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HL: Fachverband Halbleiterphysik

HL 49: Ultrafast Phenomena

HL 49.8: Vortrag

Mittwoch, 28. März 2012, 11:15–11:30, EW 202

Squeezed thermal phonons precurse nonthermal melting of silicon — •Tobias Zier, Eeuwe S. Zijlstra, and Martin E. Garcia — Theoretical Physics, University of Kassel, Germany

Femtosecond laser pulses can be used to practically instantaneously manipulate bonds in solids through the creation of a hot electron plasma. At sufficiently high fluences, some phonon modes may even become unstable, causing acceleration of the atoms, followed by a disordering within several 100's of femtoseconds. This ultrafast solid-to-liquid phase transition is called nonthermal melting and has been observed in silicon, germanium, gallium arsenide, indium antimonide, and bismuth. It is, however, not known which physical process leads up to nonthermal melting at fluences below the melting threshold. Here we show for silicon that in this regime the room temperature phonons become thermally squeezed. We found that the origin of this effect is the sudden femtosecond-laser induced softening of interatomic bonds, which can also be described in terms of a modification of the potential energy surface. We further found in ab initio molecular dynamics simulations on laser-excited potential energy surfaces that the atoms move in the same directions during the first stages of nonthermal melting at high fluences and thermal phonon squeezing at lesser fluences. Our results demonstrate that thermal phonon squeezing is the precursor to nonthermal melting in silicon. Based on the general nature of the underlying bond softening mechanism we believe that this relation between thermal squeezing and nonthermal melting is not material specific, but should occur in all materials exhibiting the latter process.