# Highlights

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)
weiterlesen
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)
weiterlesen
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
weiterlesen
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)
weiterlesen
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)
weiterlesen
Awards and Honors

### Roderich Moessner ist Gottfried Wilhelm Leibniz-Preisträger 2013

Geehrt wurden Moessners Beiträge zur Erforschung von stark wechselwirkenden Quantensysteme. Die DFG begründet ihre Entscheidung: Das Forschungsfeld ist sowohl in der Grundlagenforschung als auch für zukünftige Anwendungen eines der spannendsten Gebiete der modernen Festkörperphysik und stellt besonders für die Theoretische Physik eine enorme Herausforderung dar. Roderich Moessner hat diese Herausforderung vor allem auf dem Gebiet der frustrierten Quantenspinsysteme angenommen, auf dem er zu den weltweit führenden Wissenschaftlern zählt. Er konnte als erster die über 70 Jahre alte Hypothese von der Existenz magnetischer Monopole operationalisieren. Moessner sagte voraus, dass im Spineis magnetische Dipole in magnetische Monopole zerfallen und identifizierte zugleich ein System, in dem dieser Effekt beobachtet werden sollte. Tatsächlich konnte dies nur ein Jahr später von anderen Wissenschaftlern experimentell nachgewiesen werden. Auch Moessners Arbeiten zur Resonating Valence Bond-Phase im Quanten-Dimermodell in magnetisch frustrierten Systemen waren wegweisend.
weiterlesen
Awards and Honors

### Mitgliedschaft in der Akademie der Wissenschaften und der Literatur, Mainz

Prof. Dr. Frank Jülicher wurde zum ordentlichen Mitglied der Mathematisch-naturwissenschaftlichen Klasse der Akademie der Wissenschaften und der Literatur, Mainz gewählt.
weiterlesen
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)
weiterlesen
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)
weiterlesen
Awards and Honors

### German Life Science Award

Dr. Michael Hiller, Gurppenleiter am Max-Planck-Institut für Physik komplexer Systeme und am Max-Planck-Institut für molekulare Zellbiologie und Genetik in Dresden, erhält den German Life Science Awrad. Der Bioinformatiker vergleicht das Erbgut von Menschen, Affen und anderen Tieren, um die Unterschiede zwischen den Arten mit den Abweichungen in ihren Genen zu korrelieren. Seine Ergebnisse könnten auch der Medizin nützen.
weiterlesen