Dottorato in FISICA

Titolo della tesi: Generalized plasma waves in layered systems and their spectroscopic signatures

In a layered and strongly anisotropic system the hybrid modes provided by the propagation of electromagnetic waves in the matter identify two well separated energy scales connected to a large intra-layer and to a small inter-layer plasma frequency. Despite the wide interest in their detection and manipulation by means of different experimental protocols, ranging from strong-field THz spectroscopy to EELS and RIXS, the physical ingredients underlying a unified description of plasma waves in such systems, valid at arbitrary energy and momentum, have been for long time poorly understood. In this thesis work I will review our recent progresses in the theoretical description of the nature and the spectroscopic signatures of what we named generalized plasma waves in layered systems, with a particular emphasis on copper-oxide (cuprate) superconductors. I will first focus on their general properties, showing that the anisotropy of the current response leads to two intertwined hybrid light-matter modes with mixed longitudinal and transverse character, while a purely longitudinal plasmon is only recovered for wavevectors larger than the crossover scale set in by the plasma-frequency anisotropy. In the second part of the thesis I will discuss how these excitations emerge in different spectroscopies, with particular focus on recent experiments with strong THz field polarized either perpendicular or parallel to the superconducting copper-oxide planes and on EELS spectroscopy measuring the density response at arbitrary energy and momenta in the metallic state.