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12 May 2015 Thermal and nonthermal melting of silicon exposed to femtosecond pulses of x-ray irradiation
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Silicon irradiated with an ultrashort laser pulse can experience two competing damage processes: the ultrafast ’nonthermal melting’ or the picoseconds ‘thermal melting’. The first one occurs if the density of excited electrons within the conduction band overcomes a certain threshold value, which leads to modification of the atomic potential energy surface and triggers a phase transition. The second one heats a material due to the electron-ion (electron-phonon) coupling, which in case of atomic temperature exceeding melting temperature also induces a phase transition. Our recently developed code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, Phys. Rev. B 91 (2015) 054113), can model both effects simultaneously. Nonadiabatic electron-ion coupling is treated within tight binding molecular dynamics model beyond the Born-Oppenheimer approximation. Two different channels of phase transition emerge at different irradiation dose: thermal melting of silicon into low-density-liquid phase occurs for deposited energies above ~0.65 eV/atom; nonthermal melting into high-density liquid takes place for doses higher than ~0.9 eV/atom. Here we discuss in detail electronic processes during such phase transitions. Evolution of the electronic structure is presented.
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Nikita Medvedev, Zheng Li, and Beata Ziaja "Thermal and nonthermal melting of silicon exposed to femtosecond pulses of x-ray irradiation", Proc. SPIE 9511, Damage to VUV, EUV, and X-ray Optics V, 95110I (12 May 2015);

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