiatomic compounds studied. The determination of e/a and the test of the interference condition, both of which play a key role in the Hume-Rothery stabilization mechanism, were reliably made for all intermetallic compounds, as long as the ionicity is less than 50%.

Mechanical properties of boron-nitride nanotubes after intense femtosecond-laser excitation.

A femtosecond-laser pulse constitutes an unconventional tool to manipulate solids and nanostructures, for it may excite materials in a transient nonthermal state with hot electrons and atoms close to their initial temperature. Here we study the Young's modulus and the electronic band gap of a (5, 0) zigzag boron-nitride nanotube (BNNT) after an ultrashort laser pulse excitation using density functional theory, where the effect of a femtosecond-laser pulse is modelled by an instantaneous rise of the electronic temperature. At room temperature, before the laser pulse, we obtain a Young's modulus of 763 GPa, which decreases with increasing electronic temperature.

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.

Ultrafast evolution of the excited-state potential energy surface of TiO2 single crystals induced by carrier cooling.

We investigate the influence of carrier cooling dynamics in TiO(2) on the excited-state potential energy surface along the A(1g) optical phonon coordinate after above band-gap excitation using ultrashort ultraviolet pulses. The large amplitude coherent oscillation observed in a pump-probe transient reflectivity measurement shows a phase shift of -0.2π with respect to a purely instantaneous displacive excitation.

Anharmonic noninertial lattice dynamics during ultrafast nonthermal melting of InSb.

We compute the potential energy surface of femtosecond-laser-excited InSb along the directions in which the crystal becomes soft. Using dynamical simulations the time dependence of the atomic coordinates is obtained. We find that at high excitation densities the anharmonicity of the potential energy surface becomes significant after approximately 100 fs.