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

Effect of magnetic order on the superfluid response of single-crystal ErNi2_{2}B2_{2}C: A penetration depth study

We report measurements of the in-plane magnetic penetration depth $\Delta \lambda$(T) in single crystals of ErNi$_{2}$B$_{2}$C down to $\sim$0.1 K using a tunnel-diode based, self-inductive technique at 21 MHz. We observe four features: (1) a slight dip in $\Delta \lambda$(T) at the N$\acute{e}$el temperature $T_{N}$ = 6.0 K, (2) a peak at $T_{WFM}$ = 2.3 K, where a weak ferromagnetic component sets in, (3) another maximum at 0.45 K, and (4) a final broad drop down to 0.1 K. Converting to superfluid density $\rho_{s}$, we see that the antiferromagnetic order at 6 K only slightly depresses superconductivity. We seek to explain some of the above features in the context of antiferromagnetic superconductors, where competition between the antiferromagnetic molecular field and spin fluctuation scattering determines increased or decreased pairbreaking. Superfluid density data show only a slight decrease in pair density in the vicinity of the 2.3 K feature, thus supporting other evidences against bulk ferromagnetism in this temperature range.Comment: 15 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

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

A combined particle trap/HTDMA hygroscopicity study of mixed inorganic/organic aerosol particles

International audienceAtmospheric aerosols are often mixtures of inorganic and organic material. Organics can represent a large fraction of the total aerosol mass and are comprised of water-soluble and insoluble compounds. Increasing attention was paid in the last decade to the capability of mixed inorganic/organic aerosol particles to take up water (hygroscopicity). We performed hygroscopicity measurements of internally mixed particles containing ammonium sulfate and carboxylic acids (citric, glutaric, adipic acid) in parallel with an electrodynamic balance (EDB) and a hygroscopicity tandem differential mobility analyzer (HTDMA). The organic compounds were chosen to represent three distinct physical states. During hygroscopicity cycles covering hydration and dehydration measured by the EDB and the HTDMA, pure citric acid remained always liquid, adipic acid remained always solid, while glutaric acid could be either. We show that the hygroscopicity of mixtures of the above compounds is well described by the Zdanovskii-Stokes-Robinson (ZSR) relationship as long as the two-component particle is completely liquid in the ammonium sulfate/citric acid and in the ammonium sulfate/glutaric acid cases. However, we observe significant discrepancies compared to what is expected from bulk thermodynamics when a solid component is present. We explain this in terms of a complex morphology resulting from the crystallization process leading to veins, pores, and grain boundaries which allow for water sorption in excess of bulk thermodynamic predictions caused by the inverse Kelvin effect on concave surfaces

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

The influence of traffic and wood combustion on the stable isotopic composition of carbon monoxide

Carbon monoxide in the atmosphere is originating from various combustion and oxidation processes. Recently, the proportion of CO resulting from the combustion of wood for domestic heating may have increased due to political measures promoting this renewable energy source. Here, we used the stable isotope composition of CO (&amp;delta;&lt;sup&gt;13&lt;/sup&gt;C and &amp;delta;&lt;sup&gt;18&lt;/sup&gt;O) for the characterization of different CO sources in Switzerland, along with other indicators for traffic and wood combustion (NO&lt;sub&gt;x&lt;/sub&gt;-concentration, aerosol light absorption at different wavelengths). We assessed diurnal variations of the isotopic composition of CO at 3 sites during winter: a village site dominated by domestic heating, a site close to a motorway and a rural site. The isotope ratios of wood combustion emissions were studied at a test facility, indicating significantly lower &amp;delta;&lt;sup&gt;18&lt;/sup&gt;O of CO from wood combustion compared to traffic emissions. At the village and the motorway site, we observed very pronounced diurnal &amp;delta;&lt;sup&gt;18&lt;/sup&gt;O-variations of CO with an amplitude of up to 8&amp;permil;. Solving the isotope mass balance equation for three distinct sources (wood combustion, traffic, clean background air) resulted in diurnal patterns consistent with other indicators for wood burning and traffic. The average night-time contribution of wood-burning to total CO was 70% at the village site, 49% at the motorway site and 29% at the rural site based on the isotope mass balance. The results, however, depend strongly on the pure source isotope values, which are not very well known. We therefore additionally applied a combined CO/NO&lt;sub&gt;x&lt;/sub&gt;-isotope model for verification. Here, we separated the CO emissions into different sources based on distinct CO/NO&lt;sub&gt;x&lt;/sub&gt; emissions ratios for wood combustion and traffic, and inserted this information in the isotope mass balance equation. Accordingly, a highly significant agreement between measured and calculated &amp;delta;&lt;sup&gt;18&lt;/sup&gt;O-values of CO was found (&lt;i&gt;r&lt;/i&gt;=0.67, &lt;i&gt;p&lt;/i&gt;&lt;0.001). While different proxies for wood combustion all have their uncertainties, our results indicate that the oxygen isotope ratio of CO (but not the carbon isotope ratio) is an independent sensitive tool for source attribution studies

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