Speaker: Urs Staub, PSI
Program Description
Manipulating the electronic, magnetic and the crystal structure by ultrashort electric field pulses is of great importance for novel schemes of writing data and or electronic devices. Understanding the fundamental concept of these interactions allows us to explore its potential. In this presentation, different ways of manipulating properties of materials that re studied by ultrafast x-ray diffraction, are discussed. It is shown how an electronic excitation affects the electronic ordering of half doped manganite [1] and through which steps the photo induced phase transition passes before the system recovers. This is put in context to the results obtained on the magnetic structure e.g. obtained by a group at the ALS [2, 3]. Then we address how the electronic and crystal structure reacts, when the energy is directly dumped into the phonon system. [4] These cases are compared to the case, where we excite an insulting oxide above its bandgap to drive a purely structural phase transition. For this purpose, we study the octahedral rotation phase transition in Ca:SrTiO3 and compared it to EuTiO3. Both systems are isostructural having the same phase transition from high-temperature cubic to the low-temperature tetragonal phase. The transition is driven by a softening of phonon mode without electronic ordering in thermodynamic equilibrium. After photo excitation, these systems behave very different. Whereas in Ca:SrTiO3, the octahedra rotate on ultrafast timescales into the direction of the high-temperature symmetry, in EuTiO3 the octahedra rotate on ultrafast time scales in the opposite direction, leading to an ultrafast increase of the order parameter. Concepts to describe this unusual situation are presented.
