398 research outputs found
Raman scattering evidence for a cascade-like evolution of the charge-density-wave collective amplitude mode
The two-dimensional rare-earth tri-tellurides undergo a unidirectional
charge-density-wave (CDW) transition at high temperature and, for the heaviest
members of the series, a bidirectional one at low temperature. Raman scattering
experiments as a function of temperature on DyTe and on LaTe at 6 GPa
provide a clear-cut evidence for the emergence of the respective collective CDW
amplitude excitations. In the unidirectional CDW phase, we surprisingly
discover that the amplitude mode develops as a succession of two mean-field,
BCS-like transitions in different temperature ranges
Plasmon Evolution and Charge-Density Wave Suppression in Potassium Intercalated Tantalum Diselenide
We have investigated the influence of potassium intercalation on the
formation of the charge-density wave (CDW) instability in 2H-tantalum
diselenide by means of Electron Energy-Loss Spectroscopy and density functional
theory. Our observations are consistent with a filling of the conduction band
as indicated by a substantial decrease of the plasma frequency in experiment
and theory. In addition, elastic scattering clearly points to a destruction of
the CDW upon intercalation as can be seen by a vanishing of the corresponding
superstructures. This is accompanied by a new superstructure, which can be
attributed to the intercalated potassium. Based on the behavior of the c-axis
upon intercalation we argue in favor of interlayer-sites for the alkali-metal
and that the lattice remains in the 2H-modification
Temperature dependence of the excitation spectrum in the charge-density-wave ErTe and HoTe systems
We provide optical reflectivity data collected over a broad spectral range
and as a function of temperature on the ErTe and HoTe materials, which
undergo two consecutive charge-density-wave (CDW) phase transitions at
= 265 and 288 K and at = 157 and 110 K, respectively. We
observe the temperature dependence of both the Drude component, due to the
itinerant charge carriers, and the single-particle peak, ascribed to the
charge-density-wave gap excitation. The CDW gap progressively opens while the
metallic component gets narrow with decreasing temperature. An important
fraction of the whole Fermi surface seems to be affected by the CDW phase
transitions. It turns out that the temperature and the previously investigated
pressure dependence of the most relevant CDW parameters share several common
features and behaviors. Particularly, the order parameter of the CDW state is
in general agreement with the predictions of the BCS theory
Evidence for coupling between collective state and phonons in two-dimensional charge-density-wave systems
We report on a Raman scattering investigation of the charge-density-wave
(CDW), quasi two-dimensional rare-earth tri-tellurides Te (= La, Ce,
Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe and CeTe under
externally applied pressure. The observed phonon peaks can be ascribed to the
Raman active modes for both the undistorted as well as the distorted lattice in
the CDW state by means of a first principles calculation. The latter also
predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition.
The integrated intensity of the two most prominent modes scales as a
characteristic power of the CDW-gap amplitude upon compressing the lattice,
which provides clear evidence for the tight coupling between the CDW condensate
and the vibrational modes
Time-series analysis of two hydrothermal plumes at 9°50′N East Pacific Rise reveals distinct, heterogeneous bacterial populations
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geobiology 10 (2012): 178-192, doi:10.1111/j.1472-4669.2011.00315.xWe deployed sediment traps adjacent to two active hydrothermal vents at 9°50’N on the
East Pacific Rise (EPR) to assess variability in bacterial community structure associated with
plume particles on the time scale of weeks to months, to determine if an endemic population of
plume microbes exists, and to establish ecological relationships between bacterial populations
and vent chemistry. Automated rRNA intergenic spacer analysis (ARISA) indicated there are
separate communities at the two different vents and temporal community variations between
each vent. Correlation analysis between chemistry and microbiology indicated that shifts in the coarse particulate (>1 mm) Fe/(Fe+Mn+Al), Cu, V, Ca, Al, 232Th, and Ti as well as fine-grained
particulate (<1 mm) Fe/(Fe+Mn+Al), Fe, Ca and Co are reflected in shifts in microbial
populations. 16S rRNA clone libraries from each trap at three time points revealed a high
percentage of Epsilonproteobacteria clones and hyperthermophilic Aquificae. There is a shift
towards the end of the experiment to more Gammaproteobacteria and Alphaproteobacteria, many
of whom likely participate in Fe and S cycling. The particle attached plume environment is
genetically distinct from the surrounding seawater. While work to date in hydrothermal
environments has focused on determining the microbial communities on hydrothermal chimneys
