Monday, 11 December 2017

Excitations and relaxation dynamics in multiferroic GeV 4 S 8 studied by terahertz and dielectric spectroscopy


Reschke, S., Zhe Wang, F. Mayr, E. Ruff, P. Lunkenheimer, V. Tsurkan, and A. Loidl. "Excitations and relaxation dynamics in multiferroic GeV 4 S 8 studied by terahertz and dielectric spectroscopy." Physical Review B 96, no. 14 (2017): 144418

Abstract
We report on THz time-domain spectroscopy on multiferroic GeV4S8, which undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits antiferromagnetic order below 14.6 K. The THz experiments are complemented by dielectric experiments at audio and radio frequencies. We identify a low-lying excitation close to 0.5 THz, which is only weakly temperature dependent and probably corresponds to a molecular excitation within the electronic level scheme of the V4 clusters. In addition, we detect complex temperature-dependent behavior of a low-lying phononic excitation, closely linked to the onset of orbitally driven ferroelectricity. In the high-temperature cubic phase, which is paramagnetic and orbitally disordered, this excitation is of relaxational character becomes an overdamped Lorentzian mode in the orbitally ordered phase below the Jahn-Teller transition, and finally appears as well-defined phonon excitation in the antiferromagnetic state. Abrupt changes in the real and imaginary parts of the complex dielectric permittivity show that orbital ordering appears via a structural phase transition with strong first-order character and that the onset of antiferromagnetic order is accompanied by significant structural changes, which are of first-order character, too. Dielectric spectroscopy documents that at low frequencies, significant dipolar relaxations are present in the orbitally ordered, paramagnetic phase only. In contrast to the closely related GaV4S8, this relaxation dynamics that most likely mirrors coupled orbital and polar fluctuations does not seem to be related to the dynamic processes detected in the THz regime.


… The time-domain THz transmission experiments using a TPS Spectra 3000 spectrometer

for full paper see https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.144418

for more information about TeraView see https://www.azom.com/suppliers.aspx?SupplierID=12145

Wednesday, 6 December 2017

Vibrational states of nano-confined water molecules in beryl investigated by first-principles calculations and optical experiments

Belyanchikov, Mikhail, Elena Zhukova, Sergei Tretiak, Andriy Zhugayevych, Martin Dressel, Frank Uhlig, Jens Smiatek, Maria Fyta, Victor G. Thomas, and Boris Gorshunov. "Vibrational states of nano-confined water molecules in beryl investigated by first principles calculations and optical experiments." Physical Chemistry Chemical Physics (2017).
Abstract
Using quantum mechanical calculations within density functional theory, we provide a comprehensive analysis of infrared-active excitation of water molecules confined in nanocages of a beryl crystal lattice. We calculate infrared-active modes including the translational, librational, and mixed-type resonances of regular and heavy water molecules. The results are compared to the experimental spectra measured for the two principal polarizations of the electric field: parallel and perpendicular to the crystallographic c-axis. Good agreement is achieved between calculated and measured isotopic shifts of the normal modes. We analyze the vibrational modes in connection with the structural characteristics and arrangements of water molecules within the beryl crystal. Specific atomic displacements are assigned to each experimentally detected vibrational mode resolving the properties of nano-confined water on scales not accessible by experiments. Our results elucidate the applicability and efficiency of a combined experimental and computational approach for describing and an in-depth understanding of nano-confined water, and pave the way for future studies of more complex systems.

Graphical abstract: Vibrational states of nano-confined water molecules in beryl investigated by first-principles calculations and optical experiments

... dielectric permittivity and optical conductivity were directly determined using quasioptical
spectrometer based on monochromatic and continuously frequency tunable radiation generators –
backward- wave oscillators 33 and a pulsed THz time-domain teraview TPS Spectra ...



for full paper see http://pubs.rsc.org/en/content/articlelanding/2017/cp/c7cp06472a/unauth#!divAbstract

Monday, 4 December 2017

Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry

Dong, Junliang, Alexandre Locquet, Marcello Melis, and D. S. Citrin. "Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry." Scientific Reports 7, no. 1 (2017): 15098.

Abstact
The process by which art paintings are produced typically involves the successive applications of preparatory and paint layers to a canvas or other support; however, there is an absence of nondestructive modalities to provide a global mapping of the stratigraphy, information that is crucial for evaluation of its authenticity and attribution, for insights into historical or artist-specific techniques, as well as for conservation. We demonstrate sparsity-based terahertz reflectometry can be applied to extract a detailed 3D mapping of the layer structure of the 17th century easel painting Madonna in Preghiera by the workshop of Giovanni Battista Salvi da Sassoferrato, in which the structure of the canvas support, the ground, imprimatura, underpainting, pictorial, and varnish layers are identified quantitatively. In addition, a hitherto unidentified restoration of the varnish has been found. Our approach unlocks the full promise of terahertz reflectometry to provide a global and detailed account of an easel painting’s stratigraphy by exploiting the sparse deconvolution, without which terahertz reflectometry in the past has only provided a meager tool for the characterization of paintings with paint-layer thicknesses smaller than 50 μm. The proposed modality can also be employed across a broad range of applications in nondestructive testing and biomedical imaging.

Figure 1



.......Methods. THz reflective imaging. A typical THz time-domain system (Teraview TPS Spectra 3000) is employed in this study............ 


for full paper see
https://www.nature.com/articles/s41598-017-15069-2

for more information about TeraView see https://www.azom.com/suppliers.aspx?SupplierID=12145

Wednesday, 29 November 2017

Experimental and Theoretical Study of Strong Low-Terahertz Absorption of Thymine

Zhang, W-D., A. Bykhovski, J. A. Deibel, and E. R. Brown. "Experimental and Theoretical Study of Strong Low-Terahertz Absorption of Thymine." Journal of Infrared, Millimeter, and Terahertz Waves 38, no. 12 (2017): 1521-1529.


Abstract
The absorption coefficient of a nucleobase-thymine-in powder form was measured with terahertz spectroscopy in both frequency- and time-domain experiments. For frequencies below 3 THz, a strong signature was observed at 1.27 THz. Furthermore, molecular-dynamic simulations were conducted to reveal that the 1.27 THz absorption signature is related to a transverse optical phonon mode. The simulations also indicated that bound water molecules are vital to the vibrational mode.


... 3 Experimental Methods and Results: Time-Domain Spectroscopy. Next to verify the frequency-domain result, we measured thymine samples with a time-domain system (Teraview TPS Spectra 3000) operating from 0.1 to ∼4.0 THz, and having a dynamic range of ∼60 dB ...


For more information about TeraView visit https://www.azom.com/suppliers.aspx?SupplierID=12145

For full paper see https://link.springer.com/article/10.1007/s10762-017-0420-z

Monday, 27 November 2017

Terahertz non-destructive imaging of cracks and cracking in structures of cement-based materials


....The THz imaging is throughout the whole loading procedure in Section 3.2. All of the TPI experiments were conducted using the TeraView TPS 3000 THz spectrometer,27 the main machine of which is shown in FIG. 5. .....



For full paper see http://aip.scitation.org/doi/full/10.1063/1.4996053