Fluctuations and non-hermiticity in nematic materials

Abstract

Order-parameter fluctuations of Fermi liquids are a rich research field from the theoretical andexperimental point of view. Although the seminal zero sound excitation, a sound mode propagating in a three-dimensional fermion liquid, was computed by Landau and measured in experiments with He3 at low temperatures in the 50 and 60's, this field regained relevance with the increasing interest in low-dimensional systems. In this project, we will focus on the study of the dynamical nematic fluctuations of the normal phase of real materials, such as iron-based superconductors or cuprates. Using bosonization techniques, we have recently advanced in the study of nematic fluctuations. In particular, we have found an interesting collective mode structure with topological singularities called "exceptional points", which leave unique experimental signatures in transport and dynamical properties of Fermi liquids. Now, we propose to phenomenologically study these non-hermitian degeneracies on real set ups. Using quantum field theoretical approaches, we will compute the dynamical response of the Ising-nematic order parameter. The effect of collective mode excitations will be studied in detail. The understanding of this degeneracy and of the relation between nematic order and non-hermiticity could help us to have a deeper insight into the intricate phase diagram of these materials. In this sense, the iron-based pnictide superconductor FeSe has a very rich phase diagram with the advantage that it doesn't have magnetic long-range order. Thisfact allows us to focus on the nematic and elastic degrees of freedom in a cleaner way. We expect to compare our results with experimental data recently obtained. (AU)