The Experts below are selected from a list of 2172 Experts worldwide ranked by ideXlab platform
Alfredo Pasquarello - One of the best experts on this subject based on the ideXlab platform.
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Ab initio molecular dynamics in a finite homogeneous electric field
Physical Review Letters, 2002Co-Authors: Paolo Umari, Alfredo PasquarelloAbstract:We treat homogeneous electric fields within density functional calculations with periodic boundary conditions. A nonlocal energy functional depending on the applied field is used within an ab initio molecular dynamics scheme. The reliability of the method is demonstrated in the case of bulk MgO for the Born effective charges, and the high- and low-frequency dielectric constants. We evaluate the static dielectric constant by performing a damped molecular dynamics in an electric field and avoiding the calculation of the dynamical matrix. Application of this method to Vitreous Silica shows good agreement with experiment and illustrates its potential for systems of large size.
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identification of raman defect lines as signatures of ring structures in Vitreous Silica
Physical Review Letters, 1998Co-Authors: Alfredo Pasquarello, Roberto CarAbstract:By analyzing the vibrational eigenmodes of a model structure obtained previously by quantum molecular dynamics, the two sharp features appearing in the Raman spectrum of Vitreous Silica could conclusively be assigned to three- and four-membered ring structures embedded in the network. The projections of the eigenmodes onto oxygen breathing motions in these rings yield well defined peaks at frequencies in striking correspondence to the positions of the Raman lines. The good agreement between calculated and measured shifts upon isotopic substitution further confirms this assignment.
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dynamic structure factor of Vitreous Silica from first principles comparison to neutron inelastic scattering experiments
Physical Review B, 1998Co-Authors: Alfredo Pasquarello, Johannes Sarnthein, Roberto CarAbstract:Using a first-principles approach, we study the vibrational properties of Vitreous SiO2 which are measured in neutron-scattering experiments. We adopt a model structure consisting of corner-sharing tetrahedra, which was previously generated using first-principles molecular dynamics. We calculate the dynamic, structure function S(q,E) as a function of wave vector q and energy E by taking explicitly into account the correlations between different atoms as given by the normal modes. The effects of temperature and finite displacements are also considered. Overall, the agreement with experiment is very good, as illustrated by the comparison for the density of states. However, the calculated and measured S(q,E) differ in some cases up to a factor of 2 in absolute intensity. Nevertheless, the oscillations in S(q,E) describing the correlations between the motions of the atoms are accurately reproduced. The neutron effective density of states obtained directly from S(q,E) yields a good representation of the actual density of states. By introducing a comprehensive scheme, we clarify the relation between neutron and infrared spectre. In particular, we show that the neutron density of states does not distinguish between longitudinal and transverse excitations. Other properties such as the mean-square displacements and the elastic structure factor are also evaluated and found to be in good agreement with experiment.
Tomohiko Yamaguchi - One of the best experts on this subject based on the ideXlab platform.
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in situ conductance measurements for alignments of Silica particles under alternating electric fields
Langmuir, 2007Co-Authors: Satoshi Nishimura, Hideo Matsumura, Katsunori Kosuge, Tomohiko YamaguchiAbstract:The alignments of Silica particles formed in sinusoidal electrical fields of 1 kHz were assessed using an optical microscope with measuring the electric conductance of a Silica dispersion between two Pt electrodes in a Vitreous Silica glass cell. We confirmed that the electric conductance of the Silica dispersion between the two electrodes in the cell reflected the surface conductance of the Silica particles settling at the bottom of the cell. More interestingly, we observed that the electric conductance of the Silica dispersion in the cell increased when pearl chains of the Silica particle were formed along the direction of the electric field. However, no clear change in the electric conductance of the dispersion was observed at higher electric field strengths where a transition from pearl chains to zigzag band patterns and circulating movements of the Silica particles in the zigzag bands formed.
Lothar Wondraczek - One of the best experts on this subject based on the ideXlab platform.
