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.

The infrared spectra of protic ionic liquids: performances of different computational models to predict hydrogen bonds and conformer evolution.

The temperature dependence of the far- and mid-infrared spectrum of two prototypical protic ionic liquids (PILs) sharing a common trialkylammonium cation, but having different anions, is investigated. The exploitation of both the FIR and MIR ranges provides complementary information about the microscopic configurations and the intermolecular interactions, which determine the structure and the properties of ILs. The analysis of the data collected for all the measured frequencies in a wide temperature range reveals several phase transitions and allows the evaluation of the conformer distribution in the different physical states. The difference in the average energy between the H-bonded configurations and the dispersion-governed ones was also determined for the two PILs. Moreover, a computational model for ionic couples based on the ωB97X-D functional and a polar solvent is here successfully exploited for the description of the hydrogen bonding between anion and cation. For the attribution of vibrational lines of the conformers of the cation, the picture based on single ion calculations at the B3LYP level is more valuable and provides better agreement with the experiments.

Crystallization of mixtures of hydrophilic ionic liquids and water: Evidence of microscopic inhomogeneities.

HYPOTHESIS: We compare the effects of water, either intentionally added or due to absorption from the air, on the phase diagram of the hydrophilic 1-butyl-3-methylimidazolium dicyanamide ionic liquid, extending previous investigations to lower temperatures (down to 140 K), with a special attention to the changes of the environment of water molecules and the interface between water and ionic liquid as a function of temperature. EXPERIMENTS: Combined infrared spectroscopy and ab-initio calculations provide information about the phase transitions and the intermolecular changes occurring in the liquid. FINDINGS: The temperature dependence of the mid-infrared spectrum in the temperature range between 140 and 330 K indicates that in both cases the liquid undergoes a glass transition, but, when the water content is only due to absorption from air, a cold crystallization takes place on heating between ≈240 and ≈265 K, while it is suppressed when water is intentionally added in a greater amount. The analysis of the OH stretching bands indicates the existence of two different "liquid like" water environments. When cold crystallization takes places the water molecules, which seem less coordinated to the other H2O molecules and more related to the anions, appear to be part of the crystallized sample. In both cases, it seems that at microscopic level the sample is not homogeneous, but more likely it is composed of separated clusters or regions of bulk water confined in the ionic liquid.

Spectroscopic evidence of a new energy scale for superconductivity in H3S.

The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided fresh impetus to the search for superconductors at ever higher temperatures. Although this systems displays all the hallmarks of superconductivity, the mechanism through which it arises remains to be determined. Here we provide a first optical spectroscopy study of this superconductor. Experimental results for the optical reflectivity of H3S, under hydrostatic pressure of 150 GPa, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory. Two significant features stand out: some remarkably strong infrared active phonons at around 160 meV, and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range H3S becomes more reflecting with increasing temperature, a change that is traced to superconductivity originating from the electron-phonon interaction. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.

Tailoring the physical properties of the mixtures of ionic liquids: a microscopic point of view.

A detailed investigation of the phase diagram of the mixtures of the two ionic liquids N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)imide (TMPA-TFSI) and N-trimethyl-N-hexylammonium bis(trifluoromethylsulfonyl)imide (TMHA-TFSI) has been performed in the temperature range between 140 and 330 K by means of DSC and infrared spectroscopy measurements. In the low temperature crystalline states, a large concentration of the trans-TFSI conformer is present in TMPA-TFSI, while almost only cis-TFSI is retained in TMHA-TFSI. For the mixtures (TMPA-TFSI)100-x (TMHA-TFSI)x, at concentrations close to the extremes, solid crystalline phases are still present and they show a strong predominance of the trans conformer of the TFSI ion for x < 15 or a large concentration of the cis conformer of TFSI for x > 85. At intermediate concentrations (33 < x < 67) no crystalline phase develops at low temperatures and both conformers of TFSI survive in the whole temperature range investigated here. We suggest that the competition between the two TFSI conformers at low temperatures can be the origin of the lack of crystalline phases for intermediate concentrations and can be exploited as a valid tool to tailor the physical properties of the mixtures of ionic liquids.

Direct Observation of Spatiotemporal Dynamics of Short Electron Bunches in Storage Rings.

