Phase transformations driven by femtosecond laser excitation
The coupling of electron, spin, and lattice degrees of freedom is a key problem in understanding and controlling properties of functional materials. In the planned project phase transformations in correlated solid-state systems shall be driven by femtosecond laser excitation and probed by mapping the transient band structure with time- and angle-resolved resolved photoemission (TR-ARPES). For this purpose we have recently built a higher-harmonics beamline at MBI which optimizes time- and energy-resolution (100 fs XUV pulse duration at 200 meV bandwidth for photon energies of 15 – 40 eV) for this kind of experiments. Our photoemission endstation will be upgraded to optimize ARPES with synchrotron and HHG radiation. The tuneable HHG beamline allows us to map the entire Brillouin zone and thereby follow the dynamics and transient band structure at particular k-points.
Phase transformations investigated include laser-driven ultrafast demagnetization in thin ferromagnetic films. In ferromagnets we can detect the transient exchange-splitting of the bands which directly reflects the thin film magnetization (Fig. 8). This signature is independent of domain formation and is thus complementary to the proposed X-ray diffraction experiments (P4, P5, P8) which elucidate the dynamics of domain patterns. For particular systems photoemission allows to separate surface from bulk magnetization dynamics, which are expected to differ on the femtosecond time scale due to ballistic electron i.e. spin-transport and/or different magnetic properties of surface and bulk.
The goal of the project is to determine the signatures of thermally and laser-driven phase transformations in the electronic structure of correlated materials and measure the transient surface and bulk band structure by time-resolved ARPES and establish resonant pumping schemes.
Complementary to this project X-ray scattering experiments are carried out in P5, P6, P7, and P8 (and Stöhr, Dürr). The project relates to theory on selective phonon pumping (P10). Similar correlated materials will be investigated by high-resolution ARPES (Golden, Shen) and later on studied by TR-ARPES. Additionally, there is methodological synergy with P1, P2, P3, P7.