166 research outputs found
Anomalous Angular Dependence of the Dynamic Structure Factor near Bragg Reflections: Graphite
The electron energy-loss function of graphite is studied for momentum
transfers q beyond the first Brillouin zone. We find that near Bragg
reflections the spectra can change drastically for very small variations in q.
The effect is investigated by means of first principle calculations in the
random phase approximation and confirmed by inelastic x-ray scattering
measurements of the dynamic structure factor S(q,\omega). We demonstrate that
this effect is governed by crystal local field effects and the stacking of
graphite. It is traced back to a strong coupling between excitations at small
and large momentum transfers
Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene
We have measured a strictly linear pi-plasmon dispersion along the axis of
individualized single wall carbon nanotubes, which is completely different from
plasmon dispersions of graphite or bundled single wall carbon nanotubes.
Comparative ab initio studies on graphene based systems allow us to reproduce
the different dispersions. This suggests that individualized nanotubes provide
viable experimental access to collective electronic excitations of graphene,
and it validates the use of graphene to understand electronic excitations of
carbon nanotubes. In particular, the calculations reveal that local field
effects (LFE) cause a mixing of electronic transitions, including the 'Dirac
cone', resulting in the observed linear dispersion
The Human Minor Histocompatibility Antigen1 Is a RhoGAP
The human minor Histocompatibility Antigen HMHA-1 is a major target of immune responses after allogeneic stem cell transplantation applied for the treatment of leukemia and solid tumors. The restriction of its expression to hematopoietic cells and many solid tumors raised questions regarding its cellular functions. Sequence analysis of the HMHA-1 encoding HMHA1 protein revealed the presence of a possible C-terminal RhoGTPase Activating Protein (GAP) domain and an N-terminal BAR domain. Rho-family GTPases, including Rac1, Cdc42, and RhoA are key regulators of the actin cytoskeleton and control cell spreading and migration. RhoGTPase activity is under tight control as aberrant signaling can lead to pathology, including inflammation and cancer. Whereas Guanine nucleotide Exchange Factors (GEFs) mediate the exchange of GDP for GTP resulting in RhoGTPase activation, GAPs catalyze the low intrinsic GTPase activity of active RhoGTPases, resulting in inactivation. Here we identify the HMHA1 protein as a novel RhoGAP. We show that HMHA1 constructs, lacking the N-terminal region, negatively regulate the actin cytoskeleton as well as cell spreading. Furthermore, we show that HMHA1 regulates RhoGTPase activity in vitro and in vivo. Finally, we demonstrate that the HMHA1 N-terminal BAR domain is auto-inhibitory as HMHA1 mutants lacking this region, but not full-length HMHA1, showed GAP activity towards RhoGTPases. In conclusion, this study shows that HMHA1 acts as a RhoGAP to regulate GTPase activity, cytoskeletal remodeling and cell spreading, which are crucial functions in normal hematopoietic and cancer cells
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OSL-dating of the Pleistocene-Holocene climatic transition in loess from China, Europe and North America, and evidence for accretionary pedogenesis
Loess deposits intercalated by paleosols are detailed terrestrial archives of Quaternary climate variability providing information on the global dust cycle and landscape dynamics. Their paleoclimatic significance is often explored by quantifying their mineral magnetic properties due to their sensitivity to local/regional hydroclimate variability. Detailed chronological assessment of such regional proxy records around the climatic transitions allow a better understanding of how regional records react to major global climatic transitions such as the Pleistocene-Holocene climatic transition.
Logs of high-resolution magnetic susceptibility and its frequency dependence were used as paleoclimatic proxies to define the environmental transition from the last glacial loess to the current interglacial soil as reflected in nine loess-paleosol sequences across the northern hemisphere, from the Chinese Loess Plateau, the southeastern European loess belt and the central Great Plains, USA. The onset of increase in magnetic susceptibility above typical loess values was used to assess the onset of, and developments during, the Pleistocene-Holocene climatic transition.
High-resolution luminescence dating was applied on multiple grain-sizes (4â11 ÎŒm, 63â90 ÎŒm, 90â125 ÎŒm) of quartz extracts from the same sample in order to investigate the timing of Pleistocene-Holocene climatic transition in the investigated sites.
The magnetic susceptibility signal shows a smooth and gradual increase for the majority of the sites from the typical low loess values to the interglacial ones. The initiation of this increase, interpreted as recording the initiation of the Pleistocene-Holocene climatic transition at each site, was dated to 14â17.5 ka or even earlier. Our chronological results highlight the need of combining paleoclimatic proxies (magnetic susceptibility) with absolute dating when investigating the Pleistocene-Holocene climatic transition as reflected by the evolution of this proxy in order to avoid chronostratigraphic misinterpretations in loess-paleosol records caused by simple pattern correlation.
