# Highlights

Publication Highlights

### Novel quantum phases of droplets

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)
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### Molecular Assembly Lines in Active Droplets

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)
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### Dirac Magnons, Nodal Lines, and Nodal Plane in Elemental Gadolinium

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)
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### Flowing by gelating - gelation enables the correct architecture of the mitotic spindle.

In the cell, the mitotic spindle is a structure that forms during cell division and segregates the chromosomes into the two future daughter cells. Spindles are made of dynamic filaments called microtubules that are continuously transported towards the two opposite poles of the spindle by molecular motors. However, scientists still do not understand how these poleward flows are generated and how they lead to spindle self-organization. Researchers in the group led by Jan Brugués at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) and the Center for Systems Biology Dresden (CSBD), worked together with Frank Jülicher’s research group at the MPI-PKS to understand how such poleward flows are generated in spindles. In 2018, the Brugués lab was able to show that the size of spindles is controlled by microtubule branching: In the vicinity of DNA, new microtubules branch off from mother microtubules like branches in a tree. However, such a branching process naturally leads to microtubules branching outwards from chromosomes, whereas in real spindles they branch inwards to interact with chromosomes during segregation. In the current study, published in the journal Nature Physics, the scientists combined in vitro experiments with physical models to show that the poleward flows together with a gelation process driven by motor crosslinking, allows for the correct microtubule inward branching observed in spindles. Benjamin Dalton an author in the study, explains, “The spindle is a highly dynamic structure where its building blocks are constantly created, transported and destroyed within seconds. Still, the spindle can survive for as long as an hour and the microtubule flows are remarkably constant throughout the structure. It’s difficult to reconcile these things.” David Oriola, another author in the study explains, “By tracking the movement of single microtubules using fluorescent microscopy, we found that the spindle did not behave as a simple fluid, but rather as a gel.” Combining large-scale simulations with experimental data, the researchers found that gelation is necessary for the generation of the poleward flows, and in turn, such flows are in charge of organizing the microtubule network such that microtubules point inwards rather than outwards.

Dalton et al.: A gelation transition enables the self-organization of bipolar metaphase spindles. Nature Physics, 10 February 2022, doi: 10.1038/s41567-021-01467-x
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### Reading DNA is team work

Life starts with one cell. When an organism develops, dividing cells specialize to form the variety of tissues and organs that build up the adult body, while keeping the same genetic material – contained in our DNA. In a process known as transcription, parts of the DNA – the genes ¬– are copied into a messenger molecule -the ribonucleic acid (RNA) – that carries the information needed to produce proteins, the building blocks of life. The parts of our DNA that are read and transcribed determine the fate of our cells. The readers of the DNA are proteins called transcription factors: they bind to specific sites on the DNA and activate the transcription process. How they recognize which location on the DNA they need to bind to and how these are distinguished from other random binding sites in the genome remains an open question. Scientists 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), both located in Dresden, show that thousands of individual transcription factors team up and interact with each other. They collectively wet the DNA surface by forming liquid droplets that can identify clusters of binding sites on the DNA surface.

Jose A. Morin et al: Sequence dependent surface condensation of a pioneer transcription factor on DNA. Nature Physics. 03. February 2022, doi: 10.1038/s41567-021-01462-2
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### How do our organs know when to stop growing?

The smallest fish in the world, the Paedocypris, measures only 7 millimeters. This is nothing compared to the 9 meters of the whale shark. The small fish shares many of the same genes and the same anatomy with the shark, but the dorsal and caudal fins, gills, stomach and heart, are thousands of times smaller! How do organs and tissues of this miniature fish stop growing very quickly, unlike those of their giant cousin? A multidisciplinary team led by scientists from the University of Geneva (UNIGE), Switzerland, and the Max Planck Institute for the Physics of Complex Systems (MPIPKS) was able to answer this fundamental question by studying its physics and using mathematical equations, as revealed by their work published in the journal Nature.

M. R. Michaelidi et al., Nature (2021)
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### Long-Range Photon Fluctuations Enhance Photon-Mediated Electron Pairing and Superconductivity

Recently, the possibility of inducing superconductivity for electrons in two-dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via nonadiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results in a critical temperature which can be more than 1 order of magnitude larger than the BCS prediction. Moreover, a finite average number of photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

A. Chakraborty and F. Piazza, Phys. Rev. Lett. 127, 177002 (2021)
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### Cavity QED with quantum gases: new paradigms in many-body physics

We review the recent developments and the current status in the field of quantum-gas cavity QED. Since the first experimental demonstration of atomic self-ordering in a system composed of a Bose–Einstein condensate coupled to a quantized electromagnetic mode of a high-Q optical cavity, the field has rapidly evolved over the past decade. The composite quantum-gas-cavity systems offer the opportunity to implement, simulate, and experimentally test fundamental solid-state Hamiltonians, as well as to realize non-equilibrium many-body phenomena beyond conventional condensed-matter scenarios. This hinges on the unique possibility to design and control in open quantum environments photon-induced tunable-range interaction potentials for the atoms using tailored pump lasers and dynamic cavity fields. Notable examples range from Hubbard-like models with long-range interactions exhibiting a lattice-supersolid phase, over emergent magnetic orderings and quasicrystalline symmetries, to the appearance of dynamic gauge potentials and non-equilibrium topological phases. Experiments have managed to load spin-polarized as well as spinful quantum gases into various cavity geometries and engineer versatile tunable-range atomic interactions. This led to the experimental observation of spontaneous discrete and continuous symmetry breaking with the appearance of soft-modes as well as supersolidity, density and spin self-ordering, dynamic spin-orbit coupling, and non-equilibrium dynamical self-ordered phases among others. In addition, quantum-gas-cavity setups offer new platforms for quantum-enhanced measurements. In this review, starting from an introduction to basic models, we pedagogically summarize a broad range of theoretical developments and put them in perspective with the current and near future state-of-art experiments.

F. Mivehvar et al. Adv. Phys. 70, 1 (2021)
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### Signatures of Quantum Phase Transitions after Quenches in Quantum Chaotic One-Dimensional Systems

Quantum phase transitions are central to our understanding of why matter at very low temperatures can exhibit starkly different properties upon small changes of microscopic parameters. Accurately locating those transitions is challenging experimentally and theoretically. Here, we show that the antithetic strategy of forcing systems out of equilibrium via sudden quenches provides a route to locate quantum phase transitions. Specifically, we show that such transitions imprint distinctive features in the intermediate-time dynamics, and results after equilibration, of local observables in quantum chaotic spin chains. Furthermore, we show that the effective temperature in the expected thermal-like states after equilibration can exhibit minima in the vicinity of the quantum critical points. We discuss how to test our results in experiments with Rydberg atoms and explore nonequilibrium signatures of quantum critical points in models with topological transitions.

A. Haldar et al. Phys. Rev. X 11, 031062 (2021)
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### Cavity-induced quantum spin liquids

Quantum spin liquids provide paradigmatic examples of highly entangled quantum states of matter. Frustration is the key mechanism to favor spin liquids over more conventional magnetically ordered states. Here we propose to engineer frustration by exploiting the coupling of quantum magnets to the quantized light of an optical cavity. The interplay between the quantum fluctuations of the electro-magnetic field and the strongly correlated electrons results in a tunable long-range interaction between localized spins. This cavity-induced frustration robustly stabilizes spin liquid states, which occupy an extensive region in the phase diagram spanned by the range and strength of the tailored interaction. This occurs even in originally unfrustrated systems, as we showcase for the Heisenberg model on the square lattice.

A. Chiocchetta et al. Nat. Commun. 12, 5901 (2021)
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