Highlights of Max Planck Institute for the Physics of Complex Systems https://www.pks.mpg.de/ here are the highlights of Max Planck Institute for the Physics of Complex Systems en_GB Max Planck Institute for the Physics of Complex Systems Sat, 10 Apr 2021 21:27:59 +0200 Sat, 10 Apr 2021 21:27:59 +0200 TYPO3 EXT:news news-658 Tue, 02 Feb 2021 10:38:00 +0100 MPI-PKS kicks off the year with 3 new research groups https://www.pks.mpg.de/research/divisions-and-groups A warm welcome to three new research group leaders at the institute! Pierre Haas joins us from the University of Oxford and heads the research group "Self-Organization of Multicellular Systems". The group is based jointly at the Center for Systems Biology Dresden, the MPI-PKS and the MPI-CBG and will focus on the mechanics of cells and tissues. In particular, Pierre's group is interested in deriving the continuum theories that represent the rich mechanical behavior of tissues during development and thus allow understanding how robust development is compatible with mechanical constraints and biological variability. While the research of the group is theoretical, it will work in close collaboration with experimental groups at the MPI-CBG and beyond. Matt Eiles originally joined MPI-PKS from Purdue University and was a Distinguished PKS Postdoctoral Fellow until December, now heading the new group "Correlations and Transport in Rydberg Matter" associated with the Finite Systems Division. Via the study of Rydberg matter the group aims to answer fundamental questions about atomic structure, low-energy collisions and scattering, and the behavior of ultracold gases, while also raising new questions related to localization, transport, highly correlated systems, quantum chaos, semiclassical dynamics, and quantum simulation. A strong relationship with ongoing experimental work and the flexibility of Rydberg atoms to shed insight into new theoretical inquiries motivate their research. Marko Popovic comes to Dresden from the EPFL Lausanne and establishes the research group "Order and Disorder in Driven Systems" associated with the Biological Physics Division. The group will investigate mechanical and rheological properties of out-of-equilbrium systems, with an emphasis on development of biological tissues and the role of structural disorder. Furthermore, emergence of order in biological systems and its relation to tissue mechanical properties will be of particular interest. Welcome to the institute and have a great start!

