The recent technological development of THz spectroscopy makes it possible to probe properties of quantum matter, which cannot be observed in equilibrium. This is of considerable interest in the ﬁeld of nematic unconventional superconductors, where controlled probing of the non-equilibrium dynamics yields access to understanding ground state properties of the underlying system.
In the first part of my talk I will discuss the electronic structure of the nematic order in FeSe and discuss the particular role of the non-local dxy orbital nematicity. In addition to a pure dxz /dyz nematic order, known previously, provides a natural explanation for the unusual Fermi surface reconstruction and correctly reproduces the strongly anisotropic momentum dependence of the superconducting gap. We predict a Lifshitz transition of an electron pocket mediated by temperature and sulphur doping, whose feedback on various properties I will discuss by analysing available experimental data.
In the second part of my talk I will discuss the short-time dynamics in such a nematic superconductor with multiple attractive superconducting channels. I will analyze the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) its interaction with nematic order/fluctuations as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. I show that the polarization and intensity of the pulse can be used to control the symmetry of the non-equilibrium state as well as frequencies and relative intensities of the contributions of different collective modes
 Luke C. Rhodes, Jakob Böker, Marvin A. Müller, Matthias Eschrig and Ilya M. Eremin, npj Quantum Mater. 6, 45 (2021)
 S. Bötzel, and Ilya. M. Eremin, Frontiers in Physics, submitted
 Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 100, 140501(R) (2019)
 Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 103, 024519 (2021)
 Marvin A. Müller and Ilya M. Eremin, Phys. Rev. B 104, 144508 (2021).