Highlights

Vedika Khemani, Achilleas Lazarides, Roderich Moessner, and S.L. Sondhi Phys. Rev. Lett. 116, 250401 (2016)

Clean and interacting periodically driven systems are believed to exhibit a single, trivial “infinite-temperature” Floquet-ergodic phase. In contrast, here we show that their disordered Floquet many-body localized counterparts can exhibit distinct ordered phases delineated by sharp transitions. Some of these are analogs of equilibrium states with broken symmetries and topological order, while others—genuinely new to the Floquet problem—are characterized by order and nontrivial periodic dynamics. We illustrate these ideas in driven spin chains with Ising symmetry.

Martin Gerlach, Francesc Font-Clos, and Eduardo G. Altmann, Phys. Rev. X 6, 021009 (2016)

Quantifying the similarity between symbolic sequences is a traditional problem in information theory which requires comparing the frequencies of symbols in different sequences. In numerous modern applications, ranging from DNA over music to texts, the distribution of symbol frequencies is characterized by heavy-tailed distributions (e.g., Zipf’s law). The large number of low-frequency symbols in these distributions poses major difficulties to the estimation of the similarity between sequences; e.g., they hinder an accurate finite-size estimation of entropies. Here, we show analytically how the systematic (bias) and statistical (fluctuations) errors in these estimations depend on the sample size N and on the exponent γ of the heavy-tailed distribution. Our results are valid for the Shannon entropy (α=1), its corresponding similarity measures (e.g., the Jensen-Shanon divergence), and also for measures based on the generalized entropy of order α. For small α’s, including α=1, the errors decay slower than the 1/N decay observed in short-tailed distributions. For α larger than a critical value α*=1+1/γ <= 2, the 1/N decay is recovered. We show the practical significance of our results by quantifying the evolution of the English language over the last two centuries using a complete α spectrum of measures. We find that frequent words change more slowly than less frequent words and that α=2 provides the most robust measure to quantify language change.

See also coverage in Physics Today and Physics.

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.