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news-659 Mon, 01 Feb 2021 22:52:39 +0100 The aging of protein droplets https://science.sciencemag.org/content/370/6522/1317 Many proteins form small droplets that separate out of the cell’s cytoplasm, just like oil de-mixes from water. Since their discovery, these liquid-like protein droplets have been identified in myriad important biological phenomena ranging from embryonic development over neuro-degeneration to DNA regulation. For instance, the localization of protein droplets in a specific location during early development of a worm is believed to determine which cells will become the sexual organs of the adult worm. In another example, protein condensation into droplets under cell stress is associated with the growth of fibers related to neurodegeneration as seen in ALS (Amyotrophic lateral sclerosis) disease. Observations have shown that the material properties of these protein condensates change with time. However, an appropriate measurement and description of the material properties and their evolution over time was missing so far. Researchers from the MPI of Molecular Cell Biology and Genetics (MPI-CBG) and the MPI for the Physics of Complex Systems (MPI-PKS), together with their colleagues from the TU Dresden, EMBL Heidelberg and the IMBA Vienna have now filled this gap. In their study, recently published in Science, the scientists show that protein droplets exhibit aging behavior in which they change slowly from liquid-like behavior to a more solid-like state. Louise Jawerth, postdoc in the Frank Jülicher and Tony Hyman groups and first author of the publication, explains, “In order to carefully measure and characterize the time-dependent material properties we first developed a new optical trap technique (Jawerth et al physical review letters 2018 121 (25), 258101)”. The former ELBE postdoctoral fellow at the CSBD continues, ”We then found that droplet aging shows a strongly increasing viscosity, which leads to more solid-like behaviour. Our study suggests that the time-dependent material properties arise from unspecific mechanisms such as jamming of molecules.” Frank Jülicher, director at the MPI-PKS and member of the CSBD and the “Physics of Life” (PoL) Cluster of Excellence at the TU Dresden, adds, “Similar aging has been seen in other materials with time-dependent properties such as traditional glass, plastics, rubbers, or common household items like toothpaste or mayonnaise. By establishing a connection to glass forming systems, we open a new window in which we can utilize a wealth of understanding of these other systems to understand protein droplets.” The second supervisor of the study, Anthony Hyman, director at the MPI-CBG and member of the CSBD, summarizes, “The analogy to traditional glasses also suggests that the unspecific interactions resulting in a solid-like state require less energy to dissolve than, for instance, in comparison to a gel comprised of very strong bonds. Furthermore, it may be used in cells as novel stress sensors. Further study on aging in protein droplets may help to answer fundamental biological questions, for example in embryonic development or DNA transcription, and to better understand neurodegenerative diseases. Vice versa, it may also lead to insights into glass-like aging more generally, which is considered one of the big unsolved questions in condensed matter physics.” L. Jawerth et al., Science 370, 1317 (2020) Publication Highlights news-657 Fri, 18 Dec 2020 08:00:51 +0100 Many-Body Delocalization via Emergent Symmetry https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.240401 Many-body localization (MBL) provides a mechanism to avoid thermalization in many-body quantum systems. Here, we show that an emergent symmetry can protect a state from MBL. Specifically, we propose a $Z_2$ symmetric model with nonlocal interactions, which has an analytically known, SU(2) invariant, critical ground state. At large disorder strength, all states at finite energy density are in a glassy MBL phase, while the lowest energy states are not. These do, however, localize when a perturbation destroys the emergent SU(2) symmetry. The model also provides an example of MBL in the presence of nonlocal, disordered interactions that are more structured than a power law. Finally, we show how the protected state can be moved into the bulk of the spectrum. N. S. Srivatsa et al., Phys. Rev. Lett. 125, 240401 (2020).

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news-656 Tue, 15 Dec 2020 10:53:00 +0100 Carlsberg Foundation Young Researcher Fellowship for Anne E. B. Nielsen https://www.carlsbergfondet.dk/en Anne E. B. Nielsen receives 670 thousand euro from the Carlsberg Foundation to search for and investigate new types of non-thermal behaviors in strongly correlated quantum many-body systems. The Carlsberg Foundation Young Researcher Fellowships are three-year grants that allow the grantee to establish an independent research group working on a research topic within natural science, social science, or the humanities.

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Awards and Honors News
news-655 Mon, 14 Dec 2020 10:46:19 +0100 Protein condensates as aging Maxwell fluids https://science.sciencemag.org/content/370/6522/1317 Cells organize their biochemistry by forming liquid-like condensates of biomolecules that can act as tiny reactors to localize chemical reactions. Researchers from the Physics of Complex Systems together with with colleagues of the MPI of molecular cell biology and genetics in Dresden investigate the physical nature and material properties of protein condensates using microrheology techniques. They find that many protein condensates increase their viscosity with time and slow their dynamics, while always remaining soft and liquid-like. This is suggestive of glassy behaviors and could have important implications for cellular dynamics. Jawerth et al. Science 370, 1317 (2020)

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news-654 Fri, 11 Dec 2020 09:17:05 +0100 The surprising mise en abyme of a Rydberg molecule https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.123401 We predict a new class of Rydberg molecules, comprised of a Rydberg atom and a ground state atom, whose energy spectrum conforms to a certain mise en abyme: nested within successive levels of the infinite electronic Rydberg series lie several finite vibrational Rydberg series. These intertwined series arise from Coulomb potentials in the electronic and nuclear degrees of freedom. The emergence of nuclear Coulomb interaction in a Rydberg molecule is surprising because it is otherwise found only in a heavy Bohr atom, consisting of a cation and an anion. We establish a connection between heavy Bohr atoms and Rydberg molecules via a dressed ion-pair model. In this new perspective, the low-energy scattering of the Rydberg electron off of the ground state atom dresses the neutral atom with a fractional negative charge. Coulomb forces between this dressed anion and the Rydberg cation lead to the formation of Rydberg molecules. In the class of Rydberg molecules we predict, this charge is nearly independent of the internuclear distance, yielding a Coulombic potential. Although this property and the effective ion-pair binding mechanism are very unusual, these molecules need not be: our work shows that these are generic, and can be formed from a Rydberg atom and any neutral polarizable object. P. Giannakeas et al., Phys. Rev. Lett. 125, 123401 (2020)

