Search results for "PHONON"
showing 10 items of 466 documents
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.
Magneto-optical Investigations of Nanostructured Materials Based on Single Molecule Magnets Monitor Strong Environmental Effects
2007
The determination of the magnetic properties of molecular magnets in environments similar to those used in spintronic devices is fundamental for the development of applications. Single-molecule magnets (SMMs) are molecular cluster systems that display magnetic hysteresis of dynamical origin at low temperature. As they behave like perfectly monodisperse nanomagnets and show clear macroscopic quantum effects in their magnetic properties, they are extremely appealing candidates for the forthcoming generation of molecular devices: they have been proposed as efficient systems for quantum computation, ultra-high-density magnetic recording media, and molecular spintronic systems. These attractive …
Nanowires: A route to efficient thermoelectric devices
2019
Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy in electronics is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Nanowires can meet nicely these two requirements because enhanced …
Correlation analysis of vibration modes in physical vapour deposited Bi 2 Se 3 thin films probed by the Raman mapping technique
2021
In this work, the Raman spectroscopy mapping technique is used for the analysis of mechanical strain in Bi2Se3 thin films of various (3-400 nm) thicknesses synthesized by physical vapour deposition on amorphous quartz and single-layer graphene substrates. The evaluation of strain effects is based on the correlation analysis of in-plane (E2g) and out-of-plane (A21g) Raman mode positions. For Bi2Se3 films deposited on quartz, experimental datapoints are scattered along the line with a slope of similar to 0.85, related to the distribution of hydrostatic strain. In contrast to quartz/Bi2Se3 samples, for graphene/Bi2Se3 heterostructures with the same thicknesses, an additional negative slope of …
<title>Raman scattering by phonons and magnons in Ni<formula><inf><roman>c</roman></inf></formula>Mg<for…
2003
ABSTRACT The Raman scattering by phonons and magnons was studied at room temperature in polycrystalline solid solutions NiMg1O and pure NiO. The experimental Raman spectrum of NiO consists of six well resolved bands, whose originis due to the disorder-induced one-phonon scattering (bands at 400 and 500 cm'), two-phonon scattering (bands at 750,900 and 1 100 cm) and two-magnon scattering (band at 1500 cm). In NiMg1O solid solutions, a relative increase ofone-phonon scattering is observed upon a dilution of nickel oxide by magnesium ions: at room temperature, the two-magnon band becomes invisible for c<O.7, whereas the two-phonon contribution disappears at c<O.5. Such behaviour isexplained by…
Thermoelectric properties of spark-plasma sintered nanoparticular FeSb2prepared via a solution chemistry approach
2013
Nanoparticular FeSb2 was prepared in solution from cyclopentadienyl iron(ii) dicarbonyl dimer [Fe(Cp(CO)2)]2 and antimony nanoparticles. Spark plasma sintering was used as consolidation method to maintain the particle size. The thermoelectric performance of FeSb2 is limited by its high thermal conductivity. In this work, the thermal conductivity was suppressed by nearly 80% compared to the bulk value by introducing grain boundary scattering of phonons on the nanoscale. The thermoelectric properties of the consolidated FeSb2 emphasize the possibility of altering thermal transport of promising thermoelectric compounds by phonon scattering by engineering the interfaces at the nanoscale.
Anisotropic lattice thermal conductivity in topological semimetal ZrGeX (X = S, Se, Te): a first-principles study
2021
Abstract Topological semimetals have attracted significant attentions owing to their potential applications in numerous fields such as low-power electron devices and quantum computation, which are closely related to their thermal transport properties. In this work, the phonon transport properties of topological Dirac nodal-line semimetals ZrGeX (X = S, Se, Te) with the PbClF-type structures are systematically studied using the first-principles calculations combined with the Boltzmann transport theory. The obtained lattice thermal conductivities show an obvious anisotropy, which is caused by the layer structures of ZrGeX (X = S, Se, Te). The room-temperature lattice conductivity of ZrGeTe al…
Stability of the fergusonite phase in GdNbO 4 by high pressure XRD and Raman experiments
2017
Abstract We describe the results of high pressure x-ray diffraction and Raman measurements on gadolinium orthoniobate. The ambient pressure monoclinic fergusonite phase remains stable in a remarkable large pressure range. There is no significative evolution of the monoclinic distortion up to 25 GPa , the maximum pressure achieved. Instead, the anisotropic compressibility is associated to the stiffness of NbO 4 tetrahedra in respect to the GdO 8 polyhedra. The high pressure evolution of external modes parallels the wavenumber dependence on ionic radius along the lanthanide series. The chemical pressure analogy is attributed to the compression of GdO 8 polyhedra. There is no evidence of any p…
Crystal structure and phase transition of TlReO4: a combined experimental and theoretical study
2020
Abstract The present work describes a density-functional theory (DFT) study of TlReO4 in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P21/c) is observed at room temperature while two isostructural tetragonal structures (space group I41/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic str…
Luminescence of rutile structured crystalline silicon dioxide (stishovite)
2014
Abstract Luminescence spectrum of synthetic mono-crystalline stishovite comprises a slow blue band at ~400 nm (~3.1 eV) and a fast UV band at ~260 nm (~4.7 eV), as well as some bands in near-infrared range of spectra. The NIR luminescence of stishovite crystal, excited with lasers 532 nm, 248 nm and 193 nm as well as x-ray, possesses several sharp lines. A zero phonon line is situated at 787 nm (1.57 eV) and grows with cooling. An anti-Stokes line is located at 771 nm (1.68 eV). This line disappears with cooling. In a powder sample of stishovite created by shock waves generated by the impact of a 50-m-diameter meteorite in Arizona 50,000 years ago, the PL broad blue band is situated at 425 …