The biodiversity of dictyostelids in mountain forests: a case study in the French Alps

Forest management can seriously modify the biodiversity of forest dwelling species, but the consequences are poorly known for certain taxa, particularly soil fauna, for which few studies have been published. We compared the biodiversity of dictyostelids cellular slime moulds in a managed and an unmanaged forest in the French Alps and analysed the influence of environmental factors on species richness and abundance of dictyostelids. To our knowledge, this study is the first one undertaken in the European Alps. We must better understand the influence of various environmental factors on the biodiversity of these organisms if we want to accurately define their functional role in the soil. In our study, dictyostelids showed lower levels of diversity compared to previously published results. The mean species richness of dictyostelids was marginally higher in unmanaged than in managed forests and biodiversity indices were significantly correlated with elevation and pH. This suggests that environmental factors have a predominant effect on the biodiversity of dictyostelids and that the effect of forest management is secondary

Phonon Linewidths and Electron Phonon Coupling in Nanotubes

We prove that Electron-phonon coupling (EPC) is the major source of broadening for the Raman G and G- peaks in graphite and metallic nanotubes. This allows us to directly measure the optical-phonon EPCs from the G and G- linewidths. The experimental EPCs compare extremely well with those from density functional theory. We show that the EPC explains the difference in the Raman spectra of metallic and semiconducting nanotubes and their dependence on tube diameter. We dismiss the common assignment of the G- peak in metallic nanotubes to a Fano resonance between phonons and plasmons. We assign the G+ and G- peaks to TO (tangential) and LO (axial) modes.Comment: 5 pages, 4 figures (correction in label of fig 3

Theoretical Models Relating Acoustic Tube-Wave Attenuation To Fracture Permeability - Reconciling Model Results With Field Data

Several recent investigations indicate that tube-wave amplitude attenuation in acoustic full-waveform logs is correlated with permeability in fractured rocks. However, there are significant differences between predictions based on theoretical models for tubewave propagation and experimental waveform amplitude data. This investigation reviews the results of existing theoretical models for tube-wave attenuation in fractured rock and compares model predictions with acoustic full-waveform data where extensive independent fracture-permeability data are available from straddle-packer permeability tests. None of the tube-wave models presented in the literature predicts attenuation at fracture apertures as small as those producing attenuation in the field; and most models predict tube-wave reflections, which are rarely measured at frequencies greater then 5 kHz. Even the unrealistic assumption that all of the tube-wave energy loss is caused by viscous dissipation in fracture openings does not result in predicted apertures being as small as those indicated by packer permeability measurements in most situations. On the basis of these results, it is concluded that plane-fracture models cannot account for the measured tube-wave attenuation where natural fractures intersect fluidfilled boreholes. However, natural fractures are fundamentally different from plane parallel passages. This difference appears to explain the small equivalent flow apertures and lack of reflections associated with fractures in waveform-log data. Permeable fracture openings modeled as irregular tubes embedded between asperities along the fracture face are predicted to produce significant tube-wave attenuation when tube diameters exceed 1.0 cm, but arrays of such tubes conduct fluid flow equivalent to that through plane fractures less than 2 mm in effective flow aperture. Although the theory predicts some reflection from simple cylindrical passages, scattering from irregular distributions of natural fracture openings probably accounts for the infrequency with which coherent tube-wave reflections occur in field data.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-86ER13636

Modeling Borehole Stoneley Wave Propagation Across Permeable In-Situ Fractures

The characterization of hydraulic transmissivity of permeable fracture reservoirs is a very important task in the exploration of water resources and hydrocarbons. Previous studies that model the permeable structure as a single fluid-filled fracture failed to explain the observed significant Stoneley wave attenuation across the permeable structure. In this paper, the structure is modeled as a permeable fracture zone and synthetic Stoneley wave seismograms in the vicinity of the structure are calculated. The results show that Stoneley waves can be strongly attenuated or even eliminated without significant reflection, because of the dissipation of wave energy into the permeable zone. Several field cases are also modeled and the theoretical results are compared with the field data. It is shown that low- and medium-frequency Stoneley waves (1 kHz data from Moodus, Conneticut, and 5 kHz data from Monitoba, Canada) are very sensitive to the permeability of the fractures and can be used to assess permeability from in-situ logging data, if the fracture porosity and zone thickness can be measured. At high frequencies, however, Stoneley waves are not very sensitive to permeability but are mainly affected by the sum of the fracture openings expressed as the product of fracture zone thickness and porosity in the fracture zone. This finding is demonstrated by a logging data set (Monitoba, Canada) obtained using high-frequency Stoneley waves at 34 kHz.United States. Dept. of Energy (Grant DE-FG0286ER13636)Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Raman excitation spectroscopy of carbon nanotubes: effects of pressure medium and pressure

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added - water, hexane, sulphuric acid - and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously.Comment: 6 pages, 3 Figures, Proceedings of EHPRG 2011 (Paris

