Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Ground state of the Kagome-like S=1/2 antiferromagnet, Volborthite Cu3V2O7(OH)2.2H2O

Volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagome-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4K$\pm$0.2K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% correspond to a well defined short range order, possibly of the $\sqrt{3} \times \sqrt{3}$ type. While the freezing involve all the Cu$^{2+}$ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.Comment: 4 pages, 4 figures, to appear in PR

Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.Comment: 10.5 pages, 15 figures. Submitted to Phys. Rev.

Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl and Br. While the phase transition observed in the Cl system at T_o=18.2 K is consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.Comment: 4 pages, 4 figure

Observations of total peroxy nitrates and aldehydes: measurement interpretation and inference of OH radical concentrations

We describe measurements of total peroxy nitrates (ΣPNs), NO₂, O₃ and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July–16 September 2001. We observed a strong photochemically driven variation of ΣPNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of ΣPNs on aldehydes, OH, NO and NO₂ showing that the steady state calculations are accurate to ±30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of ΣPNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the ΣPNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7×10⁶ molecule cm^−3 at noon during the 8 weeks of the experiment

Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated Electron

Influence de la déforestation sur le fonctionnement hydrologique de petits bassins versants tropicaux

Les régions tropicales subissent une déforestation importante. En Amérique du Sud,la forêt est généralement remplacée par une prairie, C'est pourquoi nous avons étudié le comportement hydrologique de 2 petits (1,5 ha) bassins versants. Un bassin (bassin B) est recouvert par une forêt primaire, tandis que le second (bassin A) a été défriché et transformé en prairie (Digitaria swazilandensis, programme ÉCÉREX, ORSTOM/CTFT). Ces bassins, situés en Guyane Française, sont proches (500 m), escarpés et principalement constitués par des sols à drainage vertical ralenti. Le climat est de type tmpical humide avec une température moyenne (26 °C) et des précipitations moyennes annuelles (3500 à 3900 mm/an) élevées. L'évapotranspiration réelle et potentielle de la forêt primaire sont respectivement égales à 1470 mm/an et 1565 mm/an, En période d'étiage, nous avons observé un écoulement permanent à l'exutoire du bassin A, alors que le bassin B en est dépourvu. Deux crues (24 mai 1992 et 15 mai 1993) ont été étudiées, simultanément sur les 2 bassins. Pendant les crues, nous avons prélevé des échantillons d'eau des précipitations (pluie et pluviolessivat), des ruisseaux et du sol. Sur ces sites, l'eau circulant dans les couches peu profondes du sol présente une concentration élevée en K+ et faible en Cl-. Une signature opposée caractérise l'eau des couches pmfondes du sol. L'analyse des relations existant entre les traceurs chimiques (K+, Cl-) et isotopique l80) ainsi l'étude des propriétés hydrodynamiques du sol permet de décomposer qualitativement l'hydrogramme de crue en 3 réservoirs: sol superficiel (écoulement hypodermique), sol intermédiaire (de 0 à - 0,4 m), sol profond (bassin B) ou nappe (bassin A). Une décomposition quantitative a été effectuée en utilisant des traceurs chimique (Cl-) et isotopique l80). Nous avons ainsi montré que les crues sur les 2 bassins sont dominées par l'écoulement issu des couches intermédiaires du sol qui représente environ la moitié de l'écoulement total de crue. Cependant,les mécanismes de génération des crues diffèrent sur les 2 bassins. Sur le bassin A, les couches profondes du sol sont saturées avant la crue et participent donc à la totalité de la crue. Au contraire, sur le bassin B, les couches profondes de sol atteignent la saturation peu de temps avant le pic de crue et participent donc essentiellement aux écoulement pendant la décrue. Ces résultats confirment les études hydrologiques réalisées précédemment (FRITSCH, 199Ù) et permettent d'identifier les mécanismes de genèse des crues et ainsi de mettre en évidence l'effet de la déforestation.The tropical regions are subjected to fast deforestation. In South America, the tropical rain forest is being replaced by grassland. Thus, we have studied the hydrological behaviour of two small (1.5 ha) watersheds. One basin (hereafter named "B" basin) is still covered by primary forest while the second one (hereafter named "A" basin) was cleared and transformed to grassland (Digitaria swazilandensis, ÉCÉREX program, supported by ORSTOM/CTFT). These basins, located in French Guyana, are close to one another (500 m), steep, and are principally constituted of soils showing lateral drainage. The tropical humid climate is characterized by a high mean interannual temperature (26¡C), which varies slightly from month to month, and by a high mean annual precipitation (3500 to 3900 mm yr-1). Precipitation mainly occurs during the main wet season from May to June and during a secondary wet season from December to January. Real evapotranspiration of the natural forest is 1470 mm yr-1 and potential evapotranspiration is 1565 mm yr-1. During the low-water level period, we