# Electron Theory

For many phenomena of solid state physics there are processes on various scales of length, energy and time, starting from the electronic level over the atomic and mesoscopic scale to the macroscopic scale. Therefore, a comprehensive description of solid state properties requires a multi-scale analysis. We therefore combine the methods of ab-initio density functional electron theory with phenomenological methods like statistical mechanics, micromagnetism, lattice theory and elasticity theory. In the centre of interest are the static and dynamical properties of spin- and orbital magnetism of magnetic materials of various dimensions, i.e., volume materials and nanostructured materials, which are interesting both from the viewpoint of fundamental research and for technological applications (storage materials, spintronics, etc.).

The main part of our research is the investigation of dissipative magnetization dynamics on the time scale of nanoseconds to about picoseconds (fast dynamics, for instance the vortex dynamics investigated intensively in our department) and on the timescale of about 100 femtoseconds (demagnetization or change of the magnetization direction after optical femtosecond-laser pulses). For the fast dynamics the limitations of Gilbert's equation which is often used for its description are outlined, and extensions of this equation are introduced. Furthermore, the solutions of the classical Gilbert equation for arbitrary perturbations of the system by external magnetic fields are treated in the formalism of Green's functions. For ultrafast dynamics especially the influence of electron-phonon- and electron-magnon-scatterings is investigated, both with Markovian rate equations and with quantum-kinetical methods (density matrix formalism).

In addition to this theoretical fundamental research we support from the viewpoint of theory the main experimental activities of our department on the field of spectroscopy and microscopy with polarized X-rays, and to give impacts for further activities in the future when X-ray sources of even higher intensity will be available.

**k**to one with wave vector

**k'**by absorption or emission of a phonon or magnon with wave vector

**q**.

**k**to one with wave vector

**k'**by absorption or emission of a phonon or magnon with wave vector

**q**.