Ultrafast+structural+phase+transitions+and+coherent+phonons+in+solids+and+nanostructures

Ultrafast structural phase transitions and coherent phonons in solids and nanostructures

Harald Jeschke Institut für Theoretische Physik, Goethe-Universität Frankfurt am Main, Germany

Laser induced femtosecond structural changes in covalent materials are studied theoretically. In an attempt to treat both ionic motion and the changes in the electronic structure with sufficient accuracy, we employ molecular dynamic simulations on the basis of a time-dependent potential energy surface derived from a tight-binding Hamiltonian. The shape and spectral composition of the laser pulse is explicitly taken into account. We show applications of this approach to the laser excitation of bulk diamond and ultrathin graphite and silicon films. By implementing a nonorthogonal tight binding scheme for germanium, we can address the softening of phonon modes in laser induced nonequilibrium and the time scales of the structural response. Recent experiments are in good agreement with predictions from our theoretical approach. Finally, we discuss the possibilities of extending the method from optical to XUV frequencies and beyond.