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news-649 Fri, 30 Oct 2020 09:28:15 +0100 Power-law population heterogeneity governs epidemic waves https://www.mpg.de/15962200/heterogenous-population-herd-immunity Differences between individuals reduce the number of infections required for herd immunity In rapidly spreading epidemics such as the current coronavirus pandemic, it is usually expected that a majority of the population will be infected before herd immunity is achieved and the epidemic abates. The estimate of when the threshold for this is reached is usually based on models that assume all individuals in a population are identical. Researchers at the Max Planck Institute for the Physics of Complex Systems in Dresden have used a new model to demonstrate that herd immunity can be achieved at a lower threshold if some individuals are more easily infected than others. J. Neipel et al., PLoS ONE 15(10): e0239678 (2020).

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news-643 Tue, 06 Oct 2020 10:38:00 +0200 New Research Group Correlations and Topology https://www.pks.mpg.de/research/divisions-and-groups A warm welcome to Ashley Cook who joined us from the University of California in Berkeley and heads the new research group on correlations and topology! The research will focus on the effects of correlations and topology in condensed matter systems, with special emphasis on searching for novel phases of matter and exploration of mechanisms for experimental realization of exotic phases of matter. This group aims to accelerate the process of transitioning from introduction of novel phases of matter into the literature to experimental realization and finally applications, merging what have previously been more disparate areas of expertise. While the research group focuses on theoretical condensed matter physics, it will work in close collaboration with experimental groups, in particular those at MPI-CPfS.

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Institute's News
news-642 Mon, 05 Oct 2020 11:04:00 +0200 Call for Distinguished PKS Postdoctoral Fellowship 2021 now open! https://www.pks.mpg.de/fileadmin/user_upload/MPIPKS/Contact/Work_with_us/PKSFellow2021.pdf Application deadline: 25 November 2020. Distinguished PKS postdoctoral fellows appear personally along with the departments and groups on the main research page of the institute and are expected to have at least one year of postdoctoral experience at an institution other than the one at which their PhD was awarded. Applications for this fellowship directly after completion of the PhD might be considered in exceptional cases. Please click on the link- button to see the full advertisement!

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Institute's News
news-640 Thu, 24 Sep 2020 08:24:19 +0200 Interaction-Induced Transparency for Strong-Coupling Polaritons https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.133604 The propagation of light in strongly coupled atomic media takes place through the formation of polaritons—hybrid quasiparticles resulting from a superposition of an atomic and a photonic excitation. Here we consider the propagation under the condition of electromagnetically induced transparency and show that a novel many-body phenomenon can appear due to strong, dissipative interactions between the polaritons. Upon increasing the photon-pump strength, we find a first-order transition between an opaque phase with strongly broadened polaritons and a transparent phase where a long-lived polariton branch with highly tunable occupation emerges. Across this nonequilibrium phase transition, the transparency window is reconstructed via nonlinear interference effects induced by the dissipative polariton interactions. Our predictions are based on a systematic diagrammatic expansion of the nonequilibrium Dyson equations which can be controlled, even in the nonperturbative regime of large single-atom cooperativities, provided the polariton interactions are sufficiently long-ranged. Such a regime can be reached in photonic crystal waveguides thanks to the tunability of interactions, allowing us to observe the interaction-induced transparency transition even at low polariton densities. J. Lang et al., Phys. Rev. Lett. 125, 133604 (2020)

