125 research outputs found
Ăvolution temporelle de lâenvasement des retenues de barrages de Tunisie
La mobilisation et lâexploitation des eaux de surface sont des pratiques anciennes en Tunisie. Les amĂ©nagements rĂ©alisĂ©s au cours du siĂšcle dernier sont exposĂ©s Ă un alluvionnement plus ou moins accĂ©lĂ©rĂ©. La capacitĂ© de stockage des retenues des barrages est sujette Ă une rĂ©duction progressive au cours du temps. Cette perte de capacitĂ©, parfois Ă©levĂ©e, dĂ©passe les prĂ©visions de lâalluvionnement de la retenue. La quantification des sĂ©diments piĂ©gĂ©s se base soit sur les bilans de matiĂšre solide Ă lâĂ©chelle dâune retenue, soit sur des levĂ©s bathymĂ©triques ou topographiques. Les ModĂšles NumĂ©riques de Terrain (MNT) des retenues sont de plus en plus utilisĂ©s. Les diffĂ©rentes Ă©valuations sont entachĂ©es dâune incertitude. Les retenues des barrages tunisiens perdent annuellement 0,5 % Ă 1 % de leur capacitĂ© par alluvionnement. Lâanalyse des rĂ©sultats de mesures montre que lâalluvionnement est liĂ© Ă lâhydrologie du cours dâeau, Ă la gestion de la rĂ©serve dâeau et aux manoeuvres de dĂ©vasement. La comparaison entre les rĂ©sultats de mesures de lâalluvionnement et les prĂ©visions des projets met en Ă©vidence des diffĂ©rences parfois relativement importantes qui sont dues au rĂ©gime hydrologique des cours dâeau. En effet, une crue exceptionnelle peut provoquer un alluvionnement nettement supĂ©rieur Ă la moyenne annuelle en rĂ©gime hydrologique normal. Les retenues mĂ©andriformes sont comblĂ©es rapidement alors que les retenues linĂ©aires offrent la possibilitĂ© de soutirage de quantitĂ©s importantes de sĂ©diments. Les moyennes annuelles des pertes de capacitĂ© des barrages en exploitation et des barrages projetĂ©s jusquâĂ 2010 permettent de quantifier les volumes des sĂ©diments piĂ©gĂ©s Ă 500 Mm3. En 2030, la perte de capacitĂ© de stockage des barrages en exploitation pourrait atteindre 43 % de leur capacitĂ© initiale. Les amĂ©nagements amont et les travaux de conservation des eaux et des sols permettent de rĂ©duire le taux dâalluvionnement et de prolonger la durĂ©e de vie des grands rĂ©servoirs.The mobilization and exploitation of surface water are ancient practices in Tunisia. Installations carried out during last century are exposed to a greater or lesser degree of accelerated silting. The storage capacity of reservoirs is progressively reduced over the course of time. This sometimes accelerated loss of capacity of reserves by silting exceeds the forecasts of hydraulic installation projects. The quantification of trapped sediments is based on the assessment of a reservoirâs retained solid matter, either by bathymetric or topographic rising or by Digital Models of Grounds (DMG) corresponding to different dates. These various evaluations of the sediments trapped in reservoirs are, however, sullied with uncertainty. Tunisian reservoirs lose 0.5% to 1% of their storage capacity to silting annually. The analysis of the results presented in this study shows that silting is related to three factors: river hydrology, water reserve management and dredging operations. The comparison between the silting results and the project forecasts highlights relatively significant differences which may be due to river hydrology. Indeed a significant rising in water level can cause an increase in silting above the annual average found during normal hydrological modes. While meandering reservoirs are quickly filled with sediment, linear reservoirs can undergo dredging to remove a significant quantity of sediment. The annual averages in storage capacity losses of the dam under exploitation and its projections up to the year 2010 enable us to quantify the volumes of the sediment trapped as 500 Mm3. In 2030, the loss of storage capacity of the studied reservoirs may reach 43% of their initial storage capacities. Installations upstream and soil and water conservation efforts may reduce the rate of silting and prolong the lifespan of large dams
Recueil des données acquises : observations hydrologiques et pluviométriques du barrage de Saboun tangérois marocain (nord du Maroc) : période 1997-2001
Comparison of the Fermi-surface topologies of kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated analogue
We have measured details of the quasi one-dimensional Fermi-surface sections
in the organic superconductor kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated
analogue using angle-dependent millimetre-wave techniques. There are
significant differences in the corrugations of the Fermi surfaces in the
deuterated and undeuterated salts. We suggest that this is important in
understanding the inverse isotope effect, where the superconducting transition
temperature rises on deuteration. The data support models for superconductivity
which invoke electron-electron interactions depending on the topological
properties of the Fermi surface
Real space imaging of the metal - insulator phase separation in the band width controlled organic Mott system -(BEDT-TTF)Cu[N(CN)]Br
Systematic investigation of the electronic phase separation on macroscopic
scale is reported in the organic Mott system
-(BEDT-TTF)Cu[N(CN)]Br. Real space imaging of the phase
separation is obtained by means of scanning micro-region infrared spectroscopy
using the synchrotron radiation. The phase separation appears near the Mott
boundary and changes its metal-insulator fraction with the substitution ratio
in
-[(-BEDT-TTF)(-BEDT-TTF)]Cu[N(CN)]Br,
of which band width is controlled by the substitution ratio between the
hydrogenated BEDT-TTF molecule (-BEDT-TTF) and the deuterated one
(-BEDT-TTF). The phase separation phenomenon observed in this class of
organics is considered on the basis of the strongly correlated electronic phase
diagram with the first order Mott transition.Comment: 10 pages, 8 figure
Influence of internal disorder on the superconducting state in the organic layered superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br
We report high-sensitivity AC susceptibility measurements of the penetration
depth in the Meissner state of the layered organic superconductor
kappa-(BEDT-TTF)2Cu[N(CN)2]Br. We have studied nominally pure single crystals
from the two different syntheses and employed controlled cooling procedures in
order to minimize intrinsic remnant disorder at low temperatures associated
with the glass transition, caused by ordering of the ethylene moieties in
BEDT-TTF molecule at T_G = 75 K. We find that the optimal cooling procedures
(slow cooling of -0.2 K/h or annealing for 3 days in the region of T_G) needed
to establish the ground state, depend critically on the sample origin
indicating different relaxation times of terminal ethylene groups. We show
that, in the ground state, the behavior observed for nominally pure single
crystals from both syntheses is consistent with unconventional d-wave order
parameter. The in-plane penetration depth lambda_in(T) is strongly linear,
whereas the out-of-plane component lambda_out(T) varies as T^2. In contrast,
the behavior of single crystals with long relaxation times observed after slow
(-0.2 K/h) cooling is as expected for a d-wave superconductor with impurities
(i.e. lambda_in(T) propto lambda_out(T) propto T^2) or might be also reasonably
well described by the s-wave model. Our results might reconcile the
contradictory findings previously reported by different authors.Comment: 13 pages, 10 figures, submitted to Phys. Rev.
