Talks

Criticality in nature

Hallmarks of criticality, such as power-laws and self-similarity typical for order-disorder phase transitions, have been found in many complex systems in nature ranging from fault networks to biological networks. In particular, biological neuronal systems are often thought to be organized in such a way that they optimize their information processing and storage capabilities. This is supported by studies, which have indicated that neuronal networks in vivo and in vitro can self-organize to a critical state. In this talk, I will critically review some of the evidence for criticality — potentially emerging not in the form of a critical point but as an extended critical region or Griffith’s phase — both from a data point of view and a theoretical modelling point of view. The focus will be on the failure of rock samples and fluid-induced seismicity, such as encountered in enhanced geothermal systems and hydraulic fracturing, as well as information processing and memory consolidation in neuronal systems and the spreading of zoonotic diseases.

Raman scattering from moiré phonons

We develop a theoretical framework for probing moiré phonon modes using Raman spectroscopy, and illustrate it with the example of twisted bilayer graphene (TBG). moiré phonons arise from interlayer sliding motion in twisted 2D materials and correspond to fluctuations of the stacking order in reconstructed moiré superlattices. These include both gapless phason modes and a new set of low-energy optical modes, which are accessible via Raman spectroscopy. We show that the Raman response of TBG exhibits a series of low-frequency peaks that clearly distinguish it from that of decoupled layers. We further examine the role of anharmonic interactions in shaping the phonon linewidths and demonstrate the strong dependence of the Raman spectra on both the twist angle and the polarization of the incident light. Our findings establish Raman spectroscopy as a powerful tool for exploring moiré phonons in a broad class of twisted van der Waals systems.

Atomic monolayers as 2D topological quantum materials

Confining electrons to two dimensions (2D) is known to enhance electronic correlations and promote non-trivial topological phases. Atomic monolayers on semiconductor substrates represent the ultimate 2D limit of such confinement and thus have recently come into focus as "third-generation 2D designer quantum materials", following the examples of graphene and monolayer transition metal dichalcogenides. Here I will focus on atomic monolayers as 2D topological insulators (2D-TIs) which host 1D metallic and spin-polarized edge states as hallmark of the quantum spin Hall (QSH) effect. My examples range from bismuthene (Bi/SiC(0001)) [1-3], the 2D-TI with the largest band gap realized to date, to indenene (In/SiC(0001)), a triangular lattice of In atoms with emergent honeycomb physics [4,5]. Using ARPES as well as STM/STS we have studied their electronic structure and especially their topological edge states, revealing interesting insights into their protection (or loss thereof) against single-particle backscattering. I will also demonstrate how circular dichroism in ARPES can serve as a tool to identify non-trivial topology in the bulk states [6]. Finally, I will discuss the stabilization of these monolayers in ambient conditions via van der Waals capping [7,8], paving the way towards ex situ experiments and the realization of transport devices. [1] Science 357, 287 (2017) [2] Nat. Phys. 16, 47 (2020) [3] Nat. Commun. 13, 3480 (2022) [4] Nat. Commun. 12, 5396 (2021) [5] arXiv:2503.11497 [6] Phys. Rev. Lett. 132, 196401 (2024) [7] Nat. Commun. 15, 1486 (2024) [8] arXiv:2502.01592

Microwave studies of complex systems

A review is given on the microwave studies performed in the Marburg quantum chaos group starting from the very beginning about 1990 up to the shut-down two years ago. This includes test of random matrix theory and periodic orbit theory in chaotic microwave resonators, the emission patterns of distorted dielectric resonators, studies of microwave equivalents of graphene-like structures, or the generation of freak waves in a lab size version of the ocean.

Colloquium

Tunable Matter -- Many More is More Different

In 1972 Phil Andersen articulated the motto of condensed matter physics as “More is different.” However, for most many-body systems the behavior of a trillion bodies is nearly the same as that of a thousand. Here I argue for a class of condensed matter, “tunable matter," in which many more is different. The ultimate example of tunable matter is the brain, whose cognitive capabilities increase as size increases from 302 neurons (C. Elegans) to a million neurons (honeybees) to 100 billion neurons (humans). I propose that tunable matter provides a unifying conceptual framework for understanding not only a wide range of systems that perform biological functions, but also physical systems capable of being trained to develop special collective behaviors without using a processor.

Colloquium

t.b.a.