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news-639 Tue, 22 Sep 2020 12:05:00 +0200 ERC Starting grant for Christoph Weber https://erc.europa.eu/news/StG-recipients-2020 The European Research Council (ERC) has selected 436 early-career top researchers from 40 nationalities across Europe in the 2020 'Starting Grant' competition. The prestigious grants enable the best young researchers in Europe to build their own teams and to conduct pioneering research across all disciplines. Among the awardees is Christoph Weber, research group leader at the MPI-PKS and the Center for Systems Biology Dresden. With his project „FuelledLife”, Christoph and his group want to understand how living cells regulate phase separation and to unveil the role of phase separation for the emergence of life. "Living cells rely on the compartmentalisation of thousands of different molecules and their chemical reactions,” Christoph explains. “Remarkably, many of such compartments form by phase separation of polymers and are controlled by sequence-specific interactions but also cellular fuel driving the system away from thermodynamic equilibrium. If we knew how such polymers evolve in time and compartmentalise in multi-component mixtures, we would better understand the role of phase separation in living cells and how prebiotic cells could have emerged at the origin of life. My team and I will develop a theory for phase separation and chemical reactions in multi-component mixtures that are driven by irreversible, fuel-driven reactions. Our theoretical studies will let us understand how living cells regulate phase separation at the origin of life and determine the prerequisites of a protocell to divide, replicate and undergo selection." Christoph will receive 1.5 million Euros over a period of five years. Congratulations to the Weber group!

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Awards and Honors
news-638 Mon, 21 Sep 2020 12:36:17 +0200 Quantum many-body dynamics in two dimensions with artificial neural networks https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.100503 In the last two decades the field of nonequilibrium quantum many-body systems has seen a rapid development driven, in particular, by the remarkable progress in quantum simulators, which provide access to dynamics in quantum matter with an unprecedented control. However, the efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter remains a key challenge for current computational methods especially beyond one spatial dimension. In this work we present a versatile and efficient machine learning inspired approach based on a recently introduced artificial neural network encoding of quantum many-body wave functions. We identify and resolve key challenges for the simulation of real-time evolution, which previously imposed significant limitations on the accurate description of large systems and long-time dynamics. As a concrete example, we study the dynamics of the paradigmatic two-dimensional transverse field Ising model, where we observe collapse and revival oscillations of ferromagnetic order and demonstrate that the reached time scales are comparable to or exceed the capabilities of state-of-the-art tensor network methods. M. Schmitt and M. Heyl, Phys. Rev. Lett. 125, 100503 (2020)

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news-637 Thu, 03 Sep 2020 12:05:00 +0200 European funding to unravel scientific mysteries https://erc.europa.eu/news/StG-recipients-2020 Steffen Rulands receives ERC Starting Grant The prestigious ERC starting grants allow the best young researchers in Europe to build their own teams and to conduct pioneering research across all disciplines. Among this year’s awardees is Steffen Rulands, research group leader at the Max Planck Institute for the Physics of Complex Systems, and the Center for Systems Biology Dresden! With the interdisciplinary project “AHH-OMICS” Steffen and his group aim to apply theoretical methods originally developed in the physics of solids to understand the mechanisms underlying the behaviour of cells during development, regeneration and ageing. Steffen explains, “The recent breakthroughs of single-cell sequencing technologies for the first time give us the opportunity to probe the inner life of cells with unprecedented molecular detail. Biological function, however, relies on how many molecules work together in space and time, which up to now cannot be inferred from these experiments.” The theoretical physicist continues, “It’s like a car, where from detailed knowledge of all car parts we still cannot understand how an engine works. We need methods from statistical physics to do that! The same holds true for the cell – from detailed measurements of molecules we don’t learn about biological function. The theoretical tools that will be developed in my project will bridge this gap.” The ERC project will combine the novel single-cell technologies with methods from statistical and solid-state physics to understand collective processes regulating cellular behaviour. With this conceptually new approach the Rulands group will overcome existing limitations in an emerging technology and pioneer the application of methods from statistical physics to single-cell genomics. Steffen Rulands will receive 1.5 million Euros from the ERC over a period of five years. The European Research Council (ERC) has selected 436 early-career top researchers from 40 nationalities across Europe in their 2020 'Starting Grant' competition. The funding, worth in total €677 million, is part of the EU’s Research and Innovation programme, Horizon 2020. Congratulations to the Rulands group!