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boson peak heterogeneity and intermediate range order in binary sio 2 al 2 o 3 glasses
Scientific Reports, 2018Co-Authors: Mariana F Ando, Omar Benzine, Jeanluc Garden, Katrin Wondraczek, Stephan Grimm, Kay Schuster, Lothar WondraczekAbstract:In binary aluminoSilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. We determine the vibrational density of states VDoS g(ω) using low-temperature heat capacity data. From correlation with low-frequency Raman spectroscopy, we obtain the Raman coupling coefficient. Both experiments allow for the extraction of the average dynamic correlation length as a function of alumina content. We find that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (Vitreous Silica) to 7 mol%. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale.
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Boson peak, heterogeneity and intermediate-range order in binary SiO2-Al2O3 glasses
Nature Publishing Group, 2018Co-Authors: Mariana F Ando, Omar Benzine, Jeanluc Garden, Katrin Wondraczek, Stephan Grimm, Kay Schuster, Zhiwen Pan, Lothar WondraczekAbstract:Abstract In binary aluminoSilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. We determine the vibrational density of states VDoS g(ω) using low-temperature heat capacity data. From correlation with low-frequency Raman spectroscopy, we obtain the Raman coupling coefficient. Both experiments allow for the extraction of the average dynamic correlation length as a function of alumina content. We find that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (Vitreous Silica) to 7 mol%. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale
Satoshi Nishimura - One of the best experts on this subject based on the ideXlab platform.
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in situ conductance measurements for alignments of Silica particles under alternating electric fields
Langmuir, 2007Co-Authors: Satoshi Nishimura, Hideo Matsumura, Katsunori Kosuge, Tomohiko YamaguchiAbstract:The alignments of Silica particles formed in sinusoidal electrical fields of 1 kHz were assessed using an optical microscope with measuring the electric conductance of a Silica dispersion between two Pt electrodes in a Vitreous Silica glass cell. We confirmed that the electric conductance of the Silica dispersion between the two electrodes in the cell reflected the surface conductance of the Silica particles settling at the bottom of the cell. More interestingly, we observed that the electric conductance of the Silica dispersion in the cell increased when pearl chains of the Silica particle were formed along the direction of the electric field. However, no clear change in the electric conductance of the dispersion was observed at higher electric field strengths where a transition from pearl chains to zigzag band patterns and circulating movements of the Silica particles in the zigzag bands formed.
J R Manson - One of the best experts on this subject based on the ideXlab platform.
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anomalous phonon behavior blueshift of the surface boson peak in Silica glass with increasing temperature
Physical Review Letters, 2008Co-Authors: Walter Steurer, Andreas Apfolter, J R Manson, Elin Sondergard, Wolfgang E. Ernst, Markus Koch, Bodil HolstAbstract:We present helium atom scattering measurements of the boson peak at the surface of Vitreous Silica between 127.0 and 368.5 K. The most probable energy shows a strong temperature dependence and increases linearly with temperature in the measured range. The observed blueshift of the surface boson peak (shift rate 0:008 � 0:002 meV=K) is a factor of 4 to 10 times stronger than shift rates measured in the bulk by inelastic neutron and Raman scattering. We suggest that the anomalous shift direction of the boson peak to higher energies with increasing temperature has the same origin as the unusual temperature dependence of the bulk modulus of Silica glass.
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observation of the boson peak at the surface of Vitreous Silica
Physical Review Letters, 2007Co-Authors: Walter Steurer, Andreas Apfolter, Bodil Holst, Elin Sondergard, Wolfgang E. Ernst, Markus Koch, J R MansonAbstract:The boson peak is an excess in the phonon density of states compared to the Debye model that appears in almost all glasses. It has been repeatedly measured in the bulk by a variety of methods, but its origin is still highly debated. Here we present first experimental evidence of the boson peak on the v-SiO2 surface. The measurements were obtained by helium atom scattering. The boson peak appears as a dispersionless mode of � 4 meV in the recorded time-of-flight spectra. It is clearly identified as an excess contribution to the low energy Debye-like region in the surface phonon spectral density which is extracted from the timeof-flight spectra using a straightforward theoretical model. DOI: 10.1103/PhysRevLett.99.035503
