Search results for "Anharmonicity"
showing 10 items of 118 documents
Interpretation of the Cu K-edge EXAFS spectra of Cu3N using ab initio molecular dynamics
2020
Financial support provided by ERDF project No. 1.1.1.2/VIAA/l/16/147 (1.1.1.2/16/I/001) under the activity “Post-doctoral research aid” realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. This work was supported by a grant from the Swiss National Supercomputing Centre (CSCS) under the project ID s681 .
Reduction of phonon thermal conductivity in nanowires and nanoribbons with dynamically rough surfaces and edges
2009
We present an analytical model and molecular-dynamics simulations of the phonon heat transport in nanowires and nanoribbons with anharmonic lattices and dynamically rough surfaces and edges. In agreement with recent experiments on heat transport in single-crystalline silicon nanowires with rough surfaces, our model and simulations predict finite and length-independent phonon thermal conductivity in such quasi–one-dimensional systems, in contrast to anomalous phonon thermal conductivity of corresponding momentum-conserving systems with atomically smooth surfaces, divergent with the system length. Within our model, the main cause of thermal conductivity reduction is the momentum-nonconserving…
Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPa
2021
The authors acknowledge the ESRF program committee (Grenoble, France) for the opportunity to perform XAFS and XRD measurements. We are grateful to Prof. Dr Marek Tkacz from the Institute of Physical Chemistry, PAS Kasprzaka 44/52, 01-224 Warsaw, Poland, for high quality YH3 samples and to Dr. José A. Flores-Livas for a fruitful discussion. A.P.M. and A.A.I. acknowledge the Russian Foundation for the Basic Research (grant No 18-02-40001_mega) for financial support. J.P., A.K., and I.P. would like to thank the support of the Latvian Council of Science project No. lzp-2018/2-0353. ISSP UL acknowledge the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-20l 6-2017-TeamingPh…
Phonons in YBa2Cu3O7?x crystals with site-selective oxygen isotope substitution: Frequencies, normal vectors, non-linear properties
1995
Lattice vibrations of YBa2Cu3O7−x crystals with a site-selective isotope substitution18O→16O are investigated theoretically. It is shown that shifts of the frequencies of A1g oxygen vibrations due to this substitution are smaller than under complete one. However, phonon bands and normal vectors are changed much stronger, especially in the case when apical oxygens are substituted. On the basis of the presented analysis anharmonic properties and transition temperatures of the selectively substituted crystals are also briefly discussed.
Raman studies of isotope effects in Si and GaAs
1999
Abstract We have measured by Raman scattering changes of the optic phonon energy and line width in Si and GaAs with isotopic composition. The phonon energies of isotopically pure samples show the expected dependence on the average atomic mass in Si and the reduced mass in GaAs, respectively, as well as small anharmonic contributions. In isotopically disordered samples we find frequency shifts of 1.15(20) cm−1 for 28Si0.530Si0.5 and 0.31(20) cm−1 for the TO phonon of natGaAs, induced by mass disorder which also contributes to the line broadening. We give theoretical estimates of these effects.
Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII
2016
The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and iterative solution of the linearized Boltzmann transport equation(BTE) for phonons. Within the temperature range 100-350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than silicon diamond structure (d-Si) by factors of 1/2 and 1/5, respectively. The main reason for the lower lattice thermal conductivity of the clathrate structure II in comparison to d-Si was found to be the harmonic phonon spectra, while in the case of the clathrate structure VIII, the difference is mainly due to the harmonic phonon spe…
High-temperature emission spectroscopy of methane
2008
International audience; A high-enthalpy source (HES) has been developed in Rennes either to heat gases up to 2000K in local thermodynamic equilibrium (LTE) or to generate hypersonic expansions. The HES prototype has been associated with a high-resolution Bruker IFS 120 HR Fourier transform spectrometer to record emission spectra of hot gases, in LTE conditions. A series of emission spectra of methane has been obtained at 1005, 1365, 1485, 1625 and 1820K in the pentad spectral region located around 3000 cm1, at Doppler-limited resolution (0.02 cm1). Spectra have been corrected for the transmission function that strongly affects the infrared radiation emitted by the hot gas. Line-integrated a…
Retinal vibrations in bacteriorhodopsin are mechanically harmonic but electronically anharmonic: evidence from overtone and combination bands
2021
AbstractVibrations of the chromophore in the membrane protein bacteriorhodopsin (BR), a protonated Schiff base retinal, have been studied for decades, both by resonance Raman and by infrared (IR) difference spectroscopy. In spite the light-induced IR difference spectrum between the K intermediate (13-cis retinal) and the initial BR state (all-trans retinal) being first published almost 40 years ago, we present here unreported bands in the 2500 to 1800 cm−1 region. We show that the bands between 2500 and 2300 cm−1 originate from overtone and combination transitions of retinal C-C stretches. We assigned some of the newly reported bands below 2300 cm−1 to the combination of retinal C-C stretch…
Evolution towards centrosymmetry of the nonlinear-optical materialRbTiOPO4in the temperature range 293–973 K: Alkaline displacements and titanyl defo…
1998
The crystal structure of rubidium titanyl phosphate, ${\mathrm{RbTiOPO}}_{4}$ (space group ${\mathrm{Pna}2}_{1}$), has been refined at room temperature, and at 473, 673, 873, and 973 K, by using single-crystal x-ray-diffraction techniques. The data show a large anharmonic motion of the rubidium ions increasing with temperature. To describe the importance of this motion in the phase-transition procedure and in the ionic conductivity phenomenon, two models are developed. The study of the deviation to the centrosymmetric structure (space group Pnan) shows that the change towards centrosymmetry of the titanyl groups correlates directly with the observed variation of the second-harmonic generati…
Local structure and lattice dynamics of cubic Y2O3: an x-ray absorption spectroscopy study
2015
Classical molecular dynamics was used to interpret the temperature dependent (300-1273 K) Y K-edge x-ray absorption spectra of cubic c-Y2O3 and to validate two force-field models. It was shown that both models reproduce well thermal disorder effect, however are less accurate in description of static octahedra distortion around yttrium atoms. The temperature dependencies of mean-square displacements and mean-square relative displacements were obtained from molecular dynamics simulations for the first two shells of yttrium. The importance of anharmonic effects was demonstrated by comparison of molecular dynamics and lattice dynamics results.