# Highlights

Publication Highlights

### Avalanche outbreaks emerging in cooperative contagions

During human history the world has witnessed an immense loss of lives caused by infectious diseases. The number of casualties becomes even more concerning the cases of syndemic diseases (cooperative contagion), when two or more diseases co-infect individuals in a host population. For example the 1918 Spanish pandemic killed 20-40 million people mainly because of secondary bacterial infections. Contemporary syndemics that pose a major threat to public health include coinfection of HIV, Hepatitis B, C and TB. Here we modeled pathogens that spread and interact on networks, i.e. contact networks between individuals. These interactions can be cooperative and effectively change the way syndemic diseases spread and proliferate in populations. We showed that cooperation of the spreading infections can cause abrupt unexpected outbreaks at smaller epidemic thresholds, while underlying network can amplify or suppress this effect. W. Cai, L. Chen, F. Ghanbarnejad, and P. Grassberger Nature Physics (2015)
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Publication Highlights

### Scaling and Regeneration of Self-Organized Patterns

Biological patterns generated during development and regeneration often scale with organism size. Some organisms, e.g., flatworms, can regenerate a rescaled body plan from tissue fragments of varying sizes. Inspired by these examples, we introduce a generalization of Turing patterns that is self-organized and self-scaling. A feedback loop involving diffusing expander molecules regulates the reaction rates of a Turing system, thereby adjusting pattern length scales proportional to system size. Our model captures essential features of body plan regeneration in flatworms as observed in experiments. S. Werner, T. Stückemann, M. B. Amigo, J. C. Rink, F. Jülicher, B. M. Friedrich Phys. Rev. Lett. 114, 138101 (2015)
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Publication Highlights

### Active gel physics

The mechanical behaviour of cells is largely controlled by a structure that is fundamentally out of thermodynamic equilibrium: a network of crosslinked filaments subjected to the action of energy-transducing molecular motors. The study of this kind of active system was absent from conventional physics and there was a need for both new theories and new experiments. The field that has emerged in recent years to fill this gap is underpinned by a theory that takes into account the transduction of chemical energy on the molecular scale. This formalism has advanced our understanding of living systems, but it has also had an impact on research in physics per se. Here, we describe this developing field, its relevance to biology, the novelty it conveys to other areas of physics and some of the challenges in store for the future of active gel physics. J. Prost, F. Jülicher, J-F. Joanny Nature Physics 11, 111-117 (2015)
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Publication Highlights

### Lévy walks

Random walk is a fundamental concept with applications ranging from quantum physics to econometrics. Remarkably, one specific model of random walks appears to be ubiquitous across many fields as a tool to analyze transport phenomena in which the dispersal process is faster than dictated by Brownian diffusion. The Lévy-walk model combines two key features, the ability to generate anomalously fast diffusion and a finite velocity of a random walker. Recent results in optics, Hamiltonian chaos, cold atom dynamics, biophysics, and behavioral science demonstrate that this particular type of random walk provides significant insight into complex transport phenomena. This review gives a self-consistent introduction to Lévy walks, surveys their existing applications, including latest advances, and outlines further perspectives. V. Zaburdaev, S. Denisov, and J. Klafter Rev. Mod. Phys. 87, 483 (2015)
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Publication Highlights

### A Doppler effect in embryonic pattern formation

During embryonic development, temporal and spatial cues are coordinated to generate a segmented body axis. In sequentially segmenting animals, the rhythm of segmentation is reported to be controlled by the time scale of genetic oscillations that periodically trigger new segment formation. However, we present real-time measurements of genetic oscillations in zebrafish embryos showing that their time scale is not sufficient to explain the temporal period of segmentation. A second time scale, the rate of tissue shortening, contributes to the period of segmentation through a Doppler effect. This contribution is modulated by a gradual change in the oscillation profile across the tissue. We conclude that the rhythm of segmentation is an emergent property controlled by the time scale of genetic oscillations, the change of oscillation profile, and tissue shortening. Daniele Soroldoni, David J. Jörg, Luis G. Morelli, David L. Richmond, Johannes Schindelin, Frank Jülicher, Andrew C. Oates Science, 11 July 2014
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Publication Highlights

