74 research outputs found
Higgs-mode radiance and charge-density-wave order in 2H-NbSe
Despite being usually considered two competing phenomena, charge-density-wave
and superconductivity coexist in few systems, the most emblematic one being the
transition metal dichalcogenide 2H-NbSe. This unusual condition is
responsible for specific Raman signatures across the two phase transitions in
this compound. While the appearance of a soft phonon mode is a well-established
fingerprint of the charge-density-wave order, the nature of the sharp sub-gap
mode emerging below the superconducting temperature is still under debate. In
this work we use the external pressure as a knob to unveil the delicate
interplay between the two orders, and consequently the nature of the
superconducting mode. Thanks to an advanced extreme-conditions Raman technique
we are able to follow the pressure evolution and the simultaneous collapse of
the two intertwined charge density wave and superconducting modes. The
comparison with microscopic calculations in a model system supports the
Higgs-type nature of the superconducting mode and suggests that
charge-density-wave and superconductivity in 2H-NbSe involve mutual
electronic degrees of freedom. These findings fill knowledge gap on the
electronic mechanisms at play in transition metal dichalcogenides, a crucial
step to fully exploit their properties in few-layers systems optimized for
devices applications
Tetramethylammonium hydroxide thermochemolysis for the analysis of cellulose and free carbohydrates in a peatbog
International audienceWe have compared TMAH thermochemolysis with the classical method using acid hydrolysis for carbohydrates analysis in a peat core. Even if TMAH thermochemolysis does not analyse hemicellulosic carbohydrates and discriminate each individual carbohydrate sensu stricto, it allows the analysis of a cellulose pool hidden to acid hydrolysis and the specific analysis of free and terminal carbohydrates. Simple direct comparisons of thermochemolysis data with data generated from acid hydrolysis cannot be done because of the different mechanisms involved in each process. TMAH thermochemolysis must be viewed and used as a pertinent and complementary method for the analysis of carbohydrates protected and trapped by the organic matter in complex environmental systems
Preparation of nitrogen doped zinc oxide nanoparticles and thin films by colloidal route and low temperature nitridation process
International audienceNitrogen doped zinc oxide (ZnO) nanoparticles have been synthesized using a colloidal route and low temperature nitridation process. Based on these results, 200 nm thick transparent ZnO thin films have been prepared by dip-coating on SiO2 substrate from a ZnO colloidal solution. Zinc peroxide (ZnO2) thin film was then obtained after the chemical conversion of a ZnO colloidal thin film by H2O2 solution. Finally, a nitrogen doped ZnO nanocrystalline thin film (ZnO:N) was obtained by ammonolysis at 250°C. All the films have been characterized by scanning electron microscopy, X-ray diffraction, X-Ray photoelectron spectroscopy and UV-Visible transmittance spectroscopy
Effects of experimental warming on Carbon sink function of a temperate pristine mire : The project PEATWARM
International audienceWithin the PEATWARM project, we use Sphagnum peatlands as a model to analyse their vulnerability to climate change using an experimental system (ITEX) that simulates in situ an increase in average temperature. We aim to determine the effects of temperature increase on the vegetation, the balance of above- and belowground gas fluxes (CO2 and CH4), the microbial diversity and activity in Sphagnum mosses and in peat, and the dynamics of labile and recalcitrant organic matter of peat. The ultimate objective is the creation of a biogeochemical model of C coupled with N and S cycles that includes interactions between these key compartments. Keywords: Global warming, C, N, S cycles, ITEX manipulations, ecosystem structure and function, biogeochemical model of C
Effects of experimental warming on carbon sink function of a temperate pristine mire : the PEATWARM project.
communication oraleInternational audienceWithin the PEATWARM project, we use Sphagnum peatlands as a model to analyse their vulnerability to climate change using an experimental system (ITEX) that simulates in situ an increase in average temperature. We aim to determine the effects of temperature increase on the vegetation, the balance of above- and belowground gas fluxes (CO2 and CH4), the microbial diversity and activity in Sphagnum mosses and in peat, and the dynamics of labile and recalcitrant organic matter of peat. The ultimate objective is the creation of a biogeochemical model of C coupled with N and S cycles that includes interactions between these key compartments
Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS
Here we perform angle and time-resolved photoelectron spectroscopy on the
commensurate Charge Density Wave (CDW) phase of 1T-TaS. Data with different
probe pulse polarization are employed to map the dispersion of electronic
states below and above the chemical potential. Upon photoexcitation, the
fluctuations of CDW order erase the band dispersion near to the chemical
potential and halve the charge gap size. This transient phase sets within half
a period of the coherent lattice motion and is favored by strong electronic
correlations. The experimental results are compared to Density-Functional
Theory (DFT) calculations with a self-consistent evaluation of the Coulomb
repulsion. Our simulations indicate that the screening of Coulomb repulsion
depends on the stacking order of the TaS layers. The entanglement of such
degrees of freedom suggest that both the structural order and electronic
repulsion are locally modified by the photoinduced CDW fluctuations
Dynamics of electronic states in the Intermediate phase of 1T-TaS
This article reports a comparative study of bulk and surface properties in
the transition metal dichalcogenide 1T-TaS. When heating the sample, the
surface displays an intermediate insulating phase that persists for K
on top of a metallic bulk. The weaker screening of Coulomb repulsion and
stiffer Charge Density Wave (CDW) explain such resilience of a correlated
insulator in the topmost layers. Both time resolved ARPES and transient
reflectivity are employed to investigate the dynamics of electrons and CDW
collective motion. It follows that the amplitude mode is always stiffer at the
surface and displays variable coupling to the Mott-Peierls band, stronger in
the low temperature phase and weaker in the intermediate one
Assessment of atomic layer deposited TiO2 photocatalytic self-cleaning by quartz crystal microbalance
The self-cleaning properties emerging from photocatalytic effects consist in the elimination of an organic contamination layer by light-induced redox reactions. Quartz crystal microbalances (QCMs), monitoring the contaminant mass loss under UV illumination, were used to investigate this effect and its efficiency. A new setup dedicated to such purpose is introduced along with the results of a self-cleaning experiment performed with a 20-nm TiO2 thin film coated on a QCM by atomic layer deposition. In particular, a 10-nm paraffin oil thin film deposited under vacuum is shown to be degraded down to its complete removal according to a zeroth order photocatalytic reaction. Finally, the experimental opportunities offered by the new setup, such as a controlled environment composition, are presented.Peer reviewe
The application of DFRC method for the analysis of carbohydrates in a peat bog: Validation and comparison with conventional chemical and thermochemical degradation techniques
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