Identification by Raman spectroscopy of Mg–Fe content of olivine samples after impact at 6kms?1 onto aluminium foil and aerogel: In the laboratory and in Wild-2 cometary samples

AbstractOlivine, (Mg, Fe)2[SiO4], is a common mineral in extraterrestrial materials, whose Mg–Fe content varies from the end-members Forsterite (Mg2SiO4: denoted ‘Fo’) to Fayalite (Fe2SiO4: denoted ‘Fa’), together with minor quantities of Ca, Cr, Mn and Ni. Olivine is readily identified by Raman spectroscopy, and the Mg–Fe content can be obtained by precise measurements of the position of the two strongest Raman peaks. Here we show that this is not only true for pristine and highly crystalline olivine, but also for grains which have undergone high pressure shock processing during hypervelocity impact. We demonstrate that there are subtle changes to the Raman spectra in grains impacted at 6.1kms−1 onto aluminium foil and into low density aerogel. We quantify these changes, and also show that if no correction is made for the impact effects, the Fe:Mg molar ratio of the olivine can be significantly misinterpreted. This study was stimulated by NASA’s Stardust mission to comet 81P/Wild-2, since freshly ejected cometary dust particles were collected (via impact) onto aluminium foil and into aerogel cells at 6.1kms−1 and these samples are being investigated with Raman spectroscopy. We identify the residue in one Stardust impact crater on aluminium foil as arising from an olivine with a composition of Fo97–100

A Two-Dimensional Carbon Semiconductor

We show that patterned defects can be used to disrupt the sub-lattice symmetry of graphene so as to open up a band gap. This way of modifying graphene's electronic structure does not rely on external agencies, the addition of new elements or special boundaries. The method is used to predict a planar, low energy, graphene allotrope with a band gap of 1.2 eV. This defect engineering also allows semiconducting ribbons of carbon to be fabricated within graphene. Linear arrangements of defects lead to naturally embedded ribbons of the semiconducting material in graphene, offering the prospect of two-dimensional circuit logic composed entirely of carbon.Comment: 4 pages, 5 figure

Symmetry-broken dissipative exchange flows in thin-film ferromagnets with in-plane anisotropy

Planar ferromagnetic channels have been shown to theoretically support a long-range ordered and coherently precessing state where the balance between local spin injection at one edge and damping along the channel establishes a dissipative exchange flow, sometimes referred to as a spin superfluid. However, realistic materials exhibit in-plane anisotropy, which breaks the axial symmetry assumed in current theoretical models. Here, we study dissipative exchange flows in a ferromagnet with in-plane anisotropy from a dispersive hydrodynamic perspective. Through the analysis of a boundary value problem for a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic channel can be excited above a spin current threshold that depends on material parameters and the length of the channel. Symmetry-broken dissipative exchange flows display harmonic overtones that redshift the fundamental precessional frequency and lead to a reduced spin pumping efficiency when compared to their symmetric counterpart. Micromagnetic simulations are used to verify that the analytical results are qualitatively accurate, even in the presence of nonlocal dipole fields. Simulations also confirm that dissipative exchange flows can be driven by spin transfer torque in a finite-sized region. These results delineate the important material parameters that must be optimized for the excitation of dissipative exchange flows in realistic systems.Comment: 20 pages, 5 figure

Evolution of the Olympus Mons Caldera, Mars

Extensive high-resolution (15 to 20 m/pixel) coverage of Olympus Mons volcano permits the investigation of the sequence of events associated with the evolution of the nested summit caldera. The sequence of the intra-caldera events is well illustrated by image data collected on orbits 473S and 474S of Viking Orbiter 1. These data cover both the oldest and youngest portions of the caldera floor. The chronology inferred from the observations is presented which in turn can be interpreted in terms of the internal structure of the volcano (i.e., magma chamber depth and the existence of dikes)

Authorship Attribution Using a Neural Network Language Model

In practice, training language models for individual authors is often expensive because of limited data resources. In such cases, Neural Network Language Models (NNLMs), generally outperform the traditional non-parametric N-gram models. Here we investigate the performance of a feed-forward NNLM on an authorship attribution problem, with moderate author set size and relatively limited data. We also consider how the text topics impact performance. Compared with a well-constructed N-gram baseline method with Kneser-Ney smoothing, the proposed method achieves nearly 2:5% reduction in perplexity and increases author classification accuracy by 3:43% on average, given as few as 5 test sentences. The performance is very competitive with the state of the art in terms of accuracy and demand on test data. The source code, preprocessed datasets, a detailed description of the methodology and results are available at https://github.com/zge/authorship-attribution.Comment: Proceedings of the 30th AAAI Conference on Artificial Intelligence (AAAI'16

'Unlicensed' natural killer cells dominate the response to cytomegalovirus infection.

Natural killer (NK) cells expressing inhibitory receptors that bind to self major histocompatibility complex (MHC) class I are 'licensed', or rendered functionally more responsive to stimulation, whereas 'unlicensed' NK cells lacking receptors for self MHC class I are hyporesponsive. Here we show that contrary to the licensing hypothesis, unlicensed NK cells were the main mediators of NK cell-mediated control of mouse cytomegalovirus infection in vivo. Depletion of unlicensed NK cells impaired control of viral titers, but depletion of licensed NK cells did not. The transfer of unlicensed NK cells was more protective than was the transfer of licensed NK cells. Signaling by the tyrosine phosphatase SHP-1 limited the proliferation of licensed NK cells but not that of unlicensed NK cells during infection. Thus, unlicensed NK cells are critical for protection against viral infection