Lattice dynamics of BaFe2Se3

This paper presents a study of the lattice dynamics in BaFe₂Se₃. We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms thatPnmacannot be the space group of BaFe₂Se₃, even at room temperature. The phonons assignment requiresPmto be the BaFe₂Se₃space group, not only in the magnetic phase, but also in the paramagnetic phase at room temperature. This is due to a strong coupling between a short-range spin-order along the ladders, and the lattice degrees of freedom associated with the Fe-Fe bond length. This coupling induces a change in the bond-length pattern from an alternated trapezoidal one (as inPnma) to an alternated small/large rectangular one. Out of the two patterns, only the latter is fully compatible with the observed block-type magnetic structure. Finally, we propose a complete symmetry analysis of the BaFe₂Se₃phase diagram in the 0-600 K range.

On the energy scale involved in the metal to insulator transition of quadruple perovskite EuCu3Fe4O12: infrared spectroscopy and ab-initio calculations

Optical measurements were carried out by infrared spectroscopy on AA′₃B₄O₁₂ A-site ordered quadruple perovskite EuCu₃Fe₄O₁₂ (microscopic sample) as function of temperature. At 240 K (=T_MI), EuCu₃Fe₄O₁₂ undergoes a very abrupt metal to insulator transition, a paramagnetic to antiferromagnetic transition and an isostructural transformation with an abrupt large volume expansion. Above T_MI, optical conductivity reveals a bad metal behavior and below T_MI, an insulating phase with an optical gap of 125 meV is observed. As temperature is decreased, a large and abrupt spectral weight transfer toward an energy scale larger than 1 eV is detected. Concurrently, electronic structure calculations for both high and low temperature phases were compared to the optical conductivity results giving a precise pattern of the transition. Density of states and computed optical conductivity analysis identified Cu_3dxy, Fe_3d and O_2p orbitals as principal actors of the spectral weight transfer. The present work constitutes a first step to shed light on EuCu₃Fe₄O₁₂ electronic properties with optical measurements and ab-initio calculations.

Insulator-metal transition of VO₂ ultrathin films on silicon: evidence for an electronic origin by infrared spectroscopy

We report on the first simultaneous observations of both electronic and structural temperature-induced insulator-to-metal transition (IMT) in VO2 ultrathin films, made possible by the use of broad range transmission infrared spectroscopy. Thanks to these techniques, the infrared phonon structures, as well as the appearance of the free carrier signature, were resolved for the first time. The temperature-resolved spectra allowed the determination of the temperature hysteresis for both the structural (monoclinic-to-rutile) and electronic (insulator-to-metallic) transitions. The combination of these new observations and DFT simulations for the monoclinic structure allows us to verify the direct transition from monoclinic (M1) to rutile and exclude an intermediate structural monoclinic form (M2). The delay in structural modification compared to the primer electronic transition (325 K compared to 304 K) supports the role of free charges as the transition driving force. The shape of the free charge hysteresis suggests that the primer electronic transition occurs first at 304 K, followed by both its propagation to the heart of the layer and the structural transition when T increases. This study outlines further the potential of VO2 ultrathin films integrated on silicon for optoelectronics and microelectronics.

THz magnetoelectric atomic rotations in the chiral compound Ba3NbFe3Si2O14

We have determined the terahertz spectrum of the chiral langasite Ba3NbFe3Si2O14 by means of synchrotron-radiation measurements. Two excitations are revealed that are shown to have a different nature. The first one, purely magnetic, is observed at low temperature in the magnetically ordered phase and is assigned to a magnon. The second one persists far into the paramagnetic phase and exhibits both an electric and a magnetic activity at slightly different energies. This magnetoelectric excitation is interpreted in terms of atomic rotations and requires a helical electric polarization.

Structural and thermal characterization of glyceryl behenate by X-ray diffraction coupled to differential calorimetry and infrared spectroscopy

Physical and thermal properties of glyceryl behenate (Compritol 888 ATO) used as sustained-release matrix in pharmaceutical applications are studied by coupled time-resolved synchrotron X-ray diffraction and Differential Scanning Calorimetry combined with Infrared Spectroscopy. With these techniques, all polymorphs formed in glyceryl behenate, analyzed as received and after various thermal treatments from quenching to slow crystallization, are characterized. By using different well-controlled mixtures of mono-, di- and tribehenate, we identify each lamellar phase observed in the glyceryl behenate. Finally the influence of the crystallization rate on the formation of preferential conformations was also analyzed in order to bring insights into the polymorphism of glyceryl behenate. By changing the crystallization rate of the sample, it was shown that one can favor the formation of preferential polymorphs in the sample. In particular the crystallization at 10 degrees C/min seems to be well adapted for producing a single lamellar phase with a period of 60.9 A while a crystallization rate of 0.4 degrees C/min produces three different lamellar phases.