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We develop an electronic-temperature dependent interatomic potential Φ(T e ) for unexcited and laser-excited silicon. The potential is designed to reproduce ab initio molecular dynamics simulations by requiring force-and energy matching for each time step. Φ(T e ) has a simple and flexible analytical form, can describe all relevant interactions and is applicable for any kind of boundary conditions (bulk, thin films, clusters). Its overall shape is automatically adjusted by a self-learning procedure, which finally finds the global minimum in the parameter space. We show that Φ(T e ) can reproduce all thermal and nonthermal features provided by ab initio simulations. We apply the potential to simulate laser-excited Si nanoparticles and find critical damping of their breathing modes due to nonthermal melting. arXiv:1812.08595v1 [cond-mat.mtrl-sci]

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The excitation of lattice vibrations by ultrafast laser pulses provides a tool to steer atomic-scale motions beyond usual thermodynamic limitations. We simulate this process in armchair, chiral, and zigzag boron nitride nanotubes (BNNTs). In particular, for ultrathin zigzag-type tubes we show that three vibrational modes can be displacively excited. Since the boron nitride bonds are polar, the three coherent phonon oscillations emit terahertz (THz) radiation. In this work we focus on the (5,0) zigzag BNNT, which is the thinnest stable one, and demonstrate, by means of ab initio molecular dynamics simulations and optimization algorithms, that the relative amplitudes of the three phonon modes and therefore the corresponding shape of the emitted THz short pulse can be controlled by laser-pulse trains. Our work could serve as a basis for experimental studies using the coherent vibrations of BNNTs as optical memory devices in nanophononics, since information could be written in the phononic system by several femtosecond laser pulses and could be read out by measuring the produced THz emission.

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. For the band gap we find a value of 2.26 eV at room temperature, which increases with increasing electronic temperature and equals 3.28 eV at 28 420 K. We note that conventional means decrease the band gap of BNNTs and that a femtosecond-laser pulse is, to the best of our knowledge, the first tool that increases it. For comparison, we also present results for a (9, 0) zigzag BNNT.

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