have observed perennial runoff at the outlet at the "A" basin while the "B" basin is without permanent flow. We have studied two runoff events (24 May 1992 and 15 May 1993) in both basins. On 24 May 1992, the runoff event was caused by a rainfall lasting for about 10 hours. Total precipitation was 53.8 mm. The main event amounted to 32 mm. The main peak of the hydrograph corresponded to the heaviest rainfalls. On 15 May 1993, the runoff event was caused by a rain lasting for about 13 hours. Total precipitation was 64.0 mm. The main peak of the hydrograph (86.2 L s-1) corresponded again to the heaviest rainfalls. Spatial variability of the precipitation amount was high, especially for the most intense events that have the largest standard deviations. Interception by the canopy amounted to 5.3% of the rainfall in 1992 and 4.3% in 1993. High rapid runoff coefficients were observed, i.e., 0.28 for 24 May 1992 and 0.43 for 15 May 1993. No overland flow was observed in the watershed.Samples of rainwater, throughfall, stream water, and soil water were regularly collected in both watersheds during the runoff events. Temporal variations in the isotopic composition of the stream water at the outlet of the watershed paralleled variations in rainwater but with a distinct shift. The difference between the two signatures could be due to a mixture between:- Rainwater and water present in the watershed before the event and whose isotopic composition is different and variable over space. - Rainwater and water originating from various reservoirs whose contribution to the stream varies with time. The analysis of runoff events using the isotope tracer method revealed the existence in the stream of a mixture of water originating from rain and from one or several other reservoirs in the watershed. Isotope tracers alone were not sufficient to estimate the depth of the soil water contributing to the runoff event. On one hand, temporal variability in the isotopic composition of rainwater was very similar to the vertical spatial variability in the isotopic composition of soil water. On the other hand, surface evaporation in the watershed was negligible: the isotopic signature of water originating from soil during runoff events was the consequence of successive infiltrated rain events. Oxygen-18 content in rain water strongly varied with time but only slightly with space because of the small area of the watershed. Because of this temporal variability, an average isotope content of rainwater could not be used when calculating the contribution of "new water" at the outlet of the watershed.Using chemical and isotope tracers is a way to identify and quantify the contribution of the various water reservoirs to runoff. We were thus able to separate runoff hydrographs into simple components (water from superficial layer, intermediate layer and deep layer). In these watersheds, shallow water was characterized by relatively high concentration in potassium and very low concentration in chloride. An opposite signature characterized deep waterA "deep water" chemical tracer (chloride) - isotope tracer (18O) diagram shows the evidence of a hysteresis relationship:1. The decreasing limb of this relationship (rising segment of the hydrograph) is due to a decrease in heavy isotope content resulting from the decrease of oxygen-18 content in the precipitation and from the arrival of water from upper soil layers with low concentrations of chloride. 2. The increasing limb (falling segment of the hydrograph and recession) is associated with the arrival at the outlet of deep waters containing relatively high concentrations of chloride and heavy isotopes. Using chemical (Cl-) and isotope (18O) tracers, quantitative hydrograph separation was achieved with a simple 2- or 3- component conservative-mixing model. This information allowed qualitative hydrograph separation into 3 reservoirs: superficial soil layers, intermediate soil layers (0 to -0.4 m), deep soil layers ("B" watershed) or ground water ("A" watershed).Thus, the runoff event of both basins was dominated by the intermediate soil layers reservoir, which represents half of the total flow for both basins. However, the processes of runoff generation differ: in the "A" watershed, the deep soil layers were saturated before the rain: the contribution is significant throughout the runoff . In the "B" watershed, the deep soil layers become saturated a few times before the peak flow: their contribution dominates during the recession. These results confirm previous hydrological studies (Fritsch, 1990), which showed the high reactivity of the watershed, and give a better insight into the mechanisms involved.Some of these observations can also be used at a larger scale: 1. Identification of the reservoirs contributing to the runoff event by analyzing the relationships between oxygen-18 content and the flow rate, and between isotope and chemical tracers. 2. Simultaneous samplings along the stream in order to detect a possible zonation of the watershed. These samples must be taken during a runoff event as well as during a low-water level period to check whether the tracer concentrations in the continuous or discontinuous water table supplying the stream are heterogeneous. If the signature of the water table is heterogeneous or if the stream is supplied by several water tables with different chemical concentrations, the watershed must be divided into several homogeneous sub-watersheds

