Highlights

The search for departures from standard hydrodynamics in many-body systems has yielded a number of promising leads, especially in low dimension. Researchers at the Max Planck Institute for the Physics of Complex Systems studied one of the simplest classical interacting lattice models, the nearest-neighbour Heisenberg chain, with temperature as tuning parameter. Their numerics expose strikingly different spin dynamics between the antiferromagnet, where it is largely diffusive, and the ferromagnet, where they observe strong evidence either of spin superdiffusion or an extremely slow crossover to diffusion. At low temperatures in the ferromagnet, they observe an extremely long-lived regime of remarkably clean Kardar-Parisi-Zhang (KPZ) scaling (see figure). The anomalous behaviour also governs the equilibration after a quench, and, remarkably, is apparent even at very high temperatures.

A. J. McRoberts, T. Bilitewski, M. Haque, and R. Moessner, Phys. Rev. B 105, L100403 (2022)

An important but challenging task is to treat mesoscopic systems that are coupled to a complex environment at finite temperature. Alexander Eisfeld of the Max Planck Institute for the Physics of Complex Systems and his collaborators have derived a stochastic hierarchy of matrix product states (HOMPS) for non-Markovian dynamics, which is numerically exact and efficient. In this way the exponential complexity of the problem can be reduced to scale polynomially with the number of particles and modes of the environment. An additional feature caused by the stochastic noise is that individual trajectories stay well localized. The validity and efficiency of HOMPS is demonstrated for the spin-boson model and long chains where each site is coupled to a structured, strongly non-Markovian environment.

X. Gao, J. Ren, A. Eisfeld, and Z. Shuai, Phys. Rev. A 105, L030202 (2022)

A warm welcome to Pieter Claeys! Coming to our institute from the University of Cambridge, Pieter establishes the research group "Dynamics of quantum information". The group’s research lies at the interface of condensed matter physics and quantum information, using a variety of theoretical and numerical approaches to study the dynamics of quantum many-body systems. Research topics include the dynamics of entanglement, quantum chaos and thermalization, unitary circuit models, and general aspects of non-equilibrium quantum dynamics. The group will also focus on bridging recent advances in the dynamics of quantum systems and quantum computation.

On May 24, 2022, Prof. Tomaž Prosen from the University of Ljubljana in Slovenia received the "Physik-Preis Dresden" (Dresden Physics Prize), jointly awarded by the TU Dresden and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS). The theoretical physicist receives the award for his outstanding work on quantum mechanical many-body systems, nonequilibrium statistical physics, quantum information, classical chaos and quantum chaos. Tomaž Prosen has authored over 200 publications on this broad range of topics, which have over 7000 citations and are recognized in expert circles worldwide. In 2016, he received a prestigious ERC Advanced Grant. The award ceremony took place at a festive colloquium in the Recknagel Building of the TU Dresden, preceded by a reception. Prof. Carsten Timm, Dean of the Faculty of Physics, gave the welcome address, and Prof. Roderich Moessner of MPI-PKS delivered the laudation. The Physik-Preis Dresden was endowed in 2015 by Dresden physicist Prof. Peter Fulde, the founding director of the MPI-PKS. The prize winners are determined by a joint commission of the TU Dresden and the MPI-PKS. In addition to the central criterion of scientific excellence, it is particularly important for the decision that the work of the award winners is of special significance for the cooperation between the two DRESDEN-concept partners MPI-PKS and TU Dresden and that their connection has been further strengthened in the long term. The 2022 awardee, Prof. Tomaž Prosen, has a wide range of connections to the professorships at the Institute for Theoretical Physics at TU Dresden and at MPI-PKS due to his broad scientific orientation.

Bilateral symmetry of the vertebrate musculoskeletal system is necessary for proper function and its defects are associated with debilitating conditions, such as scoliosis. In a collaboration with biologists from the École polytechnique fédérale de Lausanne (EPFL) in Switzerland, Marko Popović of the Max Planck Institute for Physics of Complex Systems has studied the early stage of body segmentation using zebrafish as a model organism. They discovered that biochemical signalling and transcriptional processes that drive segmentation are not sufficient to explain the precision and symmetry of tissue shapes and sizes. However, they found that surface tension forces of the newly formed segments are a crucial component of the mechanism that is responsible for recovery and maintenance of the symmetric body plan. This discovery highlights the importance of physical interactions for precise and robust development of living beings.
S. R. Naganthan, M. Popovic, and A. C. Oates, Nature 605, 516-521 (2022)

We cordially welcome Christina Kurzthaler at the institute! Christina joins MPI-PKS from Princeton University and establishes the research group "Transport and flows in complex environments“. The group aims to unravel physical phenomena arising in soft and active matter, with emphasis on the role of transport and flows for biological systems and microfluidics. Its research topics range from the hydrodynamics of swimming bacteria and their interactions with their environments to the statistical physics of active transport in porous materials to the motion of colloidal suspensions in microfluidic settings. While the group's work is theoretical, it seeks to establish collaborations with experimentalists of the Dresden research community.