In recent synchrotron radiation facilities, the use of short (picosecond) electron bunches is a powerful method for producing giant pulses of terahertz coherent synchrotron radiation. Here we report on the first direct observation of these pulse shapes with a few picoseconds resolution, and of their dynamics over a long time. We thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses (a broadband Terahertz comb) stemming from the shortness of the electron bunches. Above this threshold, a large part of the emission is dominated by drifting structures, which appear through spontaneous self-organization. These challenging single-shot THz recordings are made possible by using a recently developed photonic time stretch detector with a high sensitivity. The experiment has been realized at the SOLEIL storage ring.

High sensitivity photonic time-stretch electro-optic sampling of terahertz pulses.

Single-shot recording of terahertz electric signals has recently become possible at high repetition rates, by using the photonic time-stretch electro-optic sampling (EOS) technique. However the moderate sensitivity of time-stretch EOS is still a strong limit for a range of applications. Here we present a variant enabling to increase the sensitivity of photonic time-stretch for free-propagating THz signals. The ellipticity of the laser probe is enhanced by adding a set of Brewster plates, as proposed by Ahmed et al. [Rev. Sci. Instrum. 85, 013114 (2014)] in a different context. The method is tested using the high repetition rate terahertz coherent synchrotron radiation source of the SOLEIL synchrotron radiation facility. The signal-to-noise ratio of our terahertz digitizer could thus be straightforwardly improved by a factor ≈6.5, leading to a noise-equivalent input electric field below 1.25 V/cm inside the electro-optic crystal, over the 0-300 GHz band (i.e., 2.3 µV/cm/Hz). The sensitivity is scalable with respect to the available laser power, potentially enabling further sensitivity improvements when needed.

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 (=TMI), 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 TMI, optical conductivity reveals a bad metal behavior and below TMI, 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 Cu3dxy, Fe3d and O2p 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.

The Complex Dance of the Two Conformers of Bis(trifluoromethanesulfonyl)imide as a Function of Pressure and Temperature.

Absorbance spectra of two ionic liquids, the short alkyl chain N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) and the longer chain N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI) are reported as a function of pressure and temperature. The occurrence of various phase transitions is evidenced by the changes in the relative concentration of the cisoid and transoid conformers of their common TFSI anion. The infrared spectrum of TMPA-TFSI was measured at 300 K with an applied pressure varying over the 0-5 GPa range. Above 0.2 GPa only the trans conformer is detected, suggesting the occurrence of a pressure induced crystallization. When pressure is applied to TMHA-TFSI at T = 310 K, both TFSI conformers subsist up to ∼11 GPa. However, the clear change of their intensity ratio observed around 2 GPa, suggests the onset of a glass phase as supported by measurements carried out at 4.2 GPa along a cooling/heating cycle. A careful analysis of the spectra collected along different p-T thermodynamic paths shows the occurrence of a cold crystallization at 295 K on heating from 139 K along the p = 0.5 GPa isobar. The rich phase diagrams of the two ionic liquids is the result of the competition among the anion-cation intermolecular interactions, the lower energy of trans-TFSI with respect to cis-TFSI and the smaller volume of cis-TFSI with respect to trans-TFSI.

Interaction of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide with an electrospun PVdF membrane: Temperature dependence of the concentration of the anion conformers.

We measured the temperature dependence of the infrared absorption spectrum of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY R14-TFSI) between 160 and 330 K, through all the phase transitions presented by this compound. The comparison of the experimental spectra with the calculated vibration modes of different conformers of the ions composing the ionic liquid allowed to detect the presence of both conformers of TFSI in the liquid, supercooled, and glass phases, while only the trans-conformer is retained in both solid phases. When the ionic liquid swells a polyvinylidenefluoride (PVdF) electrospun membrane, the cis-rotamer is detected in all phases, since the interaction between the polymer and the ionic liquid inhibits the complete transformation of TFSI into the trans-conformer in the solid phases. Computational results confirm that in the presence of a PVdF chain, cis-TFSI becomes the lowest energy conformer. Therefore, the interaction with the polymer alters the physical properties of the ionic liquid.

Observing microscopic structures of a relativistic object using a time-stretch strategy.