The detailed luminescence chronologies evidence the continuity of eolian mineral dust accumulation regardless of glacial or interglacial global climatic regimes. Coupled with magnetic susceptibility records this indicates that dust sedimentation and pedogenesis act simultaneously and result in a non-negligible accretional component in the formation of Holocene soils in loess regions across the Northern Hemisphere. The luminescence ages allowed the modeling of accumulation rates for the Holocene soil which are similar for European, Chinese and U.S.A. loess sites investigated and vary from 2 cm kaâ1 to 9 cm kaâ1. While accretional pedogenesis has often been implicitly or explicitly assumed in paleoclimatic interpretation of loess-paleosol sequences, especially in the Chinese Loess Plateau, our luminescence data add direct evidence for ongoing sedimentation as interglacial soils formed
What do dust sinks tell us about their sources and past environmental dynamics? A case study for oxygen isotope stages 3â2 in the Middle Rhine Valley, Germany
The study of geological archives of dust is of great relevance as they are
directly linked to past atmospheric circulation and bear the potential to
reconstruct dust provenance and flux relative to climate changes. Among the
dust sinks, loessâpalaeosol sequences (LPSs) represent the only continental
and non-aquatic archives that are predominantly built up by dust deposits
close to source areas, providing detailed information on Quaternary climatic
and terrestrial environmental changes. Upper Pleistocene LPSs of western
central Europe have been investigated in great detail showing their linkage
to millennial-scale northern hemispheric climate oscillations, but
comprehensive data on dust composition and potential sourceâsink
relationships as well as inferred past atmospheric circulation patterns for
this region are still fragmentary.
Here, we present an integrative approach that systematically combines
sedimentological, rock magnetic, and bulk geochemical data, as well as information
on Sr and Nd isotope composition, enabling a synthetic interpretation of LPS formation. We focus on the Schwalbenberg RP1 profile in the Middle Rhine
Valley in Germany and integrate our data into a robust age model that has
recently been established based on high-resolution radiocarbon dating of
earthworm calcite granules. We show that Schwalbenberg RP1 is subdivided
into a lower section corresponding to late oxygen isotope stage 3 (OIS; âŒâ40â30âka)
and an upper section dating into the Last Glacial Maximum (LGM; âŒâ24â22âka),
separated by a major stratigraphic unconformity. Sedimentological proxies of
wind dynamics (UÂ ratio) and pedogenesis (finest clay) of the lower section
attest to comparable and largely synchronous patterns of northern
hemispheric climatic changes supporting the overall synchronicity of
climatic changes in and around the North Atlantic region. The anisotropy of
magnetic susceptibility (AMS) reveals a clear correlation between finer
grain size and increasing AMS foliation within interstadials, possibly owing
to continuous accumulation of dust during pedogenic phases. Such a clear
negative correlation has so far not been described for any LPS on
stadialâinterstadial scales.
Distinct shifts in several proxy data supported by changes in isotope
composition (87Sr/86Sr and ΔNd) within the lower
section are interpreted as changes in provenance and decreasing weathering
simultaneously with an overall cooling and aridification towards the end of
OIS 3 (after âŒâ35âka) and enhanced wind activity with
significant input of coarse-grained material recycled from local sources
related to increased landscape instability (after âŒâ31.5âka).
We find that environmental conditions within the upper section, most likely
dominated by local to regional environmental signals, significantly differ
from those in the lower section. In addition, AMS-based reconstructions of
near-surface wind trends may indicate the influence of north-easterly winds
beside the overall dominance of westerlies. The integrative approach
contributes to a more comprehensive understanding of LPS formation including
changes in dust composition and associated circulation patterns during
Quaternary climate changes.</p
Effect of Polypropylene fibres on the Workability parameters of Extrudable Cementitious Material
Additive manufacturing in construction industry has been introduced as an aspiration for a more sustainable built environment and currently evolving with high demand amongst researches. This study is an investigation of the influence of polypropylene (PP) fibre addition on the workability parameters of a new extrudable concrete mixture. As the quality of final printed structure prominently depends on the fresh state properties of concrete, this investigation mainly focused on the rheological properties such as workability (flow), setting time, extrudability and buildability. These parameters were systematically investigated through a small scale experimental process with time after mixing. The selected control mix with Ground granulated blast furnace slag (GGBS) and Silica Fume (SF) was used in this analysis. The Control cementitious specimens without fibre inclusion and with fibre addition in different volume fraction of binder, ranging from 0.5% to 3% were printed. The results showed that the fibre addition of 0, 0.5 and 1.0% have the better flowability and extrudability compared to 1.5, 2 and 3%. Also, reduction in the print quality was assessed visually with increasing fibre percentage. However, results indicated that the initial setting time is comparatively low for those mixes with higher fibre inclusion which is required for better bond strength between layers. Moreover, higher fibre content caused better buildability and shape retention in the extruded samples
Approaches and challenges to the study of loessâIntroduction to the LoessFest Special Issue
In September 2016, the annual meeting of the International Union for Quaternary Research's Loess and Pedostratigraphy Focus Group, traditionally referred to as a LoessFest, met in Eau Claire, Wisconsin, USA. The 2016 LoessFest focused on thin loess deposits and loess transportation surfaces. This LoessFest included 75 registered participants from 10 countries. Almost half of the participants were from outside the United States, and 18 of the participants were students. This review is the introduction to the special issue for Quaternary Research that originated from presentations and discussions at the 2016 LoessFest. This introduction highlights current understanding and ongoing work on loess in various regions of the world and provides brief summaries of some of the current approaches/strategies used to study loess deposits
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