Interlayer Dependence of G-Modes in Semiconducting Double-Walled Carbon Nanotubes

A double-walled carbon nanotube (DWNT), a coaxial composite of two single-walled carbon nanotubes (SWNT), provides a unique model to study interactions between the two constituent SWNTs. Combining high resolution transmission electron microscopy (HRTEM), electron diffraction (ED), and resonant Raman scattering (RRS) experiments on the same individual suspended DWNT is the ultimate way to relate unambiguously its atomic structure, defined by the chiral indices of the coaxial outer/inner SWNTs, and its Raman-active vibration modes. This approach is used to investigate the intertube distance dependence of the G-modes of individual index-identified DWNTs composed of two semiconducting SWNTs. We state the main features of the dependence of the G-mode frequencies on the distance between the inner and outer layers: (i) When the interlayer distance is larger than the nominal van der Waals distance (close to 0.34 nm), a downshift of the inner-layer G-modes with respect to the G-modes in the equivalent SWNTs is measured. (ii) The amplitude of the downshift depends on the interlayer distance, or in other words, on the negative pressure felt by the inner layer in DWNT. (iii) No shift is observed for an intertube distance close to 0.34 nm

Study of collective radial breathing-like modes in double-walled carbon nanotubes: Combination of continuous two-dimensional membrane theory and Raman spectroscopy

Radial breathing modes (RBMs) are widely used for the atomic structure characterization and index assignment of single-walled carbon nanotubes (SWNTs) from resonant Raman spectroscopy. However, for double-walled carbon nanotubes (DWNTs), the use of conventional ¿RBM(d) formulas is complicated due to the van der Waals interaction between the layers, which strongly affects the frequencies of radial modes and leads to new collective vibrations. This paper presents an alternative way to theoretically study the collective radial breathing-like modes (RBLMs) of DWNTs and to account for interlayer interaction, namely the continuous two-dimensional membrane theory. We obtain an analytical ¿RBLM(do, di) relation, being the equivalent of the conventional ¿RBM(d) expressions, established for SWNTs. We compare our theoretical predictions with Raman data, measured on individual index-identified suspended DWNTs, and find a good agreement between experiment and theory. Moreover, we show that the interlayer coupling in individual DWNTs strongly depends on the interlayer distance, which is manifested in the frequency shifts of the RBLMs with respect to the RBMs of the individual inner and outer tubes. In terms of characterization, this means that the combination of Raman spectroscopy data and predictions of continuous membrane theory may give additional criteria for the index identification of DWNTs, namely the interlayer distance

Accurate determination of the chiral indices of individual carbon nanotubes by combining electron diffraction and Resonant Raman spectroscopy

The experimental approach combining high resolution transmission electron microscopy (HRTEM), electron diffraction (ED) and resonant Raman spectroscopy (RRS) on the same free-standing individual carbon nanotubes (CNT) is the most efficient method to determine unambiguously the intrinsic features of the Raman-active phonons. In this paper, we review the main results obtained by the approach regarding the intrinsic features of the phonons of single-walled (SWNT) and double-walled carbon nanotubes (DWNT). First, we detail the different methods to identify the structure of SWNTs and DWNTs from the analysis of their electron diffraction patterns (EDP). In the following, we remind the principal features of the Raman response of SWNTs, unambiguously index-identified by ED. A special attention is devoted to the effect of the inter-layer interaction on the frequencies of the Raman-active phonons in index-identified DWNTs. The information obtained on index-identified SWNT and DWNT allows us to propose Raman criteria, which help identifying CNT when the ED fails to propose a single assignment. The efficiency of the Raman criteria as the complement to the ED information for the index-assignment of a few SWNTs and DWNTs is shown. The same approach to index-assign a triple-walled carbon nanotube (TWNT), by combining ED and RRS information, is reported

O2 Loaded Germanosilicate Optical Fibers: Experimental In Situ Investigation and Ab Initio Simulation Study of GLPC Evolution under Irradiation

In this work we present a combined experimental and ab initio simulation investigation concerning the Germanium Lone Pair Center (GLPC), its interaction with molecular oxygen (O2), and evolution under irradiation. First, O2 loading has been applied here to Ge-doped optical fibers to reduce the concentration of GLPC point defects. Next, by means of cathodoluminescence in situ experiments, we found evidence that the 10 keV electron irradiation of the treated optical fibers induces the generation of GLPC centers, while in nonloaded optical fibers, the irradiation causes the bleaching of the pre-existing GLPC. Ab initio calculations were performed to investigate the reaction of the GLPC with molecular oxygen. Such investigations suggested the stability of the dioxagermirane (DIOG) bulk defect, and its back conversion into GLPC with a local release of O2 under irradiation. Furthermore, it is also inferred that a remarkable portion of the O2 passivated GLPC may form Ge tetrahedra connected to peroxy bridges. Such structures may have a larger resistance to the irradiation and not be back converted into GLPC