Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4

Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies and Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic CdCr2S4 and antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic field dependence of the eigenfrequencies show no anomalies indicating displacive polar soft mode behavior. However, significant effects are detected in the temperature dependence of the plasma frequencies indicating changes in the nature of the bonds and significant charge transfer. In HgCr2S4 we provide experimental evidence that the magnetic field dependence of specific polar modes reveal shifts exactly correlated with the magnetization showing significant magneto-dielectric effects even at infrared frequencies.Comment: 8 pages, 8 figure

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

Identifying Winter Forage Triticale (×\u3ci\u3eTriticosecale\u3c/i\u3e Wittmack) Strains for the Central Great Plains

Triticale (×Triticosecale Wittmack) is mainly used as a forage crop in the central Great Plains. A successful triticale cultivar should have high forage yield with good quality, and also high grain yield so the seed can be economically produced. The purpose of this study was to evaluate existing triticale cultivars and experimental strains for their relative value in the central Great Plains as an annual hay crop primarily for feeding to beef cattle. Two experiments (one for forage yield and one for grain yield) were planted at two locations (one representing the arid Great Plains and the second representing the or higher rainfall central Great Plains) for 2 yr. Twenty-nine triticale cultivars and strains were evaluated for forage yield and quality, and grain yield. In both experiments, year effects were significant (P \u3c 0.05) for all traits except grain yield; location effects were significant for forage yield, neutral detergent fiber (NDF), and acid detergent fiber. There was no location × strain or year × location × strain interaction for all the quality traits indicating that triticale forage quality was stable across environments. Triticale strains differed significantly for forage yield, grain yield, NDF, acid detergent lignin, and relative feed value. However, forage of all strains had good feed quality. Three strains had high grain and forage yield, and very good relative feed value suggesting that triticale improvement for both grain and forage traits is possible

Existence of orbital polarons in ferromagnetic insulating La1−x_{1-x}Srx_xMnO3_{3} (0.11<x<<x<0.14) evidenced by giant phonon softening

We present an inelastic light scattering study of single crystalline (La$_{1-y}$Pr$_y$)$_{1-x}$Sr$_{x}$MnO$_3$ ($0\leq x\leq0.14$,$y=0$ and $x=1/8$,$0\leq y\leq0.5$). A giant softening up to 20 - 30 cm$^{-1}$ of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples ($0.11\leq x \leq 0.14$) upon cooling below the Curie temperature. With increasing Pr-doping the giant softening is gradually suppressed. This is attributed to a coupling of the breathing mode to orbital polarons which are present in the FMI phase.Comment: 4 pages, 5 figure

Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon coupling constants have been estimated based on microscopic modeling using density-functional theory and analysis of the local spin density. Our observations show the importance of orbital degrees of freedom for the Fe-based superconductors with large ordered magnetic moments, while small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.Comment: 17 pages, 3 figure

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

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

Theoretical model for the superconducting and magnetically ordered borocarbides

We present a theory of superconductivity in presence of a general magnetic structure in a form suitable for the description of complex magnetic phases encountered in borocarbides. The theory, complemented with some details of the band structure and with the magnetic phase diagram, may explain the nearly reentrant behaviour and the anisotropy of the upper critical field of HoNi2B2C. The onset of the helical magnetic order depresses superconductivity via the reduction of the interaction between phonons and electrons caused by the formation of magnetic Bloch states. At mean field level, no additional suppression of superconductivity is introduced by the incommensurability of the helical phase.Comment: 8 pages, 2 figures. Published version, one important reference adde

Dimethylamine and ammonia measurements with ion chromatography during the CLOUD4 campaign

The CLOUD project investigates the influence of galactic cosmic rays on the nucleation of new particles in an environmental chamber at CERN. Dimethylamine (DMA) was injected intentionally into the CLOUD chamber to reach atmospherically relevant levels away from sources (up to 100 pptv) in order to study its effect on nucleation with sulphuric acid and water at 278 K. Quantification of DMA and also background ammonia (NH&lt;sub&gt;3&lt;/sub&gt;) was performed with ion chromatography (IC). The IC method used together with the sampling line developed for CLOUD in order to measure NH&lt;sub&gt;3&lt;/sub&gt; and DMA at low pptv levels is described; the overall sampling efficiency of the method is discussed; and, finally, mixing ratios of NH&lt;sub&gt;3&lt;/sub&gt; and DMA measured during CLOUD4 are reported

On the effects of hydrocarbon and sulphur-containing compounds on the CCN activation of combustion particles

International audienceThe European PartEmis project (''Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines'') was focussed on the characterisation and quantification of exhaust emissions from a gas turbine engine. A comprehensive suite of aerosol, gas and chemi-ion measurements were conducted under different combustor operating conditions and fuel sulphur concentrations. Combustion aerosol characterisation included on-line measurements of mass and number concentration, size distribution, mixing state, thermal stability of internally mixed particles, hygroscopicity, cloud condensation nuclei (CCN) activation potential, and off-line analysis of chemical composition. Modelling of CCN activation of combustion particles was conducted using microphysical and chemical properties obtained from the measurements as input data. Based on this unique data set, the role of sulphuric acid coatings on the combustion particles, formed in the cooling exhaust plume through either direct condensation of gaseous sulphuric acid or coagulation with volatile condensation particles nucleating from gaseous sulphuric acid, and the role of the organic fraction for the CCN activation of combustion particles was investigated. It was found that particles containing a large fraction of non-volatile organic compounds grow significantly less at high relative humidity than particles with a lower content of non-volatile OC. Also the effect of the non-volatile OC fraction on the potential CCN activation is significant. While a coating of water-soluble sulphuric acid increases the potential CCN activation, or lowers the activation diameter, respectively, the non-volatile organic compounds, mainly found at lower combustion temperatures, can partially compensate this sulphuric acid-related enhancement of CCN activation of carbonaceous combustion aerosol particles