Structural Changes

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1.1 Molecular Dynamic simulations of laser induced nonequilibrium phase transitions in solids and films

Questions addressed: Microscopic theoretical description of femtosecond structural phase transitions?

We developed a numerical approach to treat ultrafast phase transitions induced by laser pulses of arbitrary form. We perform molecular dynamics simulations on time-dependent, many-body potential energy surfaces derived from a microscopic Hamiltonian. Applying this method to diamond, we show that a nonequilibrium transition to graphite takes place for a wide range of pulse durations and intensities. This ultrafast (~100 fs) collective motion of the atoms is driven by the suppression of the diamond minimum in the potential energy surface of the laser excited system. The laser induced melting of a C60-crystal under pressure was also analyzed. In this case an ultrafast melting of the system occurs.

Using this theoretical approach, the physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses were analyzed using a microscopic electronic theory. We show that graphite has the unique property of exhibiting two distinct laser induced structural instabilities. For high absorbed energies ( > 3.3 ~eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold ( > 2.0 ~eV/atom) ablation occurs via removal of intact graphite sheets. We have also calculated the ablation thresholds of diamond and graphite as a function of the pulse duration for femtosecond pulses. For both materials we obtain smoothly increasing thresholds for increasing duration.

Moreover, the ultrafast time-dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy.

In collaboration with an experimental group at the Federal Institute for Materials Research and Testing (Berlin), the ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on our theoretical approach. We determine the thresholds of melting and ablation for two different pulse durations τ = 20fs and τ = 500fs. Experiments have been performed using 100 Ti:Sapphire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25fs and 400fs, respectively. Good agreement is obtained between theory and experiment.

We also studied laser induced femtosecond melting of graphite under high external pressure. Our results show that the laser induced melting process occurs in two steps: (i) destruction of the graphite sheets via bond breaking, and (ii) merging of the melted layers. The separation of the two steps is more evident for graphite under pressure (10~GPa), but is also present in graphite films at normal pressure. The melting product is a low density carbon phase, which remains stable under high pressure, but is unstable with an ultrashort life-time under normal pressure.

In collaboration with the experimental group of Prof. R. Falcone (UC Berkeley) we have studied the laser induced melting of silicon foils and the response of the transient liquid to ultrashort x-ray pulses. We simulated the melting process and determined the EXAFS spectra. Good agreement with the experimental result was obtained.

Methods: Molecular dynamics simulations in films and in the bulk at constant pressure, tight-binding Hamiltonian, equarion of motion for the density matrix, calculation of EXAFS-spectra using many-body theory.

Publications: [48], [46], [40], [39], [38], [36], [34], [30] (see list of publications).

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1.2 Ab-initio, all-electron description of laser induced coherent phonons in solids

Questions addressed: Is it possible to provide an accurate, quantitative description of the experimentally observed time-resolved diffraction signals? Prediction of particular laser-induced structural changes?

Using first principles, all-electron calculations and dynamical  simulations we study the behavior of the A1g and Eg coherent phonons induced in Bi by intense laser pulses. We determine the potential landscapes in the laser heated material and show that they exhibit phonon-softening, phonon-phonon coupling, and anharmonicities. As a consequence the Eg mode modulates the A1g oscillations and higher harmonics of both modes appear, which explains recent isotropic reflectivity measurements. Our results offer a unified description of the different experimental observations performed so far on bismuth.

Calculations under different assumptions for excited carrier thermalization were done and successfully compared with experiments performed by S. L. Johnson and coworkers at the PSI (Switzerland).

The method was also applied to As, where a laser induced transition from the A7 into the simple cubic crystal structure was predicted

Methods: The calculations were done using the code Wien2k, based on the full potential linearized aug- mented plane wave method. All electrons are taken into account, including the core electrons. Simulations on the calculated PES.

Publications: [20], [12], [4], [1] (see list of publications)

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