Emission of light by a single electron moving on a curved trajectory (synchrotron radiation) is one of the most well-known fundamental radiation phenomena. However experimental situations are more complex as they involve many electrons, each being exposed to the radiation of its neighbors. This interaction has dramatic consequences, one of the most spectacular being the spontaneous formation of spatial structures inside electrons bunches. This fundamental effect is actively studied as it represents one of the most fundamental limitations in electron accelerators, and at the same time a source of intense terahertz radiation (Coherent Synchrotron Radiation, or CSR). Here we demonstrate the possibility to directly observe the electron bunch microstructures with subpicosecond resolution, in a storage ring accelerator. The principle is to monitor the terahertz pulses emitted by the structures, using a strategy from photonics, time-stretch, consisting in slowing-down the phenomena before recording. This opens the way to unpreceeded possibilities for analyzing and mastering new generation high power coherent synchrotron sources.

The effect of myoglobin crowding on the dynamics of water: an infrared study.

Solutions containing 8 and 32 wt% myoglobin are studied by means of infrared spectroscopy, as a function of temperature (290 K and lower temperatures), in the mid- and far-infrared spectral range. Moreover, ultrafast time-resolved infrared measurements are performed at ambient temperature in the O-D stretching region. The results evidence that the vibrational properties of water remain the same in these myoglobin solutions (anharmonicity, vibrational relaxation lifetime…) and in neat water. However, the collective properties of the water molecules are significantly affected by the presence of the protein: the orientational time increases, the solid-liquid transition is affected in the most concentrated solution and the dynamical transition of the protein is observed, from the point of view of water, even in the least concentrated solution, proving that the water and myoglobin dynamics are coupled.

Stabilization of different conformers of bis(trifluoromethanesulfonyl)imide anion in ammonium-based ionic liquids at low temperatures.

The infrared absorption spectra of two ionic liquids with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion and ammonium with different alkyl chains as cations are reported as a function of temperature. Using the comparison with ab initio calculations of the infrared-active intramolecular vibrations, the experimental lines were ascribed to the various ions composing the ionic liquids. In the liquid state of the samples, both conformers of the TFSI ion are present. In the solid state, however, the two conformers survive in N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), while only cis-TFSI is retained in N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI). We suggest that the longer alkyl chains of the former compound stabilize the less stable conformer of TFSI by means of stronger interactions between anions and cations.

Magneto- to electroactive transmutation of spin waves in ErMnO3.

The low-energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

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.

Coherent synchrotron radiation for broadband terahertz spectroscopy.

We present the first high resolution (10(-3) cm(-1)) interferometric measurements in the 200-750 GHz range using coherent synchrotron radiation, achieved with a low momentum compaction factor. The effect of microbunching on spectra is shown, depending on the bunch current. A high signal-to-noise ratio is reached thanks to an artifact correction system based on a double detection scheme. Combined to the broad emitted spectral range and high flux (up to 10(5) times the incoherent radiation), this study demonstrates that coherent synchrotron radiation can now be used for stability-demanding applications, such as gas-phase studies of unstable molecules.

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.

A trapped water network in nanoporous material: the role of interfaces.

The infrared spectra of water confined in well controlled pore glasses were recorded as a function of the pore size ranging from 8 to 320 nm and in the 30-4000 cm(-1) spectral range using the ATR technique. The experiments prove that even in the large pores, the water network is significantly perturbed. The energy of the connectivity (or hindered translation) band (around 150 cm(-1)) is found to increase when the pore size decreases, indicating that confinement increases the H-bonding between neighbouring water molecules. Moreover, a drastic decrease of the FWHM of the connectivity band was observed upon confinement. This can be related to some ordering induced by the rigid walls of the pores. Furthermore, the partial filling of pores causes a significant modification to the water network, resembling heating of the trapped liquid and suggesting a role played by the water/air interface.

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.

Signatures of the hydrogen bonding in the infrared bands of water.

Infrared spectroscopy measurements have been completed over a wide range of frequencies allowing to measure the evolution of both intramolecular and intermolecular vibrational modes in water as a function of temperature. Emphasis is made on the high frequency OH stretching band and the so-called connectivity band that lies in the far infrared region. The substructures of the two infrared bands are analyzed in terms of different levels of connectivity of the water molecules, along the statements of the percolation model. Both band profiles appear to be related to the different degrees of connectivity of water molecules. Comparison of the data with the predictions of the percolation model shows good agreement as for the temperature evolution of liquid water. This work provides additional support to the interpretation of water bands substructures as signatures of its very specific connectivity pattern.