6. Electronic correlations and dynamics
If electrons are more strongly confined in d- or f -orbitals or in man-made nanostructures their mutual Coulomb repulsion is large. Strong correlations between the electrons are the consequence, giving rise to fascinating physics, e.g., superconductivity, quantum criticality, spin entanglement, and Mott transitions. They also make materials functional since colossal responses upon small changes of external parameters such as temperature, magnetic field, and pressure can be achieved. However, the theoretical description of such correlated matter is a big challenge. Karsten Held, who joined the faculty of physics on March 1, 2008, is one of the pioneers in the field of realistic calculations of correlated materials. A major breakthrough was achieved by merging density functional theory (DFT) in its local density approximation with dynamical mean field theory (DMFT) which accounts for the major part of the electronic correlations, i.e., the local ones. This method has been successfully employed to calculate thermodynamic and spectral properties of transition metals, their oxides, and f -electron systems. For the inclusion of long-range correlations which are essential for physical phenomena such as quantum criticality, superconductivity, and magnons an extension of DMFT,the dynamical vertex approximation, has been developed recently. A second research focus of the Held group are quantum dots with an emphasis on decoherence and Kondoesque effects. The numerical methods to track quantum dot systems are intimately connected with those to solve the DMFT equations. In both cases, quantum Monte Carlo simulations, the exact and Lanczos diagonalization as well as the numerical and perturbative renormalization group techniques are employed.
Cooperations with DFT experts of the CompMat consortium (Blaha, Mohn, Redinger) have been started and are funded by the FWF GK computational materials science. A particular subject is the better integration of DFT and DMFT and the developement of standardized program packakages for making this approach available to a wider scientific and industrial community. The microscopic calculation of exchange parameters of magnets with correlated d-electrons and mechanical materials properties offers an excellent opportunity for magnetic multiscaling and micromechanical calculations, opening the door for potential cooperations with the Fidler and Boehm group. Cooperations with the Burgdoerfer, Arnold and Juengel groups on the other hand are planned for quantum structures and quantum dots.