Eeuwe S. Zijlstra

Laser-induced phonon-phonon interactions in bismuth

Intense femtosecond laser pulses can induce a nonequilibrium state which leads to sudden and dramatic changes in the potential energy surfaces of different solids [1]. This pro perty can be used to excite and manipulate coherent lattice vibrations, as has been recently shown by several experiments [2, 3, 4, 5] and simulations [6]. In the last years, a variety of experiments on laser excitation of coherent phonons has been performed on bismuth [7, 8, 9, 10], which is a particularly interesting solid since its ground-state structure e xhibits a Peierls distortion. From the different studies done so far , a number of fundamental aspects still remain unexplained, like the detection of higher harmonics [9] and the appearance of modes that are forbidden by symmetry in isotropic reflectivi ty measurements [8]. In this letter we perform ab initio calculations which show, for the first time, the existence of a coupling between laserinduced phonon modes of different symmetry. In addition, we demonstrate that the large amplitude atomic vibrations excited by femtosecond laser pulses are affected by the anharmonic part of the potential energy surface, which creates overton es. The structure of Bi can be derived from a simple cubic atomic packing in two steps. First a simple cubic lattice is deformed by elongating it along one of the body diagonals, which is indicated by a thin line in Fig. 1. A Peierls instabil ity

Fractional Diffusion in Silicon

Microscopic processes leading to ultrafast laser-induced melting of silicon are investigated by large-scale ab initio molecular dynamics simulations. Before becoming a liquid, the atoms are shown to be fractionally diffusive, which is a property that has so far been observed in crowded fluids consisting of large molecules. Here, it is found to occur in an elemental semiconductor.

All-optical control and visualization of ultrafast two-dimensional atomic motions in a single crystal of bismuth

In a bulk solid, optical control of atomic motion provides a better understanding of its physical properties and functionalities. Such studies would benefit from active control and visualization of atomic motions in arbitrary directions, yet, so far, mostly only one-dimensional control has been shown. Here we demonstrate a novel method to optically control and visualize two-dimensional atomic motions in a bulk solid. We use a femtosecond laser pulse to coherently superpose two orthogonal atomic motions in crystalline bismuth. The relative amplitudes of those two motions are manipulated by modulating the intensity profile of the laser pulse, and these controlled motions are quantitatively visualized by density functional theory calculations. Our control-visualization scheme is based on the simple, robust and universal concept that in any physical system, two-dimensional particle motion is decomposed into two orthogonal one-dimensional motions, and thus it is applicable to a variety of condensed matter systems.

Optimized Gaussian basis sets for Goedecker–Teter–Hutter pseudopotentials

We have optimized the exponents of Gaussian s and p basis functions for the elements H, B–F and Al–Cl using the pseudopotentials of (Goedecker, Teter and Hutter 1996 Phys. Rev. B 54 1703) by minimizing the total energy of dimers. We found that this procedure causes the Gaussians to be somewhat more localized than the usual procedure, where the exponents are optimized for atoms. We further found that three exponents, equal for s and p orbitals, are sufficient to reasonably describe the electronic structure of all elements that we have studied. For Li and Be results are presented for pseudopotentials of (Hartwigsen et al 1998 Phys. Rev. B 58 3641). We expect that our exponents will be useful for density functional theory studies where speed is important.

Signatures of nonthermal melting

Intense ultrashort laser pulses can melt crystals in less than a picosecond but, in spite of over thirty years of active research, for many materials it is not known to what extent thermal and nonthermal microscopic processes cause this ultrafast phenomenon. Here, we perform ab-initio molecular-dynamics simulations of silicon on a laser-excited potential-energy surface, exclusively revealing nonthermal signatures of laser-induced melting. From our simulated atomic trajectories, we compute the decay of five structure factors and the time-dependent structure function. We demonstrate how these quantities provide criteria to distinguish predominantly nonthermal from thermal melting.

Ab-initio study of coherent phonons excited by femtosecond laser pulses in Bismuth

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.

Coherent and Incoherent Structural Dynamics in Laser-Excited Antimony

We investigate the excitation of phonons in photoexcited antimony and demonstrate that the entire electron-lattice interactions, in particular coherent and incoherent electron-phonon coupling, can be probed simultaneously. Using femtosecond electron diffraction (FED) with high temporal resolution, we observe the coherent excitation of the fully symmetric \Ag\ optical phonon mode via the shift of the minimum of the atomic potential energy surface. Ab initio molecular dynamics simulations on laser excited potential energy surfaces are performed to quantify the change in lattice potential and the associated real-space amplitude of the coherent atomic oscillations. Good agreement is obtained between the parameter-free calculations and the experiment. In addition, our experimental configuration allows observing the energy transfer from electrons to phonons via incoherent electron-lattice scattering events. The electron-phonon coupling is determined as a function of electronic temperature from our DFT calculations and the data by applying different models for the energy-transfer.

Electronic origin of bond softening and hardening in femtosecond-laser-excited magnesium

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we—surprisingly—find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials.

Ab initio molecular dynamics simulations of femtosecond-laser-induced anti-Peierls transition in antimony

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.

Melting of Al Induced by Laser Excitation of 2p Holes

Novel photon sources—such as XUV- or X-ray lasers—allow to selectively excite core excitations in materials. We study the response of a simple metal, Al, to the excitation of 2p holes using molecular dynamics simulations. During the lifetime of the holes, the interatomic interactions in the slab are changed; we calculate these using WIEN2k. We find that the melting dynamics after core-hole excitation is dominated by classical electron–phonon dynamics. The effects of the changed potential surface for excited Al atoms occur on the time scale of 100 fs, corresponding to the Debye time of the lattice.