Attractive forces are ubiquitous in nature: they glue very different objects ranging from atomic nuclei, droplets of water, to stars, galaxies, and black holes. Peter Karpov and Francesco Piazza of the Max Planck Institute for the Physics of Complex Systems have now demonstrated that highly tuneable attractive interactions can be engineered artificially using optical cavities, leading to various novel phases of quantum droplets of ultracold atoms. Upon tuning the cavity-mediated interactions it is possible to switch between superfluid droplets and incompressible water-like droplets, as well as to realise crystalline and even more exotic supersolid droplets combining superfluid and solid properties.
P. Karpov and F. Piazza, Phys. Rev. Lett. 128, 103201 (2022)

A fundamental question in biology is how complexes of several molecules can assemble reliably. Tyler Harmon and Frank Jülicher of the Max Planck Institute for the Physics of Complex Systems have now shown that a molecular assembly line can be self-organized by active droplets where it can form spontaneously. This assembly line arranges different assembly steps spatially so that a specific order of assembly is achieved and incorrect assembly is suppressed. They have shown how assembly bands are positioned and controlled and discuss the rate and fidelity of assembly as compared to other assembly scenarios.
T. S. Harmon and F. Jülicher, Phys. Rev. Lett. 128, 108102 (2022)

ERC Consolidator Grant for Jan Brugués

Today, the European Research Council (ERC) announced the winners of its latest  Consolidator Grant competition for ambitious mid-career researchers. Jan Brugués, research group leader both at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG) and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) is one of the 313 laureates who were awarded the 2022 ERC Consolidator Grants. The funding is part of the EU’s Horizon Europe programme, and the winners will receive in total 632 million Euros to tackle big scientific questions. In total, 2,652 applicants submitted proposals and 12% of them will receive the funding. Male and female applicants were equally successful in winning the grants. The future grantees will carry out their projects at universities and research centers across 24 EU Member States and associated countries. This new round of grants will create an estimated 1,900 jobs for postdoctoral fellows, PhD students and other staff at 189 host institutions.
Jan receives the grant for his project “Understanding emergent physical properties of chromatin using synthetic nuclei.” The main goal of this project is to resolve how the physics of molecular-scale activities result in the material properties of chromatin and how those contribute to chromatin organization and function. Jan Brugués explains: “With my project, I hope to provide a physical description of the material state of chromatin across different scales and contribute to reveal the basic physical principles that govern nuclear organization and function.”
Congratulations Jan!

The exploration of band topology in crystalline solids has been at the forefront of condensed matter physics for many years in efforts tying together theoretical physics, materials science and powerful spectroscopic techniques. One relatively new avenue is the exploration of band topology in the spin wave, or magnon, excitations of magnetic materials. Spin wave topology is connected to novel magnetic transport properties, little explored symmetries and surface magnetism and offers a new platform to study interactions. One of the main current directions is to find new materials with topologically interesting band structures and this new study does exactly this by establishing the existence of Dirac nodal lines and a nodal plane in the magnons of elemental gadolinium.
Gadolinium is a hexagonal closed packed magnetic metal in which, at around room temperature, the magnetic moments order into a simple ferromagnetic structure. In this new study, a team of experimentalists at Oakridge National Lab in the US together with theory collaborators explored the magnetic excitations of gadolinium in the ordered phase in unprecedented detail using inelastic neutron scattering. They found that gadolinium hosts Dirac magnons in the form of nodal lines extending along the zone corners. The existence of the nodal lines can be seen to arise from the presence of combined spin rotation and crystalline symmetries providing a first experimental example of the importance of such symmetries for band topology. In the vicinity of the nodal lines, the neutron scattering intensity can be seen to wind around the lines from strong to weak and in antiphase between the two bands (as shown in the figure). This intensity signature is a robust prediction connected to the nontrivial topology of these points. From the existence of the nodal lines one may infer the existence of magnon surface states $-$ a challenging target for future experiments.
Furthermore, the nonsymmorphic crystal symmetries together with spin rotation enforce the presence of a degenerate plane of magnon excitations. Just as the nodal lines exhibit winding of the neutron intensity in their vicinity, the nodal plane is linked to a sharp flip in the intensity on paths crossing through the plane. Both the nodal plane and the intensity jump are clearly observed in the data.

Phys. Rev. Lett. 128, 097201 (2022)