ALBERT

Beschreibung: Author(s): L. Z. Deng, B. Lv, Z. Wu, Y. Y. Xue, W. H. Zhang, F. S. Li, L. L. Wang, X. C. Ma, Q. K. Xue, and C. W. Chu We report a detailed investigation on the superconductivity in eight 1–4 unit-cell (UC) FeSe films on SrTiO 3 substrates by measuring their magnetization and sheet resistance in a field between 5×10 −2 and 7×10 4  Oe over the last one and a half years as a function of temperature and frequency, from 2 t... [Phys. Rev. B 90, 214513] Published Thu Dec 11, 2014

Beschreibung: The Pliocene Model Intercomparison Project (PlioMIP) is the first coordinated climate model comparison for a warmer palaeoclimate with atmospheric CO2 significantly higher than pre-industrial concentrations. The simulations of the mid-Pliocene warm period show global warming of between 1.8 and 3.6 C above pre-industrial surface air temperatures, with significant polar amplification. Here we perform energy balance calculations on all eight of the coupled ocean-atmosphere simulations within PlioMIP Experiment 2 to evaluate the causes of the increased temperatures and differences between the models. In the tropics simulated warming is dominated by greenhouse gas increases, with the cloud component of planetary albedo enhancing the warming in most of the models, but by widely varying amounts. The responses to mid-Pliocene climate forcing in the Northern Hemisphere midlatitudes are substantially different between the climate models, with the only consistent response being a warming due to increased greenhouse gases. In the high latitudes all the energy balance components become important, but the dominant warming influence comes from the clear sky albedo, only partially offset by the increases in the cooling impact of cloud albedo. This demonstrates the importance of specified ice sheet and high latitude vegetation boundary conditions and simulated sea ice and snow albedo feedbacks. The largest components in the overall uncertainty are associated with clouds in the tropics and polar clear sky albedo, particularly in sea ice regions. These simulations show that albedo feedbacks, particularly those of sea ice and ice sheets, provide the most significant enhancements to high latitude warming in the Pliocene.

Beschreibung: We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than approximately 400nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Beschreibung: Mercury's regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury's exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury's regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury's regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury's regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury's dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury's relatively featureless visible-near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size less than 45 micron) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury's surface composition.

Beschreibung: Author(s): Guoqing Wu, F. Zamborszky, A. P. Reyes, P. L. Kuhns, R. L. Greene, and W. G. Clark We report 63,65 Cu-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-T c superconductor Pr 1.85 Ce 0.15 CuO 4−y with an applied magnetic field (H) up to 26.42 T. A very small NQR frequency is obtained with the observation of the spectrum, which shows an extremely... [Phys. Rev. B 90, 214506] Published Thu Dec 04, 2014

Beschreibung: Author(s): I. Zaytseva, O. Abal'oshev, P. Dłużewski, W. Paszkowicz, L. Y. Zhu, C. L. Chien, M. Kończykowski, and Marta Z. Cieplak Detailed structural and transport characterization of ultrathin niobium films, as thin as 1.1 nm, reveals a change of the Hall coefficient sign from positive to negative in the thinnest films. This finding may have implications for understanding of disappearance of superconductivity in the ultrathin film limit. [Phys. Rev. B 90, 060505] Published Fri Aug 22, 2014

Beschreibung: Author(s): Yu. G. Naidyuk, O. E. Kvitnitskaya, N. V. Gamayunova, L. Boeri, S. Aswartham, S. Wurmehl, B. Büchner, D. V. Efremov, G. Fuchs, and S.-L. Drechsler We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab initio local density approximation–based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivativ... [Phys. Rev. B 90, 094505] Published Thu Sep 11, 2014

Beschreibung: Author(s): L. Simonelli, T. Mizokawa, M. Moretti Sala, H. Takeya, Y. Mizuguchi, Y. Takano, G. Garbarino, G. Monaco, and N. L. Saini We have studied local magnetic moment and electronic phase separation in superconducting K x Fe 2−y Se 2 by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properti... [Phys. Rev. B 90, 214516] Published Wed Dec 17, 2014

Beschreibung: Author(s): H. P. Wang, Y. L. Sun, X. B. Wang, Y. Huang, T. Dong, R. Y. Chen, G. H. Cao, and N. L. Wang Ba 2 Ti 2 Fe 2 As 4 O is a newly discovered superconductor with a crystal structure that could be viewed as an intergrowth of BaFe 2 As 2 and BaTi 2 As 2 O. Although neither BaFe 2 As 2 nor BaTi 2 As 2 O is superconducting, the combined structure exhibits superconductivity at T c =21.5 K without doping. This work presents the first optical spectroscopy study on single-crystalline samples of Ba 2 Ti 2 Fe 2 As 4 O showcasing valuable information on the charge-dynamical properties across the superconducting and the as of yet unspecified density-wave phase transition. [Phys. Rev. B 90, 144508] Published Mon Oct 13, 2014

Beschreibung: Author(s): S.-F. Wu, P. Richard, W.-L. Zhang, C.-S. Lian, Y.-L. Sun, G.-H. Cao, J.-T. Wang, and H. Ding We have performed polarized Raman scattering measurements on the newly discovered superconductor Ba2Ti2Fe2As4O (Tc=21 K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principles calculations. The phonon spectra suggest neither strong electron-phono... [Phys. Rev. B 89, 134522] Published Wed Apr 30, 2014