Elastic and magnetic effects on the infrared phonon spectra of MnF2

We measured the temperature dependent infrared reflectivity spectra of MnF2 between 4 K and room temperature. We show that the phonon spectrum undergoes a strong renormalization at TN. The ab-initio calculation we performed on this compound accurately predict the magnitude and the direction of the phonon parameters changes across the antiferromagnetic transition, showing that they are mainly induced by the magnetic order. In this material, we found that the dielectric constant is mostly from phonon origin. The large change in the lattice parameters with temperature seen by X-ray diffraction as well as the A2u phonon softening below TN indicate that magnetic order induced distortions in MnF2 are compatible with the ferroelectric instabilities observed in TiO2, FeF2 and other rutile-type fluorides. This study also shows the anomalous temperature evolution of the lower energy Eu mode in the paramagnetic phase, which can be compared to that of the B1g one seen by Raman spectroscopy in many isostructural materials. This was interpreted as being a precursor of a phase transition from rutile to CaCl2 structure which was observed under pressure in ZnF2.Comment: 8 pages, 8 figures, updated version accepted in PR

Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4

Utilizing magnetic susceptibility, specific heat, thermal expansion and IR spectroscopy we provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr_2S_4 at T_N1 = 15 K and T_N2= 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at T_N2 reveals a strong temperature hysteresis typical for a first-order transformation. Due to strong spin-phonon coupling both magnetic phase transitions induce a splitting of phonon modes, where at T_N1 the high-frequency and at T_N2 the low-frequency modes split. The anomalies and phonon splitting observed at T_N2 are strongly suppressed by magnetic field. Regarding the small positive Curie-Weiss temperature Theta= 8 K, we argue that this scenario of two different magnetic phases with concomitant different magneto-elastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange of equal strength.Comment: 4 pages, 4 figure

Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland

A simple statistical model to predict the number of aerosols which activate to form cloud droplets in warm clouds has been established, based on regression analysis of data from four summertime Cloud and Aerosol Characterisation Experiments (CLACE) at the high-altitude site Jungfraujoch (JFJ). It is shown that 79 % of the observed variance in droplet numbers can be represented by a model accounting only for the number of potential cloud condensation nuclei (defined as number of particles larger than 80 nm in diameter), while the mean errors in the model representation may be reduced by the addition of further explanatory variables, such as the mixing ratios of O3, CO, and the height of the measurements above cloud base. The statistical model has a similar ability to represent the observed droplet numbers in each of the individual years, as well as for the two predominant local wind directions at the JFJ (northwest and southeast). Given the central European location of the JFJ, with air masses in summer being representative of the free troposphere with regular boundary layer in-mixing via convection, we expect that this statistical model is generally applicable to warm clouds under conditions where droplet formation is aerosol limited (i.e. at relatively high updraught velocities and/or relatively low aerosol number concentrations). A comparison between the statistical model and an established microphysical parametrization shows good agreement between the two and supports the conclusion that cloud droplet formation at the JFJ is predominantly controlled by the number concentration of aerosol particles

Yield and Agronomic Traits of Waxy Proso in the Central Great Plains

Proso (Panicum miliaceum L.) is a summer annual grass capable of producing seed in 60 to 90 d. This characteristic, and its efficient use of water, makes it well suited to the short, and often hot and dry, growing season in the high plains of the central Great Plains. The introduction of novel end-use characteristics such as waxy starch can stimulate an increased market for proso. We evaluated 18 experimental F5 waxy lines derived from a cross of ‘Huntsman’ and PI436626 across seven locations. Genotype × environment variation in waxy proso was mostly a matter of changes in magnitude and not crossover interaction. When crossover interaction was implicated, it was generally slight and occurred at lower environmental means—at locations with low mean response to any given variable. Waxy progeny mean yield was lower than Huntsman but significantly higher than PI436626. Except for test weight, waxy progeny mean response for most traits was similar to check cultivars. Mean yield of one experimental line did not differ significantly from Huntsman, and 14 did not differ significantly from ‘Horizon’, the second highest yielding cultivar. In addition, regression analysis suggests that top-yielding waxy lines responded well to high-yield environments. Seed sizes for all waxy lines were smaller than the check lines, but most were significantly larger than PI436626. Waxy lines generally headed at a similar time to Huntsman and the other non-waxy checks, and most were significantly earlier than PI436626. Late maturity of PI436626 was the main factor limiting its culture in the High Plains region

