Archiv 2019

Raum: Seminar Room I, EG.076

Light-field-driven currents in graphene

Dirac fermions and flat bands in the ideal kagome metal FeSn

Signatures of the quantum vacuum

MPSD Seminar
The quantum vacuum is one of the most counter-intuitive concepts of quantum electrodynamics. Whereas the classical vacuum refers to a region of space that is devoid of any particles or fields, its quantum counterpart contains fluctuating electromagnetic fields even in the most idealised case. As predicted by macroscopic quantum electrodynamics, the structure of these virtual photons can be significantly altered by the presence of magnetodielectric bodies or media. The signature of the quantum vacuum is manifest in the interaction of virtual photons with charged matter. [mehr]

Symmetries in high harmonic generation and their application to novel ultrafast spectroscopies

MPSD Seminar
The analysis of symmetries and their associated selection rules is extremely useful in many fields of science. The field of nonlinear optics is no exception. In the early days of nonlinear optics, symmetries were used to derive whether particular nonlinear optical processes are allowed/forbidden according to the medium’s point-group. This approach is believed to be complete, and is regularly taught in graduate classes. [mehr]

Two-particle correlation functions for the theoretical description of strongly correlated electrons systems

MPSD Seminar
Green's functions represent one of the most useful tools for the theoretical description of correlated lattice electrons. In particular, the one-particle Green's function contains information about the spectral properties of the system and can be directly compared to (angular resolved) photoemission spectroscopy experiments. However, also two-particle correlations functions provide very interesting insights into the properties of correlated electron systems as they contain crucial information on response functions such as the magnetic susceptibility or the optical conductivity. In my talk, I will present an overview about the physical content as well as the applications of two-particle Green's and vertex functions in frontier condensed matter research. In particular, I will demonstrate how local frequency-dependent vertices can be used to include non-local correlations effects in interacting many-electron systems on top of the local ones of dynamical mean-field theory (DMFT). While these so-called diagrammatic extensions [1] of DMFT have been successfully exploited to describe collective phenomena such as magnetism and superconductivity, their predictive power is still limited by specific inconsistencies between the one- and the two-particle level [2]. In the final part of my talk, I will present possible solutions to these problems [3] which I will address in the framework of my Emmy Noether project at the University of Hamburg. [mehr]

Cooperative valence dynamics in Anderson Lattices observed by resonant inelastic x-ray scattering

MPSD Seminar
In rare earth intermetallics with weakly bound f-electrons and a Kondo energy scale much larger than magnetic exchange interactions or crystal field splittings, the screening of local moments may result in a non-magnetic Fermi liquid ground state [1]. At low temperatures, the quantum fluctuations between magnetic and non-magnetic valence configurations can then acquire a cooperative (lattice) character. On a phenomenological basis, a sound understanding of this Anderson Lattice phenomenon has been achieved. On the other hand, the microscopic description of the coherent coupling between Kondo-screened sites remains an outstanding theoretical challenge [2]. In experiment, the cooperative character of Anderson Lattices has only recently become directly accessible. Momentum-resolved spectroscopies, such as angle-resolved photoemission and inelastic neutron scattering, reveal the emergence of characteristic low-energy quasiparticle dynamics at low temperatures [3]. These methods probe single-particle excitations in the charge and magnetic channels, respectively. By contrast, high-resolution resonant inelastic x-ray scattering (RIXS) experiments couple to both charge and spin degrees of freedom in a non-trivial way and thus provide a more subtle point of view. If calculations of the underlying Kramers-Heisenberg term on a basis of strongly correlated f-electronic bands are achieved, RIXS may unlock unprecedented microscopic insights into the entanglement of local and itinerant charge and magnetic degrees of freedom. This would address a fundamental mechanism of quantum matter, with relevance far beyond lanthanides and actinides. I will review previous spectroscopic investigations of intermediate valence materials, present our recent RIXS results on the archetypal Anderson Lattice compound CePd3, and highlight some ideas for future x-ray scattering studies at 3rd and 4th generation light sources. [mehr]

Probing Topological Matter by «Heating»: From Quantized Circular Dichroism to Tensor Monopoles

MPSD Seminar
The intimate connection between topology and quantum physics has been widely explored in high-energy and solid-state physics, revealing a plethora of remarkable physical phenomena over the years. Building on their universal nature, topological properties are currently studied in an even broader context, ranging from ultracold atomic gases to photonics, where distinct observables and probes offer a novel view on topological quantum matter. [mehr]
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