Talks

Symmetry constraints of band topology

I will give an overview of different approaches to deduce the topology of gaps, point crossings, and nodal planes from crystalline symmetries. Nontrivial gap topology arises from band inversions, and thus usually depends on the details of a given material. Yet, we find and classify space groups that enforce a nontrivial weak topology on subsets of the Brillouin zone [1]. Rotational symmetries characterize chiral band crossings, i.e., two or more degenerate bands that carry non-zero Chern numbers [2]. Using this relation between topology rotation eigenvalues we can explain the chiralities of quadruple Weyl points. Further, we identify the possibility of a band with Chern number 5 within a fourfold point crossing and propose BaAsPt as a candidate material. Beyond zero-dimensional point crossings, two-dimensional nodal planes can also be enforced by symmetry. Their topology is deducible from symmetries and their eigenvalues [3]. We relate the anomalous Hall effect in the noncollinear magnet CoNb3S6 to a gapped topological nodal plane [4]. [1] K. Parshukov et al., arXiv:2408.00042 (2024) [2] K. Alpin, et al., Phys. Rev. Research 5, 043165 (2023) [3] M.A. Wilde, et al., Nature 594 (7863), 374-379 (2021) [4] N.D. Khanh et al., arXiv:2403.01113 (accepted in Nat.Comm.)

IMPRS Seminar: Serial diffractive imaging to study nanoscale structural dynamics

X-ray free electron lasers provide a unique opportunity to measure ultrafast, high resolution dynamics using diffractive imaging methods. In particular, I will focus on measurements on ensembles of nanosystems where the high peak brightness enables a serial (one-at-a-time) imaging approach which, when combined with modern machine learning analysis algorithms, provides much richer information than conventional ensemble measurements. I will talk about three problems where we use this method to observe hitherto unseen properties of nanosystems, (i) large polaron formation in CsPbBr3 quantum dots, (ii) dehydration dynamics on MS2 bacteriophage capsids and (iii) plasmon-induced structural deformations of gold nanoparticles. Finally, I will discuss the possibility of using such an approach to observe ultrafast dynamics where precise optical triggering is not possible.

IMPRS Seminar: Reconstruction of three-dimensional molecular electronic density from coherent diffraction images using Machine Learning

As the three-dimensional (3D) electron density profile recovery technique for a single macro-molecule from an ensemble of its 2D coherent diffraction images, generated using an X-ray free-electron laser (XFEL), I have applied an unsupervised machine learning algorithm namely Generative Adversarial Network (GAN). In this optimization procedure a constantly modifying three-dimensional structure is used to create diffraction images which have similar distribution to the given set of diffraction images. As the training procedure converges this structure followed by a 3D Phase Retrieval step should develop into an equivalent version of the target electronic density profile of the molecule under consideration.

Particle-hole symmetries in condensed matter

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Low-energy excitations in glasses: Universal statistics, localization and the boson peak

Glassy systems exhibit various universal anomalies compared to their crystalline counterparts, manifested in their vibrational, thermodynamic, transport and strongly dissipative properties. At the heart of understanding these phenomena resides the need to quantify glassy disorder, which is self-generated during the non-equilibrium glass formation process, and to identify the emerging elementary excitations. In this talk, I will review recent progress made in relation to this basic problem. Using a combination of theory, computer simulations and experimental data, I will elucidate the statistical and micro-mechanical nature and properties of low-frequency glassy (non-phononic) excitations, including their universal statistics, spatial localization, non-equilibrium history dependence, relations to spatially extended phonons and the emergence of a boson peak.

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IMPRS

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IMPRS Seminar

IMPRS Seminar