Theory of Condensed Matter: Hard Condensed Matter

Programm für das Wintersemester 2019/2020

Tuesdays, 10:00 Uhr s.t.

Ort: Institut für Physik, SPICE, Seminar room K

18.12.19Joel Moore, University of California, Berkeley, and Lawrence Berkeley National Laboratory
This talk starts by reviewing known examples of how topological materials generate new kinds of electrodynamic couplings and effects. Three-dimensional topological insulators realize a particular electromagnetic coupling known as “axion electrodynamics”, and understanding this leads to an improved understanding of magnetoelectricity in all materials. We then turn to how topological Weyl and Dirac semimetals can show unique electromagnetic responses; we argue that in linear response the main observable effect solves an old problem via the orbital moment of Bloch electrons, and how in nonlinear optics there should be a new quantized effect, which may have been seen experimentally. This nonlinear effect has a natural quantum e^3/h^2 and appears in chiral Weyl semimetals over a finite range of frequencies. We discuss interaction and disorder corrections to nonlinear responses in closing. Work with F. de Juan, A. Grushin, T. Morimoto, D. Parker, J. Orenstein, C. Felser, T. Torchinsky, and others.

21.01.20Souvik Paul, Christian-Albrechts-Universität zu Kiel
Magnetic skyrmions, localized spin structure with topological protection, have become a research hotspot as they show promise for future memory and logic devices. The key challenges for applications are to achieve small bits and stability of those skyrmionic bits. Research shows that transition-metal interfaces (TMI) and multilayers are a very promising class of systems to realize nanometer-sized and stable magnetic skyrmions. Therefore, a lot of effort has been put to tailor the properties of these systems for application. In this direction, using first-principles methods, we have proposed ultrathin films, Fe/Rh and Rh/Fe bilayers on Re(0001) substrate, which show various spin structures at the interface including isolated skyrmions, depending on the stacking order of Fe/Rh and Rh/Fe bilayers. This study would encourage the experimentalist to check our predictions and would generate more investigations on other bilayers on Re(0001). The other topic I would focus on is the effect of higher-order exchange interactions (HOI) on the stability of skyrmions. HOI are shown to stabilize magnetic ground states in transition-metal ultrathin films, however, their role on the stability of metastable skyrmions has note been investigated yet. We showed that the HOI increase the stability of skyrmions by a large amount at TMI. This study opens up a route to tune skyrmions stability and lifetime in ultrathin films. [1] Nat. Nanotechnol. 8, 899–911 (2013) [2] Sci. Rep. 4, 6784 (2014) [3] Nat. Commun. 5, 4652 (2014) [4] arXiv:1912.03465 [5] Nat. Phys. 7, 713 (2011) [6] Phys. Rev. Lett. 120, 207201 (2018) [7] Phys. Rev. Lett. 120, 207202 (2018) [8] arXiv:1912.03474

Koordination: Kontakt:

Prof. Dr. Jairo Sinova
Institut für Physik, SPICE

Hjördis Pusch