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Awards and Honors
news-636 Fri, 28 Aug 2020 10:38:00 +0200 New Research Group at the Center for Systems Biology Dresden starts in January 2021 https://www.mpi-cbg.de/research-groups/current-groups/pierre-haas/research-focus/ Starting in January 2021, Pierre Haas will lead the research group 'Self-organization of Multicellular Systems' at the Center for Systems Biology Dresden. The research of the group will focus on the mechanics of cells and tissues, with a particular interest in deriving the continuum theories that represent the rich mechanical behaviour of tissues during development, and to understand how robust development is compatible with mechanical constraints and variability. While the research of the group is theoretical, it will work in close collaboration with experimental groups at MPI-CBG and beyond.

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Institute's News
news-635 Wed, 19 Aug 2020 10:53:00 +0200 Anne E. B. Nielsen receives the H.C. Ørsted Research Talent Prize 2020 http://hcoersted.dk/h-c-oersted-prisen/prismodtagere/ The H.C. Ørsted Prize and the two H.C. Ørsted Research Talent Prizes are awarded annually to celebrate the Danish physicist and chemist Hans Christian Ørsted's influence on culture, art, thinkers, and scientists all over the world. The prizes are awarded by the H.C. Ørsted Association and Langeland municipality with support from the energy company Ørsted. H.C. Ørsted discovered in 1820 that an electric current produces a magnetic field, and to celebrate the 200 years anniversary of this influential discovery, the prizes are this year awarded to scientists, whose research is related to electromagnetism and its applications in a broad sense. Anne E. B. Nielsen receives the H.C. Ørsted Research Talent Prize, which is donated with a diploma and 10,000 Danish kroner, for her many innovative contributions to the fields of anyon research and quantum light, where she investigates possibilities that arise by combining quantum mechanics and electromagnetism. The prize ceremony took place on H.C. Ørsted's birthday on August 14 in Rudkøbing on the Danish island Langeland, where H.C. Ørsted grew up.

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Awards and Honors
news-634 Fri, 14 Aug 2020 12:16:23 +0200 Realization of an anomalous Floquet topological system with ultracold atoms https://doi.org/10.1038/s41567-020-0949-y Coherent control via periodic modulation, also known as Floquet engineering, has emerged as a powerful experimental method for the realization of novel quantum systems with exotic properties. In particular, it has been employed to study topological phenomena in a variety of different platforms. In driven systems, the topological properties of the quasienergy bands can often be determined by standard topological invariants, such as Chern numbers, which are commonly used in static systems. However, due to the periodic nature of the quasienergy spectrum, this topological description is incomplete and new invariants are required to fully capture the topological properties of these driven settings. Most prominently, there are two-dimensional anomalous Floquet systems that exhibit robust chiral edge modes, despite all Chern numbers being equal to zero. Here we realize such a system with bosonic atoms in a periodically driven honeycomb lattice and infer the complete set of topological invariants from energy gap measurements and local Hall deflections. Wintersperger et al., Nature Physics (2020)

