research highlights SPECTROMETRY COMPLEX NETWORKS To better understand the structure and Connect the dots Bring the noise dynamics of galaxies, Noam Libeskind and Nature 523, 67–70 (2015) Nature Commun. (in the press); preprint at co-workers use the Cosmicflows-2 survey http://arxiv.org/abs/1408.1168 (2015) of peculiar velocities to map the movement NPG of local galaxies out to 150 parsecs. They The Black Death spread across Europe find a filament of dark matter bridging the leaving few in its wake, so it’s natural to Local Group and the massive Virgo cluster think about it as a wave of devastation. roughly 50 million light years away. It is But spare a thought for how it might have simultaneously being stretched by the travelled, had the afflicted been able to hop Virgo cluster and squeezed by the Local Void on an airplane: long-distance flights foster plus smaller voids from the sides. the appearance of clusters in the spatial This dark matter ‘superhighway’ channels evolution of a contagion, with potentially dwarf galaxies between galaxy clusters. more disastrous consequences. And now, Other studies have revealed that dwarf Dane Taylor and colleagues have used a well- galaxies are often compressed into planes known model to map contagions in space, that are possibly co-rotating. The authors showcasing a technique for predicting — show that four out of the five vast planes of and perhaps controlling — spreading on dwarf galaxies are aligned with the main complex networks. direction of compression (and the direction Many networks are connected by of expansion of the Local Void). This How many scientists would love to have a spatially constrained edges (like roads connection between large-and small-scale small and cheap spectrometer? Jie Bao and on a map, for example), but may also structures will help refine models, such as the Moungi Bawendi have come up with a solution incorporate additional edges connecting prevailing cold dark matter model. MC by integrating almost two-hundred absorptive distant nodes. Taylor et al. cast these long- filter arrays made of colloidal quantum dots range connections as a source of noise RANDOM WALKS and a charge-coupled device detector into a in an otherwise geometric network, and Competitive advantage coin-sized digital camera capable of resolving observed two competing phenomena: Phys. Rev. E 91, 062815 (2015) spectral peaks separated by mere nanometres. wavefront propagation on the underlying Such resolution and compactness may find geometric network, and the formation of Many a statistical physicist looking for applications in medicine or space exploration. clusters due to the noisy edges. Within their stochastic processes to study will have Colloidal quantum dots can be thought model, the team found regimes in which considered competitive team sports and of as artificial atoms whose absorption and the spreading was almost unaffected by the wondered about the dynamics at play, and emission properties can be designed during noise edges — hinting at a possible means of perhaps even thought about how they fabrication by controlling their size and contagion control. AK might best be modelled and understood. composition. This flexibility allowed Bao and Appreciating their complexity is one thing, Bawendi to create 195 unique broadband DWARF GALAXIES but making meaningful insights that filters covering a spectral range of 300 nm. Plane speaking might translate into quantitative predictive The spectrum was efficiently reconstructed by Mon. Not. R. Astron. Soc. 452, 1052–1059 (2015) capability is altogether more challenging. measuring a set of light intensities in parallel Aaron Clauset and colleagues have with an array detector whose elements were Matter, as well as dark matter, is not focused on a simple, yet decisive, each set to one filter. uniformly distributed throughout the characteristic of sporting contests, namely Besides their minimal size and cost, one Universe. There are clumps, filaments and the times in a game when the lead changes. advantage of these spectrometers is the voids, for example, the Local Void — a vast Using random walk theory, they showed that possibility to enhance spectral resolution and emptiness — near our own Local Group the number of lead changes in any individual range simultaneously — simply by adding of galaxies that includes the Milky Way, game follows a Gaussian distribution. more dots. IG Andromeda and many dwarf galaxies. More surprisingly, they also show that the probability that the last lead will change and VALLEYTRONICS the time when the largest lead size occurs follow the same bimodal distribution that Jump across Phys. Rev. Appl. 4, 014002 (2015) diverges at the start and the end of the game. Finally, they evaluated the probability that The electronic band structure of certain semiconductors can contain conically shaped valleys. any given lead is ‘safe’ and maintained until When there are two non-equivalent valleys, such as in graphene, the system has an additional the game is over. quantum number, known as the valley index. In much the same way that the electron spin is Most importantly for sports fans, the used in spintronic devices, valleytronic devices aim to make use of the valley index. authors demonstrated a good agreement Many of the materials that offer this valley degree of freedom are single layers, but how do with an exhaustive body of empirical data, these valley electronic states move between layers? Ryosuke Akashi and colleagues have now particularly in the case of professional shown that the stacking geometry can control how many dimensions the valley states travel in. basketball, where the number of scoring Using monolayers of a typical transition metal dichalcogenide, the authors showed that the events in each game is very high. Sport isn’t motion of valley electronic states was confined to just one of the two-dimensional monolayers random; it’s a random walk. AT for a particular type of stacking, known as 3R stacking. For 2H stacking, on the other hand, the valley states could jump between different monolayers, and travel in three dimensions. LF Written by May Chiao, Luke Fleet, Iulia Georgescu, Abigail Klopper and Andrea Taroni . 620 NATURE PHYSICS | VOL 11 | AUGUST 2015 | www.nature.com/naturephysics © 2015 Macmillan Publishers Limited. All rights reserved.