Ultrafast photoinduced dynamics in correlated materials

One of the key questions in solid state physics is the origin of the electric conductivity of a material. While simple band structure theory can explain basic electronic and optical properties of most insulators, semi-conductors and metals, other more complex materials exhibit properties based on many body interactions. The electronic properties of such highly correlated materials are often governed by strong electron-phonon coupling and correlation effects leading to phenomena like formation of charge density waves (CDW), metal insulator transitions or superconductivity. As the dynamics of the underlying elementary processes occur typically on different characteristic timescales, time-resolved spectroscopy can provide complementary and new information on these elementary processes and on the coupling between the various degrees freedom.

Time- and angle-resolved photoemission spectroscopy (trARPES) can simultaneously provide information about the single particle spectral function in the frequency domain and collective excitations (e.g. coherent phonons) in the time domain, allowing directly relating these excitations with the electronic band structure. Employing a novel position-sensitive Time-of-Flight spectrometer, we investigate the electronic structure over a contiguous (2D) area of the reciprocal space and follow the evolution of the Fermi surface real time. We study the photoinduced dynamics of TbTe₃, a metal which exhibits a Fermi surface nesting driven CDW transition. Using trARPES we can identify the amplitude mode giving rise to the CDW transition and its anisotropic (k-dependent) coupling to the electronic system in real time. Furthermore, we probe in the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+d the momentum resolved dynamics of optically excited quasiparticles and Cooper pair recombination along the Fermi surface.

Finally, we show that ultrafast changes of the lattice symmetry can be probed in the time domain via coherent phonon generation, enabling an all-optical probe of photoinduced phase transitions. We demonstrate this principle for the structural (insulator-to-metal) transition of VO₂ and show that strong electronic excitation changes the lattice potential on a sub phonon-period timescale.