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news-633 Mon, 03 Aug 2020 08:46:22 +0200 Excitonic Laughlin states in ideal topological insulator flat bands and their possible presence in moiré superlattice materials https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.035158 We investigate few- and many-body states in half-filled ideal topological insulator flat bands realized by two degenerate Landau levels which experience opposite magnetic fields. This serves as a toy model of flat bands in moiré materials in which valleys have Chern numbers $C=\pm 1$. We argue that although the spontaneously polarized Ising Chern magnet is a natural ground state for repulsive Coulomb interactions, it can be in reasonable energetic competition with correlated Laughlin states of excitons when short-distance corrections to interactions are included. This is because charge neutral excitons in these bands behave effectively as charged particles in ordinary Landau levels. In particular, the Ising Chern magnet is no longer the ground state once the strength of a short-range intravalley repulsion is about 30% larger than the intervalley repulsion. Remarkably, these excitonic Laughlin states feature valley number fractionalization but no charge fractionalization and a quantized charge Hall conductivity identical to the Ising magnet, $\sigma_{xy}=±e2/h$, and thus cannot be distinguished from it by ordinary charge transport measurements. The Laughlin state with the highest density of excitons that can be constructed in these bands is an analog of $\nu=1/4$ bosonic Laughlin state and has no valley polarization even though it spontaneously breaks time reversal symmetry. N. Stefanidis and I. Sodemann, Phys. Rev. B 102, 035158 (2020)

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news-632 Thu, 30 Jul 2020 11:04:29 +0200 Call for ELBE postdoctoral Fellowships at the Center for Systems Biology Dresden now open! https://www.pks.mpg.de/fileadmin/user_upload/MPIPKS/Contact/Work_with_us/ElbePostDocAd_fall_2020_final.pdf Application deadline: 25 September 2020. The ELBE program seeks outstanding external candidates who are passionate about bringing new ideas, concepts, or systems to the Center. ELBE postdoctoral fellows pursue an interdisciplinary research project in collaboration with members of the CSBD. They are encouraged to develop and use theoretical or computational approaches to study biological systems in close collaboration with experimental groups at the MPI-CBG and the TU Dresden. Ideal candidates should have backgrounds in physics, computer science, mathematics or a related discipline, with a strong interest in working in a cross-disciplinary life-science environment. In some exceptional cases, ELBE fellows can be analytically-minded experimentalists in the field of cell or developmental biology, who work between a lab and a CSBD research group. Please click on the link- button to see the full advertisement!

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Institute's News
news-631 Mon, 13 Jul 2020 11:16:12 +0200 Phase separation provides a mechanism to reduce noise in cells https://science.sciencemag.org/content/367/6476/464.full Expression of proteins inside cells is noisy, causing variability in protein concentration among identical cells. A central problem in cellular control is how cells cope with this inherent noise. Compartmentalization of proteins through phase separation has been suggested as a potential mechanism to reduce noise, but systematic studies to support this idea have been missing. In this study, we used a physical model that links noise in protein concentration to theory of phase separation to show that liquid droplets can effectively reduce noise. We provide experimental support for noise reduction by phase separation using engineered proteins that form liquid-like compartments in mammalian cells. Thus, phase separation can play an important role in biological signal processing and control. Klosin et al., Science 366, 464 (2020) Publication Highlights news-630 Mon, 13 Jul 2020 11:10:56 +0200 Active Forces Shape the Metaphase Spindle through a Mechanical Instability https://doi.org/10.1073/pnas.2002446117 The metaphase spindle is a dynamic structure orchestrating chromosome segregation during cell division. Recently, soft matter approaches have shown that the spindle behaves as an active liquid crystal. Still, it remains unclear how active force generation contributes to its characteristic spindle-like shape. Here we combine theory and experiments to show that molecular motor-driven forces shape the structure through a barreling-type instability. We test our physical model by titrating dynein activity in Xenopusegg extract spindles and quantifying the shape and microtubule orientation. We conclude that spindles are shaped by the interplay between surface tension, nematic elasticity, and motor-driven active forces. Our study reveals how motor proteins can mold liquid crystalline droplets and has implications for the design of active soft materials. Oriola et al., PNAS (2020)

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