This workshop endeavours to capture the fundamental aspects, new developments, and novel theoretical methods in the theory of x-ray spectroscopy. It is not intended for details of computational codes, algorithms or large scale complex systems. The prominence will be on the fundamental ideas behind theory. The workshop will be accessible for a broad audience interested in the deeper aspects and developments of physics applied to x-ray spectroscopy. There will be relevant experimental talks in conjunction with the theory. The emphasis of the workshop is on where to go from now into the future, and how x-ray spectroscopy can help to understand new exotic materials or even help to discover new materials.
Some open questions that the workshop likes to address: What do we currently understand about the spectroscopy and what not? Where lay the major problems? Where does the support of the theory become essential to understand the spectroscopy? How could we use x-rays to measure the Berry phase, topological insulators, Cooper pairing, majorana fermions, Nambu-Goldstone bosons, thermoelectrics, multifunctional materials, or core excited state dynamics, to name a few? How can we link spectral distributions to physical properties, such as by sum rules that are theory-independent?
In x-ray spectroscopy, such as XAS, XMCD, XMLD, XPS, ResPes, Auger, RIXS, NIXS, the core level provides a local probe. How much of the solid around the excited atom is actually probed? Are electron delocalization and hybridization really taken into account? To what extent is a localized core hole excited by x-rays able to measure dispersive phenomena, such as phonons and magnons?
Which new effects could occur from breaking selection rules (e.g., by higher multipole transitions or by intermediate resonance states) or breaking parity symmetry and time-reversal invariance? How much is there to discover with x-ray detected optical activity, and how common are toroidal and anapole moments in nature? What is the role of x-ray birefringence in highly anisotropic materials?
Simple concepts will break down at higher energy and shorter time scales of the light-matter interaction. The angular anisotropy contains crucial information about the electronic charge and magnetization distribution but relies critically on the assumptions about of the angular momentum transfer. Deep core electrons are furthermore highly relativistic, so the Dirac formalism might need to be complemented by QED effects.
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