Multiferroics have attracted strong interest for potential applications where electric fields control magnetic order. The ultimate speed of control via magnetoelectric coupling, however, remains largely unexplored. Here, we report an experiment in which we drove spin dynamics in multiferroic TbMnO3 with an intense few-cycle terahertz (THz) light pulse tuned to resonance with an electromagnon, an electric-dipole active spin excitation. We observed the resulting spin motion using time-resolved resonant soft x-ray diffraction. Our results show that it is possible to directly manipulate atomic-scale magnetic structures with the electric field of light on a sub-picosecond time scale.

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Large-scale online communities rely on computer-mediated communication between participants, enabling them to sustain interactions and exchange on a scale hitherto unknown. Yet, little research has focused on how these online communities sustain themselves and how their interactions are structured. In this paper, we theorize and empirically measure the network exchange patterns of long-duration sustainable online communities. We propose that participation dynamics follow specific forms of social exchange: direct reciprocity, indirect reciprocity, and preferential attachment. We integrate diverse findings about individual participation motivations by identifying how individual behavior manifests in network-level structures of online communities. We studied five online communities over 27 months and analyzed 38,483 interactions using exponential random graph (p*) models and mixed-effects ANCOVA analysis. In a test of competing models, we find that network exchange patterns in online community communication networks are characterized by direct reciprocity and indirect reciprocity patterns and, surprisingly, a tendency away from preferential attachment. Our findings undermine previous explanations that online exchange follows a power law distribution based on people wanting to connect to "popular" others in online communities. Our work contributes to theories of new organizational forms by identifying network exchange patterns that regulate participation and sustain online communities.

Habitat use by juvenile coho salmon (Oncorhynchus kisutch) during spring, summer, and winter was examined in Oregon coastal streams. Coho salmon fry were most abundant in backwater pools during spring. During summer, juvenile coho salmon were more abundant in pools of all types than they were in glides or riffles. During winter, juvenile coho salmon were most abundant in alcoves and beaver ponds. Because of the apparent strong preference for alcove and beaver pond habitat during winter and the rarity of that habitat in coastal streams, we concluded that if spawning escapement is adequate, the production of wild coho salmon smolts in most coho salmon spawning streams on the Oregon Coast is probably limited by the availability of adequate winter habitat.

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

C ompared to traditional organizations, online community leadership processes and how leaders emerge are not well studied. Previous studies of online leadership have often identified leaders as those who administer forums or have high network centrality scores. Although communication in online communities occurs almost exclusively through written words, little research has addressed how the comparative use of language shapes community dynamics. Using participant surveys to identify leading online community members, this study analyzes a year of communication network history and message content to assess whether language use differentiates leaders from other core community participants. We contribute a novel use of textual analysis to develop a model of language use to evaluate the utterances of all participants in the community. We find that beyond communication network position-in terms of formal role, centrality, membership in the core, and boundary spanning-those viewed as leaders by other participants, post a large number of positive, concise posts with simple language familiar to other participants. This research provides a model to study online language use and points to the emergent and shared nature of online community leadership.

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Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS2, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.Among strongly correlated electron systems, superconductors and materials exhibiting metal-insulator transitions are usually characterized by strong electronelectron and electron-phonon couplings [1][2][3]. At thermodynamic equilibrium, the corresponding many-body interactions lead to rich phase diagrams as a function of temperature, pressure or doping. Such compounds also display fascinating out-of-equilibrium physics, in the form of ultra-fast symmetry changes known as photoinduced phase transitions [4][5][6], and occurrence of new, transient states [6][7][8].Charge density wave (CDW) states are broken symmetry states of metals arising from electron-phonon interactions. They are characterized by a periodic modulation of both atomic positions and electron density. The metal-to-CDW phase transition is characterized by the growth of a complex-valued order parameter p = A exp iΦ , which reflects both the amplitude A and the phase Φ of the periodic modulation [3]. A number of photo-induced phase transitions that have been achieved in CDW compounds correspond to a suppression of the CDW order, i.e. a transition between a CDW state and a metallic state free of any structural modulation [5,[9][10][11][12][13][14][15]. Among those, the photo-induced suppression of the CDW state in blue bronze was shown to involve a coherent motion of atoms along the normal coordinates of the CDW amplitude mode [5]. In this case, the amplitude mode allows continuous variations of the modulus of the order parameter |p|, the metallic state corresponding to |p|=0. In the present work, we focus on the photo-induced phase transition between the nearly commensurate (NC) and the incommensurate (I) CDW states in 1T-TaS 2 , which exhibit two distinct order parameters. When thermallyinduced, this first-order phase transition involves a discontinuous change of atomic positions, and a coexistence of NC and I phase domains over a 3 K range [16,17]. It is thus expected that the photo-induced I phase appears through non-coherent atomic motions, by a nucleation/growth process. We report that the photo-induced NC → I phase is completed within 100 ps after laser excitation. At this 100 ps delay, the photo-induced I-CDW phase is found divided into domains with a typical size of 150Å. Its ordering kinet...