Antimony is an interesting elemental crystal because, in its ground state, it is stabilized by a Peierls distortion. Here we perform density-functional-theory molecular dynamics simulations of this intriguing material before and after femtosecond-laser excitation using a simulation box with N = 864 atoms and periodic boundary conditions, where the atoms are treated in the Γ-point approximation and the electrons are integrated over 8 k points. After an appropriate initialization of the atoms in the harmonic approximation we thermalize our system during 20 picoseconds. Then an intense femtosecond-laser excitation is simulated by instantaneously raising the electronic temperature to 8000 Kelvin. Our results show a laser-induced anti-Peierls transition.
By means of first principles calculations we studied the intense femtosecond-laser excitation of several boron nitride nanotubes and a boron-nitride doped graphene layer up to irradiation levels where these structures disintegrate. We performed molecular dynamics simulations using our in-house Code for Highly excited Valence Electron Systems (CHIVES). For different boron-nitride nanotubes we determined the damage threshold in terms of the electronic temperature and the absorbed energy per atom. We found that all nanotubes studied were destroyed in the first 200 fs after an ultrafast laser excitation heating the electrons to 108 mHa (34103 K). Some tubes also disintegrated at lower electronic temperatures. For the boron-nitride doped graphene we found that at a laser-induced electronic temperature of 100 mHa (31577 K) bonds break and the boron-nitride dimer leaves the structure.
Using the Born-Oppenheimer approximation we calculate potential energy surfaces of photoexcited Bismuth-bulk. We determine phonon frequencies and potential anharmonicities for the case of high density of excited carriers.
In particular, we focus on the phonon modes A1g and Eg. We find strong softening of the A1g-frequency for increasing excited carrier density. Furthermore, from the analysis of the lattice motion upon excitation we show
that there is a coupling between the A1g and Eg modes, which is consistent with recent experimental findings.