Raman and Brillouin scattering studies of bulk 2H-WSe2.

Raman and Brillouin spectroscopy were used to probe optic and acoustic phonons in bulk 2H-WSe2. Raman spectra collected under different polarization conditions allowed assignment of spectral peaks to various first- and second-order processes. In contrast to some previous studies, a Raman peak at ∼259 cm(-1)was found not to be due to the A(1g) mode but to a second-order process involving phonons at either the M or K point of the Brillouin zone. Resonance effects due to excitons were also observed in the Raman spectra. Brillouin spectra of 2H-WSe2 contain a single peak doublet arising from a Rayleigh surface mode propagating with a velocity of [Formula: see text] m s(-1). This value is comparable to that estimated from Density Functional Theory calculations and also to those for the transition metal diselenides 2H-TaSe2 and 2H-NbSe2. Unlike these two materials, however, peaks arising from scattering via the elasto-optic mechanism were not observed in Brillouin spectra of WSe2 despite its lower opacity.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors.

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

Laboratory and field validation of a combined NO2-SO2 Radiello passive sampler.

A combined NO2-SO2 Radiello radial-type diffusive sampler was validated under controlled laboratory conditions and compared with NO2-SO2 results of 3 other type of samplers in a field comparison at two locations Ghent-Mariakerke and Borgerhout in Flanders. Laboratory exposures at different temperatures (-5, 10 and 30 degrees C) and relative humidities (0, 50 and 80% RH) in combination with varying concentration levels and exposure times were carried out, with a focus on extreme conditions. Concentration level and exposure time were changed together following suppliers linear working range of samplers and assuring absolute amounts of compounds on the sampler corresponding to those of environmental levels. The average uptake rate for NO2 for 24 hour exposures at 10 degrees C and 50% RH and tested concentration levels (+/-73, 146 and 293 ppb NO2) was 0.076 +/- 0.011 ng ppb(-1) min(-1). Uptake rates during all experiments were lower than the uptake rate given in the instruction manual of the sampler. A significant effect of temperature and relative humidity on NO2 uptake rate was observed. The temperature effect from 10 to 30 degrees C corresponds to the temperature effect given by the supplier of the samplers. High relative humidity (70 to 80%) caused a strong non-reproducible decrease of uptake rate for NO2 at 24 hour experiments but this effect was not observed at longer exposures except for the tests at -5 degrees C. At the tested temperature below zero in combination with high relative humidity the sampler showed anomalous behaviour for NO2. The possible effect of concentration level and exposure time for NO2 needs further research. The average uptake rate for SO2 calculated from all exposures is 0.478 +/- 0.075 ng of sulfate ion each ppb min of SO2 and accords to suppliers uptake rate. No clear effects of temperature, relative humidity or concentration level/exposure time on the uptake rate for SO2 were found, partly due to the large scatter of results. Although NO2 accuracy of Radiello samplers was better during field campaigns than during laboratory validation, IVL and OGAWA samplers gave better results for NO2. In the field, IVL samplers showed best agreement with the continuous analyzers for both NO2 and SO2.

Dumbbell rattling in thermoelectric zinc antimony.

Inelastic neutron scattering measurements on thermoelectric Zn4Sb3 reveal a dominant soft local phonon mode at 5.3(1) meV. The form factor of this local mode is characteristic for dumbbells vibrating preferably along the dumbbell axis and can be related to a vibration of Sb dimers along the c axis. The Lorentzian width of the mode corresponds to short phonon lifetimes of 0.39(2) ps and yields an estimate of the thermal conductivity that agrees quantitatively with recent steady state measurements. Heat capacity measurements are consistent with an Einstein mode model describing the local Sb-dimer rattling mode with an Einstein temperature of 62(1) K. Our study suggests that soft localized phonon modes in crystalline solids are not restricted to cagelike structures and are likely to be a universal feature of good thermoelectric materials.

Goldstone-mode phonon dynamics in the pyrochlore Cd2Re2O7.

We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200 K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.

Four-well tunneling States and elastic response of clathrates.

We present resonant ultrasound elastic constant measurements of the Eu8Ga16Ge30 and Sr8Ga16Ge30 clathrates. The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLS's are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLS's explains the observed harmonic oscillator type specific heat. In addition the quadrupolar interaction of FLS's with elastic strains explains the pronounced depression observed in elastic constant measurements. In the case of the Sr clathrate, we explain the elastic properties assuming the same type of interaction, but with Sr Einstein mode.

Metallic "ferroelectricity" in the pyrochlore Cd2Re2O7.

A class of materials known as "ferroelectric metals" was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)]], but to date no examples of this class of materials have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ferroelectric metal in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a twofold degenerate mode with E(u) symmetry and that motions of the O ions forming the O octahedra dominate the energetics of the transition.

Disparate atomic displacements in skutterudite-type LaFe3CoSb12, a model for thermoelectric behavior.

Mean-square atomic displacements in lanthanum triiron cobalt dodecaantimonide, determined as a function of temperature using single-crystal neutron diffraction, show that the La atom exhibits an anomalously large displacement at room temperature, U(eq) = 0.0196 (9) Å(2), because it is too small to fill the atomic cage formed by the corner-linked octahedral framework of M(4)Sb(12), M = Fe, Co. Site-occupancy refinements show 25% vacancies on the La site and an actual Fe:Co ratio of 2.17:1. Analysis of the temperature dependence of the atomic displacements identifies a significant temperature-independent component for the La atom ascribed to static disorder, which amounts to 19% of the room-temperature value. The large-amplitude rattling of the La atom can be effectively linked to the dramatic decrease of the lattice contribution to the thermal conductivity, which is a key factor for improving the thermoelectric behavior of these materials. This structure-property relationship offers a new paradigm for the exploration of thermoelectric materials.