Spin-driven Phase Transitions in ZnCr2_2Se4_4 and ZnCr2_2S4_4 Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction

The crystal and magnetic structures of the spinel compounds ZnCr$_2$S$_4$ and ZnCr$_2$Se$_4$ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr$_2$Se$_4$ exhibits a first order phase transition at $T_N=21$ K into an incommensurate helical magnetic structure. Magnetic fluctuations above $T_N$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr$_2$S$_4$ shows two magnetic transitions at $T_{N1}=15$ K and $T_{N2}=8$ K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group $Fd$\={3}$m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I4_1$ / $amd$) for $T_{N2} < T < T_{N1}$ into a low temperature orthorhombic phase (space group $I m m a$) for $T < T_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr$_2$S$_4$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr$_2$Se$_4$.Comment: 11 pages, 13 figures, to be published in PR

Source apportionment of submicron organic aerosols at an urban site by linear unmixing of aerosol mass spectra

International audienceSubmicron ambient aerosol was characterized in summer 2005 at an urban background site in Zurich, Switzerland, during a three-week measurement campaign. Highly time-resolved samples of non-refractory aerosol components were analyzed with an Aerodyne aerosol mass spectrometer (AMS). Positive matrix factorization (PMF) was used for the first time for AMS data to identify the main components of the total organic aerosol and their sources. The PMF retrieved factors were compared to measured reference mass spectra and were correlated with tracer species of the aerosol and gas phase measurements from collocated instruments. Six factors were found to explain virtually all variance in the data and could be assigned either to sources or to aerosol components such as oxygenated organic aerosol (OOA). Our analysis suggests that at the measurement site only a small (1 originates from freshly emitted fossil fuel combustion. Other primary sources identified to be of similar or even higher importance are charbroiling (10?15%) and wood burning (~10%), along with a minor source interpreted to be influenced by food cooking (6%). The fraction of all identified primary sources is considered as primary organic aerosol (POA). This interpretation is supported by calculated ratios of the modelled POA and measured primary pollutants such as elemental carbon (EC), NOx, and CO, which are in good agreement to literature values. A high fraction (60?69%) of the measured organic aerosol mass is OOA which is interpreted mostly as secondary organic aerosol (SOA). This oxygenated organic aerosol can be separated into a highly aged fraction, OOA I, (40?50%) with low volatility and a mass spectrum similar to fulvic acid, and a more volatile and probably less processed fraction, OOA II (on average 20%). This is the first publication of a multiple component analysis technique to AMS organic spectral data and also the first report of the OOA II component

Characterization of positive clusters in the CLOUD nucleation experiments

The mechanism of new particle formation (NPF) events is still poorly understood. The CLOUD experiment is studying at which conditions this process occurs. During the CLOUD 7 campaign (fall 2012) the evolution of the nucleation rate with different reagents (sulphuric acid, ammonia, dimethylamine and α-pinene) was tested. Here we investigate the composition of freshly formed positive ions during the nucleation process, which allows us to understand the formation mechanisms of these clusters. The time evolution of the clusters is shown demonstrating the correlation between the formation of these clusters and the nucleation process

Registration of ‘Homestead’ Canada Wildrye

‘Homestead’ (Reg. No. CV-255, PI 655522) Canada wildrye (Elymus canadensis L.) was developed cooperatively by USDA-ARS and the University of Nebraska and was released in 2008 for use in the Great Plains and the Midwest USA, a region for which no adapted cultivars were previously available. It was developed by means of the Ecotype Selection Breeding System from a collection made in a remnant prairie in Eastern Nebraska USA. Homestead, which was tested as NE3, is adapted to Plant Adaptation Region (PAR) 251-5 (Temperate Prairie Parkland–Plant Hardiness Zone 5), which is its origin, and in which it has been evaluated in both space-transplanted and sward trials. This region is equivalent to USDA Plant Hardiness Zone 5 of the tallgrass-prairie ecoregion of the Midwest, USA. When grown in its area of adaptation, it produces more forage than the previously available, unadapted cultivar of the species and its forage has higher in vitro dry matter digestibility than another adapted experimental strain to which it was compared in sward forage yield trials. Its primary use will be as a native cool-season grass component of conservation, roadside, and grassland seeding mixtures