Every one of us has had the experience of behaving differently in the crowd than when we are on our own. This is also true for electrons, for example, in solids. Just as the l'ola in one stage can only establish itself in a system of many people, there are also effects in the theory of condensed matter that only manifest themselves in a many-body system. From a quantum theoretical point of view, these lead to new so-called quasi-particles, which have effective masses and charges that differ considerably from those of the particles on which they are based. There may even be completely new types of quasiparticles that have no equivalent among the elementary particles.
Members of the Theoretical Physics I group have contributed to many aspects of many-particle physics:
For example, the extreme increase of the effective mass of highly correlated electrons in f-bands has been investigated, which leads to the localization of the charge carriers, analogous to the small polaron. For the calculation of these materials called heavy fermion systems, conventional LDA programs are combined with special approaches for the self-energy of the highly correlated f-electrons within the framework of renormalized band structure calculations.
Motivated by the crystal structure of the heavy-fermion compound LiV2O4 (see figure), another exciting phenomenon has been theoretically predicted: quasi-particles with a half-integer effective charge in frustrated systems. In the borderline case of large mutual repulsion, the Verwey-Anderson tetrahedron rule applies in such systems, which states that only two electrons per tetrahedron occupying the corner places form a half-filled band. In this band, however, the motion of the electrons is so highly correlated by the above rule that it effectively appears to be the motion of quasiparticles with a half-integer charge.
Of particular interest in connection with nanostructured, amorphous and organic semiconductors is the description of their transport properties, e.g. by means of non-equilibrium green functions.
In addition, we deal with fundamental questions of density functional theory and the further development of numerical methods in many-particle physics.
If you have questions or are interested in a bachelor, master, or doctoral thesis in this field, please contact Prof. Dr. Erich Runge.