Theory of many body effects and broken symmetry states in condensed matter
In particular unconventional superconductivity and hidden multipolar order is investigated. Furthermore the collective modes like spin excitons appearing in the gap of the ordered phase are studied. T-matrix scattering theory is used to predict and analyze the STM quasiparticle interference (QPI)spectra which are a powerful tool for determination of order parameter symmetry. Furthermore physical effects of helical Dirac surface states in topological insulators like response functions, transport properties and resonance states from impurity scattering are investigated.