# Highlights

Publication Highlights

### Dirac Strings and Magnetic Monopoles in Spin Ice Dy$_2$Ti$_2$O$_7$

While sources of magnetic fields—magnetic monopoles—have so far proven elusive as elementary particles, several scenarios have been proposed recently in condensed matter physics of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin ice state is argued to be well-described by networks of aligned dipoles resembling solenoidal tubes—classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defect looks like a magnetic monopole. We demonstrate, by diffuse neutron scattering, the presence of such strings in the spin ice Dy$_2$Ti$_2$O$_7$. This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction. D. J. P. Morris, D. A. Tennant , S. A. Grigera , B. Klemke , C. Castelnovo , R. Moessner , C. Czternasty , M. Meissner , K. C. Rule , J.-U. Hoffmann , K. Kiefer , S. Gerischer , D. Slobinsky, R. S. Perry Science 326, 411 (2009)
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Publication Highlights

### Generalized Models Reveal Stabilizing Factors in Food Webs

Insights into what stabilizes natural food webs have always been limited by a fundamental dilemma: Studies either need to make unwarranted simplifying assumptions, which undermines their relevance, or only examine few replicates of small food webs, which hampers the robustness of findings. We used generalized modeling to study several billion replicates of food webs with nonlinear interactions and up to 50 species. In this way, first we show that higher variability in link strengths stabilizes food webs only when webs are relatively small, whereas larger webs are instead destabilized. Second, we reveal a new power law describing how food-web stability scales with the number of species and their connectance. Third, we report two universal rules: Food-web stability is enhanced when (i) species at a high trophic level feed on multiple prey species and (ii) species at an intermediate trophic level are fed upon by multiple predator species. T. Gross, L. Rudolf, S.A. Levin, U. Dieckmann Science 325, 747 (2009)
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Publication Highlights

### Germline P Granules Are Liquid Droplets That Localize by Controlled Dissolution/Condensation

In sexually reproducing organisms, embryos specify germ cells, which ultimately generate sperm and eggs. In Caenorhabditis elegans, the first germ cell is established when RNA and protein-rich P granules localize to the posterior of the one-cell embryo. Localization of P granules and their physical nature remain poorly understood. Here we show that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which we used to estimate their viscosity and surface tension. As with other liquids, P granules rapidly dissolved and condensed. Localization occurred by a biased increase in P granule condensation at the posterior. This process reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell. Such phase transitions may represent a fundamental physicochemical mechanism for structuring the cytoplasm. C. P. Brangwynne, C. R. Eckmann, D. S. Courson, A Rybarska, C. Hoege, J. Gharakhani, F.Jülicher, A. A. Hyman Science 324, 1729 (2009)
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Publication Highlights

### Magnetism: Monopoles on the move

Magnetic materials provide a new context for observing magnetic monopoles. Numerical simulations now establish an experimentally measurable signature of their dynamics - one that has in fact already been seen in a spin-ice compound. R. Moessner and P. Schiffer Nature Physics 5, 250 (2009)
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