and the basaltic lavas that form the surrounding seafloor, little comparable data exists on the
plume environment that physically and chemically connects them. By employing sediment traps
for a time series approach to sampling, we show that bacterial community composition on plume
particles changes on time scales much shorter than previously known.This work was supported by the NSF Marine
Geology and Geophysics program, the Science and Technology program, and the Gordon and
Betty Moore Foundation
Coupled Electronic and Nuclear Motions during Azobenzene Photoisomerization Monitored by Ultrafast Electron Diffraction
Ultrafast electron diffraction is a powerful technique that can resolve molecular structures with femtosecond and angstrom resolutions. We demonstrate theoretically how it can be used to monitor conical intersection dynamics in molecules. Specific contributions to the signal are identified which vanish in the absence of vibronic coherence and offer a direct window into conical intersection paths. A special focus is on hybrid scattering from nuclei and electrons, a process that is unique to electron (rather than X-ray) diffraction and monitors the strongly coupled nuclear and electronic motions in the vicinity of conical intersections. An application is made to the cis to trans isomerization of azobenzene, computed with exact quantum dynamics wavepacket propagation in a reactive two-dimensional nuclear space
X-ray Absorption Linear Dichroism at the Ti K-edge of TiO2 anatase single crystal
Anatase TiO2 (a-TiO2) exhibits a strong X-ray absorption linear dichroism
with the X-ray incidence angle in the pre-edge, the XANES and the EXAFS at the
titanium K-edge. In the pre-edge region the behaviour of the A1-A3 and B peaks,
originating from the 1s-3d transitions, is due to the strong -orbital
polarization and strong orbital mixing. An unambiguous assignment of the
pre-edge peak transitions is made in the monoelectronic approximation with the
support of ab initio finite difference method calculations and spherical tensor
analysis in quantitative agreement with the experiment. It is found that A1 is
mostly an on-site 3d-4p hybridized transition, while peaks A3 and B are
non-local transitions, with A3 being mostly dipolar and influence by the 3d-4p
intersite hybridization, while B is due to interactions at longer range.
Finally, peak A2 which was previously assigned to a transition involving
pentacoordinated titanium atoms exhibits a quadrupolar angular evolution with
incidence angle. These results pave the way to the use of the pre-edge peaks at
the K-edge of a-TiO2 to characterize the electronic structure of related
materials and in the field of ultrafast XAS where the linear dichroism can be
used to compare the photophysics along different axes.Comment: 43 pages, 19 figure
A First Look at Dissolved Ge Isotopes in Marine Sediments
The removal of chemical species from seawater during the precipitation of authigenic minerals is difficult to constrain but may play a major role in the global biogeochemical cycles of some elements, including silicon (Si) and germanium (Ge). Here, we present Ge/Si, δ^(74)Ge, and supporting chemical data of pore waters and core incubations at three continental margin sites in California and the Gulf of Mexico. We used these data to partition Ge release and uptake by the various allogenic (delivered via sedimentation) and authigenic (formed in situ) phases in these sediments. About half of the pore water Ge (δ^(74)Ge_(pw) = 1.3–2.4‰) is supplied by biogenic silica dissolution (δ^(74)Ge ~ 3‰), with the other half contributed by lithogenic particulates (δ^(74)Ge ~ 0.6‰). The highest Ge/Si (~3μmol/mol) and lowest δ^(74)Ge (1.3–1.9‰) are observed at the Fe redox horizon, suggesting a supply from detrital Ge-rich Fe oxides. The precipitation of authigenic phases (most likely aluminosilicate clays) in deeper sediments preferentially incorporates Ge over Si, resulting in low pore water Ge/Si (~0.3μmol/mol). The lack of corresponding δ^(74)Ge_(pw) trend indicates negligible Ge isotope fractionation during this process. Ge fluxes measured via core incubations were variable and appeared strongly controlled by Fe redox behavior near the sediment-water interface. In some cases, reductive Fe oxide dissolution appeared to enhance the benthic Ge flux by over 100% and released fractionated low δ74Ge of ~−0.7‰, resulting in overall benthic δ^(74)Ge_(inc) between –0.2 and 3.6‰, depending on Fe oxide contribution to Ge flux. We estimate that detrital inputs supply 12–31% of total dissolved Ge to continental margin pore fluids globally, resulting in an average pore water and benthic flux δ^(74)Ge between 2.2 and 2.7‰. Assuming 10-60% of pore water Ge is captured by the authigenic aluminosilicate sink, the dissolved Ge flux to the ocean derived from terrigenous inputs should be roughly 2.5–6.6 Mmol/y, much higher than previously estimated. Our results imply that authigenic Si burial in continental margins should be in the range of 1–8 Tmol/y (best estimate 3.1 Tmol/y), sufficient to close the global marine Si budget
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