### Extracting information from S-curves of language change

It is well accepted that adoption of innovations are described by S-curves (slow start, accelerating period and slow end). In this paper, we analyse how much information on the dynamics of innovation spreading can be obtained from a quantitative description of S-curves. We focus on the adoption of linguistic innovations for which detailed databases of written texts from the last 200 years allow for an unprecedented statistical precision. Combining data analysis with simulations of simple models (e.g. the Bass dynamics on complex networks), we identify signatures of endogenous and exogenous factors in the S-curves of adoption. We propose a measure to quantify the strength of these factors and three different methods to estimate it from S-curves. We obtain cases in which the exogenous factors are dominant (in the adoption of German orthographic reforms and of one irregular verb) and cases in which endogenous factors are dominant (in the adoption of conventions for romanization of Russian names and in the regularization of most studied verbs). These results show that the shape of S-curve is not universal and contains information on the adoption mechanism. F. Ghanbarnejad, M. Gerlach, J. M. Miotto, and E. G. Altmann J. R. Soc. Interface 11, 20141044 (2014)
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Publication Highlights

### Raman Scattering Signatures of Kitaev Spin Liquids in A$_2$IrO$_3$ Iridates with A=Na or Li

We show how Raman spectroscopy can serve as a valuable tool for diagnosing quantum spin liquids (QSL). We find that the Raman response of the gapless QSL of the Kitaev-Heisenberg model exhibits signatures of spin fractionalization into Majorana fermions, which give rise to a broad signal reflecting their density of states, and Z$_2$ gauge fluxes, which also contribute a sharp feature. We discuss the current experimental situation and explore more generally the effect of breaking the integrability on response functions of Kitaev spin liquids. J. Knolle, Gia-Wei Chern, D.L. Kovrizhin, R. Moessner, and N. B. Perkins Phys. Rev. Lett. 113, 187201 (2014)
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Publication Highlights

### Dynamics of a Two-Dimensional Quantum Spin Liquid: Signatures of Emergent Majorana Fermions and Fluxes

We provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum spin liquid in gapless and gapped phases, as realized in Kitaev’s honeycomb model. We show that there are direct signatures—qualitative and quantitative—of the Majorana fermions and gauge fluxes emerging in this model. These include counterintuitive manifestations of quantum number fractionalization, such as a neutron scattering response with a gap even in the presence of gapless excitations, and a sharp component despite the fractionalization of electron spin. Our analysis identifies new varieties of the venerable x-ray edge problem and explores connections to the physics of quantum quenches. J. Knolle, D. L. Kovrizhin, J.T. Chalker, and R. Moessner Phys. Rev. Lett. 112, 207203 (2014)
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Publication Highlights

### Giant Spin Oscillations in an Ultracold Fermi Sea

Collective behavior in many-body systems is the origin of many fascinating phenomena in nature, ranging from the formation of clouds to magnetic properties of solids. We report on the observation of collective spin dynamics in an ultracold Fermi sea with large spin. As a key result, we observed long-lived and large-amplitude coherent spin oscillations driven by local spin interactions. At ultralow temperatures, Pauli blocking stabilizes the collective behavior, and the Fermi sea behaves as a single entity in spin space. With increasing temperature, we observed a stronger damping associated with particle-hole excitations. Unexpectedly, we found a high-density regime where excited spin configurations are collisionally stabilized. Our results reveal the intriguing interplay between microscopic processes either stimulating or suppressing collective effects in a fermionic many-body system. J.S. Krauser, U. Ebling, N. Fläschner, J. Heinze, K. Sengstock, M. Lewenstein, A. Eckardt, and C. Becker, Science 343, 157 (2014)
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Publication Highlights

### Cell-body rocking is a dominant mechanism for flagellar synchronization in a swimming alga

The eukaryotic flagellum is a best-seller of nature: These slender cell appendages propel sperm and many other microswimmers, including disease-causing protists. In mammalian airways or the oviduct, collections of flagella beat in synchrony to pump fluids efficiently. Flagellar synchronization was proposed to rely on mechanical feedback by hydrodynamic forces, but the details are not well understood. Here, we used theory and experiment to elucidate a mechanism of synchronization in the model organism Chlamydomonas, a green algal cell that swims with two flagella like a breaststroke swimmer. Our analysis shows how synchronization arises by a coupling of swimming and flagellar beating and characterizes an exemplary force–velocity relationship of the flagellar beat. Veikko F. Geyer, Frank Jülicher, Jonathon Howard, and Benjamin M. Friedrich PNAS 110 (45), 18058 (2013)
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