# Theory of many body effects and broken symmetry states in condensed matter

Theoretical work is focused on the effect of electronic correlations on quasiparticle renormalization in f-electron compounds and appearance of broken symmetries.

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.

Quasiparticle interference in non-centrosymmetric heavy fermion superconductor CePt3Si.

(a) Rashba-split normal state Fermi surface (full lines) and node lines of mixed singlet-triplet superconducting gap (broken lines). (b) equal energy surfaces in the superconducting state. (c) predicted QPI spectrum with prominent wave vectors q_i indicated. Backscattering vectors q_4, q_5 are suppressed in QPI due to helical spin structure of Rashba split bands.