Most of the problems involve many-body physics and development of electronic theories treating correctly correlations. All problems must be treated by using modern developments of theoretical physics ( many-body theory, theory of phase transitions, Green's function theory, electronic theory of solids, etc. ).
An electronic theory for the normal and the superconducting states of high- TC-superconductors and triplett-superconductors is developed. Our theory yields strong dynamical short-range antiferromagnetic correlations causing pronounced changes of the quasiparticle excitation spectrum as observed in recent photoemission experiments. The superconducting state, resulting from these spin fluctuations is of d(x2-y2) - symmetry for single and bilayer cuprate systems. In addition, we can explain the doping dependence of the transition temperature TC, the increase of TC in bilayer systems, and photoemission and tunneling measurements below TC. Presently, we study the pseudo-gap origin, and the interplay of phase- and amplitude fluctuations of the order parameter. Moreover, we have developed an electronic theory for triplett Cooper-pairing in the Sr2RuO4 compound, and for magnetism in the Ruthenates.
The investigation of this classical problem of solid state physics is two-fold: First, using statistical mechanics of Heisenberg and Ising models, the role of spin reorientation, susceptibility, domain formation, and magnetic relaxation for ultrathin ferromagnetic films during growth is investigated. Especially the interdependence of atomic structure and magnetic properties is of considerable interest. In addition, the coercivity and exchange bias of coupled ferro- antiferromagnetic bilayers is calculated as function of temperature and spin disorder. Of particular interest are the determination of magnetic properties like average energy, distribution of states, and energy scaling of dipole-coupled disordered nanostructures. Other topics are the calculation of the magnetic spiral structure near surfaces/interfaces, and the determination of the higher-harmonics of the ac-susceptibility. Second, based on electronic theories fundamental quantities like magneto-crystalline anisotropy on surfaces and thin magnetic films, as well as the linear and nonlinear magneto-optical response is determined. The origin of this magneto-crystalline is traced back to the properties of the electronic band structure. The anisotropy energy has been calculated within a non-perturbative theory.
The nonlinear magneto-optical Kerr-effect (NOLIMOKE) describes the rotation of the polarization plane in optical second harmonic generation (SHG). NOLIMOKE is well suited for a direct investigation of two-dimensional ferromagnetic structures at surfaces and interfaces of magnetic thin films and multilayers. The theoretical interest focuses on the electronic theory for the non-linear Kerr spectra and the Kerr rotation angle, where a drastic enhancement by several orders of magnitude compared to the angle in linear magneto-optics is predicted. This prediction has subsequently been experimentally confirmed. In contrast to linear optics, NOLIMOKE also allows for the direct investigation of magnetic quantum well states and of interface antiferromagnetism. Theories for these effects have been developed in our group. We extend now our theory to analyze systematically quantum well state effects in Ni/Cu, Fe/Cu, and Co/Cu films in order to calculate from first principles χijl to include plasmon excitations to SHG. Moreover, we calculate the magnetic Mie-scattering.
The electronic properties of small atomic clusters are subject of intense study in our group. Clusters are of fundamental importance fo both basic and applied physics. They are used for catalysis, photography, and magnetic storage. Clusters are the link between the atom and the solid. Therefore, the evolution of their electronic properties with increasing size exhibits dramatic transitions like metal-insulator, noncrystalline-to-crystalline, color (light absorption frequencies) changes, developement of collective excitations, etc. Our research interests are the study of optical properties of metal and nonmetal clusters, the description of the size-dependent changes of the chemical bonding, and the study of many-body effects. We also investigate collisions of clusters on surfaces and Auger-electron emission. Of particular interest is the multiple ionisation of clusters in an intense field and clusters at non-equilibrium (fragmentation, etc.).
Since the recent developement of the femto-second spectroscopy it is possible to monitor directly the atomic motion after excitation of electrons with an ultrashort (10-15 sec) laser pulse. The physics in the sub-picosecond timescale shows new phenomena, some of them related to phase coherence and memory effects - the short time dynamics of small clusters after excitation with a laser pulse is one of our research topics - th understanding of this dynamics serve as a basis for the control and "manipulation" of the time scales for chemical reactions. The excitation with ultrashort pulses also permits the systems the access to regions of the phase-space which are not easily reachable through variation of other external parameters like temperature, pressure, or static magnetic fields. So we study the possibility of generating new phase transitions, like graphite - diamond, or graphite - liquid carbon at low pressures, induced by femtosecond laser pulses. In combination with optical "pump and probe" techniques the nonlinear magnetio-optical Kerr-effect allows a time-dependent study of 2D-magnetism on the pico- through femtosecond time scale. Of particular importance for future theoretical and experimental research is the nonlinear femtosecond dynamics of hot electrons, which may provide new ultrafast magneto-optical switching mechanisms of technological relevance. We study application of pump- and probe spectroscopy to SHG and for ferromagnetic Ni and Fe. How quickly breaks the magnetization down due to hot electrons and how does the electronic temperature develop with time ?
Our group takes part in two
Collaborative Research Centers (Centers of Excellence) of the Deutsche Forschungsgemeinschaft
"Metallische dünne Filme: Struktur, Magnetismus und elektronische Eigenschaften"
"Analyse und Steuerung ultraschneller photoinduzierter Reaktionen"