Shear-Wave Elastography Assessments of Quadriceps Stiffness Changes prior to, during and after Prolonged Exercise: A Longitudinal Study during an Extreme Mountain Ultra-Marathon.

In sports medicine, there is increasing interest in quantifying the elastic properties of skeletal muscle, especially during extreme muscular stimulation, to improve our understanding of the impact of alterations in skeletal muscle stiffness on resulting pain or injuries, as well as the mechanisms underlying the relationships between these parameters. Our main objective was to determine whether real-time shear-wave elastography (SWE) can monitor changes in quadriceps muscle elasticity during an extreme mountain ultra-marathon, a powerful mechanical stress model. Our study involved 50 volunteers participating in an extreme mountain marathon (distance: 330 km, elevation: +24,000 m). Quantitative SWE velocity and shear modulus measurements were performed in most superficial quadriceps muscle heads at the following 4 time points: before the race, halfway through the race, upon finishing the race and after recovery (+48 h). Blood biomarker levels were also measured. A significant decrease in the quadriceps shear modulus was observed upon finishing the race (3.31±0.61 kPa) (p<0.001) compared to baseline (3.56±0.63 kPa), followed by a partial recovery +48 h after the race (3.45±0.6 kPa) (p = 0.002) across all muscle heads, as well as for each of the following three muscle heads: the rectus femoris (p = 0.003), the vastus medialis (p = 0.033) and the vastus lateralis (p = 0.001). Our study is the first to assess changes in muscle stiffness during prolonged extreme physical endurance exercises based on shear modulus measurements using non-invasive SWE. We concluded that decreases in stiffness, which may have resulted from quadriceps overuse in the setting of supra-physiological stress caused by the extreme distance and unique elevation of the race, may have been responsible for the development of inflammation and muscle swelling. SWE may hence represent a promising tool for monitoring physiologic or pathological variations in muscle stiffness and may be useful for diagnosing and monitoring muscle changes

Observations of total peroxy nitrates and aldehydes: measurement interpretation and inference of OH radical concentrations

International audienceWe describe measurements of total peroxy nitrates (?PNs), NO2, O3 and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July?16 September 2001. We observed a strong photochemically driven variation of ?PNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of ?PNs on aldehydes, OH, NO and NO2 showing that the steady state calculations are accurate to ±30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of ?PNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the ?PNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7×106 molecule cm?3 at noon during the 8 weeks of the experiment