The Dependence of the Superconducting Transition Temperature of Organic Molecular Crystals on Intrinsically Non-Magnetic Disorder: a Signature of either Unconventional Superconductivity or Novel Local Magnetic Moment Formation
We give a theoretical analysis of published experimental studies of the
effects of impurities and disorder on the superconducting transition
temperature, T_c, of the organic molecular crystals kappa-ET_2X and beta-ET_2X
(where ET is bis(ethylenedithio)tetrathiafulvalene and X is an anion eg I_3).
The Abrikosov-Gorkov (AG) formula describes the suppression of T_c both by
magnetic impurities in singlet superconductors, including s-wave
superconductors and by non-magnetic impurities in a non-s-wave superconductor.
We show that various sources of disorder lead to the suppression of T_c as
described by the AG formula. This is confirmed by the excellent fit to the
data, the fact that these materials are in the clean limit and the excellent
agreement between the value of the interlayer hopping integral, t_perp,
calculated from this fit and the value of t_perp found from angular-dependant
magnetoresistance and quantum oscillation experiments. If the disorder is, as
seems most likely, non-magnetic then the pairing state cannot be s-wave. We
show that the cooling rate dependence of the magnetisation is inconsistent with
paramagnetic impurities. Triplet pairing is ruled out by several experiments.
If the disorder is non-magnetic then this implies that l>=2, in which case
Occam's razor suggests that d-wave pairing is realised. Given the proximity of
these materials to an antiferromagnetic Mott transition, it is possible that
the disorder leads to the formation of local magnetic moments via some novel
mechanism. Thus we conclude that either kappa-ET_2X and beta-ET_2X are d-wave
superconductors or else they display a novel mechanism for the formation of
localised moments. We suggest systematic experiments to differentiate between
these scenarios.Comment: 18 pages, 5 figure
Pairing Symmetry Competition in Organic Superconductors
A review is given on theoretical studies concerning the pairing symmetry in
organic superconductors. In particular, we focus on (TMTSF)X and
-(BEDT-TTF)X, in which the pairing symmetry has been extensively
studied both experimentally and theoretically. Possibilities of various pairing
symmetry candidates and their possible microscopic origin are discussed. Also
some tests for determining the actual pairing symmtery are surveyed.Comment: 16 pages, 8 figures, to be published in J. Phys. Soc. Jpn., special
issue on "Organic Conductors
Tetrabenzoporphyrin and -mono-, - Cis -di- and tetrabenzotriazaporphyrin derivatives: Electrochemical and spectroscopic implications of meso CH Group replacement with nitrogen
Nonperipherally hexyl-substituted metal-free tetrabenzoporphyrin (2H-TBP, 1a) tetrabenzomonoazaporphyrin (2H-TBMAP, 2a), tetrabenzo-cis-diazaporphyrin (2H-TBDAP, 3a), tetrabenzotriazaporphyrin (2H-TBTAP, 4a), and phthalocyanine (2H-Pc, 5a), as well as their copper complexes (1b-5b), were synthesized. As the number of meso nitrogen atoms increases from zero to four, Ămax of the Q-band absorption peak becomes red-shifted by almost 100 nm, and extinction coefficients increased at least threefold. Simultaneously the blue-shifted Soret (UV) band substantially decreased in intensity. These changes were related to the relative electron-density of each macrocycle expressed as the group electronegativity sum of all meso N and CH atom groups, ĂąĂâĄR. X-ray photoelectron spectroscopy differentiated between the three different types of macrocyclic nitrogen atoms (the Ninner, (NH)inner, and Nmeso) in the metal-free complexes. Binding energies of the Nmeso and Ninner,Cu atoms in copper chelates could not be resolved. Copper insertion lowered especially the cathodic redox potentials, while all four observed redox processes occurred at larger potentials as the number of meso nitrogens increased. Computational chemical methods using density functional theory confirmed 1b to exhibit a Cu(II) reduction prior to ring-based reductions, while for 2b, Cu(II) reduction is the first reductive step only if the nonperipheral substituents are hydrogen. When they are methyl groups, it is the second reduction process; when they are ethyl, propyl, or hexyl, it becomes the third reductive process. Spectro-electrochemical measurements showed redox processes were associated with a substantial change in intensity of at least two main absorbances (the Q and Soret bands) in the UV spectra of these compounds
- âŠ