Highlights

A. Banerjee, C. A. Bridges, J.-Q. Yan, A. A. Achel, L. Li, M. B. Stone, G. E. Granroth, M. D. Lumsden, Y. Yiu, J. Knolle, S. Bhattacharjee, D. L. Kovrizhin, R. Moessner, D. A. Tennant, D. G. Mandrus & S. E. Nagler, Nature Materials 15, 733 (2016)

Quantum spin liquids (QSLs) are topological states of matter exhibiting remarkable properties such as the capacity to protect quantum information from decoherence. Whereas their featureless ground states have precluded their straightforward experimental identification, excited states are more revealing and particularly interesting owing to the emergence of fundamentally new excitations such as Majorana fermions. Ideal probes of these excitations are inelastic neutron scattering experiments. These we report here for a ruthenium-based material, α-RuCl3, continuing a major search (so far concentrated on iridium materials) for realizations of the celebrated Kitaev honeycomb topological QSL. Our measurements confirm the requisite strong spin–orbit coupling and low-temperature magnetic order matching predictions proximate to the QSL. We find stacking faults, inherent to the highly two-dimensional nature of the material, resolve an outstanding puzzle. Crucially, dynamical response measurements above interlayer energy scales are naturally accounted for in terms of deconfinement physics expected for QSLs. Comparing these with recent dynamical calculations involving gauge flux excitations and Majorana fermions of the pure Kitaev model, we propose the excitation spectrum of α-RuCl3 as a prime candidate for fractionalized Kitaev physics.

We are glad to announce the arrival of Dr. Anne Ersbak Bang Nielsen, who heads the Max Planck Research Group 'Quantum many-body systems' since 1 March 2016.

The Starting Grants are awarded by the European Research Council (ERC) on an annual basis. Applications are open to researchers with two to seven years of experience since completion of their PhD, provided they conduct their project at a research organisation located in one of the EU member states. The grants are valued at up to 1.5 million euros each.

E. Roldán, I. Neri, M. Dörpinghaus, H. Meyr and F. Jülicher Phys. Rev. Lett. 115, 250602 (2015)

We show that the steady-state entropy production rate of a stochastic process is inversely proportional to the minimal time needed to decide on the direction of the arrow of time. Here we apply Wald’s sequential probability ratio test to optimally decide on the direction of time’s arrow in stationary Markov processes. Furthermore, the steady-state entropy production rate can be estimated using mean first-passage times of suitable physical variables. We derive a first-passage time fluctuation theorem which implies that the decision time distributions for correct and wrong decisions are equal. Our results are illustrated by numerical simulations of two simple examples of nonequilibrium processes.

E. Derivery, C. Seum, A. Daeden, S. Loubéry, L. Holtzer, F. Jülicher and M. Gonzalez-Gaitan Nature 528, 280 (2015)

During asymmetric division, fate determinants at the cell cortex segregate unequally into the two daughter cells. It has recently been shown that Sara (Smad anchor for receptor activation) signalling endosomes in the cytoplasm also segregate asymmetrically during asymmetric division. Biased dispatch of Sara endosomes mediates asymmetric Notch/Delta signalling during the asymmetric division of sensory organ precursors in Drosophila1. In flies, this has been generalized to stem cells in the gut and the central nervous system, and, in zebrafish, to neural precursors of the spinal cord. However, the mechanism of asymmetric endosome segregation is not understood. Here we show that the plus-end kinesin motor Klp98A targets Sara endosomes to the central spindle, where they move bidirectionally on an antiparallel array of microtubules. The microtubule depolymerizing kinesin Klp10A and its antagonist Patronin generate central spindle asymmetry. This asymmetric spindle, in turn, polarizes endosome motility, ultimately causing asymmetric endosome dispatch into one daughter cell. We demonstrate this mechanism by inverting the polarity of the central spindle by polar targeting of Patronin using nanobodies (single-domain antibodies). This spindle inversion targets the endosomes to the wrong cell. Our data uncover the molecular and physical mechanism by which organelles localized away from the cellular cortex can be dispatched asymmetrically during asymmetric division.

We welcome Prof. Takashi Oka who joined the institute from University of Tokyo to head the research group 'Nonequilibrium Quantum Matter'. The joint group between the our institute and the neighbouring MPI for Chemical Physics of Solids (MPI- CPfS) was established in order to provide an organisational framework combining the respective expertise of the institutes, in particular enabling a young scientist to benefit from both access to experimental work at MPI-CPfS and the theory environment at MPIPKS.

The Max Planck Fellow Programme promotes cooperation between outstanding university professors and Max Planck Society researchers. The appointment of university professors as Max Planck Fellows is limited to a five-year period with the possibility of a five-year extension and entails the supervision of a small working group at a Max Planck institute. We are very happy to announce that the Max Planck Fellow Group of Prof. Roland Ketzmerick (TU Dresden) has been extended by the Max Planck Society until 2020.

What do soccer and quantum mechanics have in common? Both have surprising twists in store that are difficult to predict. Soccer, however, at least follows some rules that are more or less reliable. As a striker, Jens Hjörleifur Bárdarson controls the ball; as a physicist, he masters the rules of the quantum universe. The 35-year-old researcher at the Max Planck Institute for the Physics of Complex Systems in Dresden studies atomic particles, which display many a tricky move.

We show that the one-particle density matrix $\rho$ can be used to characterize the interaction-driven many-body localization transition in closed fermionic systems. The natural orbitals (the eigenstates of $\rho$ ) are localized in the many-body localized phase and spread out when one enters the delocalized phase, while the occupation spectrum (the set of eigenvalues of $\rho$ ) reveals the distinctive Fock-space structure of the many-body eigenstates, exhibiting a step-like discontinuity in the localized phase. The associated one-particle occupation entropy is small in the localized phase and large in the delocalized phase, with diverging fluctuations at the transition. We analyze the inverse participation ratio of the natural orbitals and find that it is independent of system size in the localized phase. S. Bera, H. Schomerus, F. Heidrich-Meisner, and J. H. Bardarson Phys. Rev. Lett. 115, 046603 (2015)

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)