Total Vibrational Density Of States Research Articles

Comparison of Raman spectra and vibrational density of states between graphene nanoribbons with different edges

Vibrational properties of graphene nanoribbons are examined with density functional based tight-binding method and non-resonant bond polarization theory. We show that the recently discovered reconstructed zigzag edge can be identified from the emergence of high-energy vibrational mode due to strong triple bonds at the edges. This mode is visible also in the Raman spectrum. Total vibrational density of states of the reconstructed zigzag edge is observed to resemble the vibrational density of states of armchair, rather than zigzag, graphene nanoribbon. Edge-related vibrational states increase in energy which corroborates increased rigidity of the reconstructed zigzag edge.

The Raman coupling coefficients of calcium and sodium silicate melts in high-frequency region - SiOT model study

The Raman coupling coefficient (CC), which was defined by Shuker and Gammon, of silicate melt in high-frequency region, is still an untouched subject but the key parameter for quantitatively processing the Raman spectra (RS) to achieve the abundance of microstructural units. In this paper, we present the results of a newly constructed model - SiOT model (Wu et al) about the Raman coupling coefficients of calcium and sodium silicate melts, especially in the high-frequency region. This new model combines classical MD simulation with decomposition of simulated configurations and vibrational analysis including Wilson's GF matrix method, electro-optical parameter method (EOPM) and bond polarizability model (BPM). The displacement dependence of the cluster polarizability through the combination of EOPM and BPM connotes the description of the frequency dependence of CC. This allows us to make a direct comparison for the first time between the calculated VDOS (vibrational density of states) and RS within the entire frequency range. A strong conclusion was given that the partial VDOS, RS and CC are all the intrinsic properties of respective Qi species, and the only variable inducing the change in total VDOS, RS and CC is the change of microstructure, i.e. the distribution of the Qi.

Capillary Pressure and Phonons in Ag, Au, Cu and Ni Nanoparticles

Spherical nanoparticles of fcc metals with diameters between 2 and 10 nm have been studied by molecular-dynamics simulations. The results show that despite of the small size of the clusters the Kelvin-equation accounts for the capillary pressure building up in the particles. Furthermore, the simulations show that the capillary pressure leads to a shift of the local vibrational density of states in the core of the particles to higher energies. The cluster's total vibrational density of states is found to be broadened by the contributions from surface atoms. The connection between this broadening and the density of states in nanocrystalline materials is discussed.

Structure and dynamics of amorphous silica surfaces

We use molecular dynamics computer simulations to study the equilibrium properties of the surface of amorphous silica. Two types of geometries are investigated: (i) clusters with different diameters (13.5, 19, and 26.5 Å) and (ii) a thin film with thickness 29 Å. We find that the shape of the clusters is independent of temperature and that it becomes more spherical with increasing size. The surface energy is in qualitative agreement with the experimental value for the surface tension. The density distribution function shows a small peak just below the surface, the origin of which is traced back to a local chemical ordering at the surface. Close to the surface the partial radial distribution functions as well as the distributions of the bond–bond angles show features which are not observed in the interior of the systems. By calculating the distribution of the length of the Si–O rings we can show that these additional features are related to the presence of two-membered rings at the surface. The surface density of these structures is around 0.6/nm2, in good agreement with experimental estimates. From the behavior of the mean-squared displacement at low temperatures, we conclude that at the surface the cage of the particles is larger than the one in the bulk. Close to the surface the diffusion constant is somewhat larger than the one in the bulk and with decreasing temperature the relative difference grows. The total vibrational density of states at the surface is similar to the one in the bulk. However, if only the one for the silicon atoms is considered, significant differences are found.

Partial vibrational spectra for Ni and Nb atoms in the NiNb amorphous system

The partial frequency distributions of the individual chemical constituents and the total vibrational density of states of the amorphous alloy Ni62Nb38 have been measured using neutron inelastic scattering. Samples with different Ni isotope composition were involved. The average interaction constant of the Nb atoms with all neighbours is estimated to be about 40% higher than that of the Ni atoms. The low-temperature heat capacity of the amorphous alloys, Ni62Nb38 and Ni44Nb56, has been investigated by the adiabatic method. An increase of the Nb concentration decreases the Debye temperature (∼ 3%) and increases the density of the electron states at the Fermi level (∼ 26%). The temperature dependence of the heat capacity calculated from the experimental vibrational density of states of amorphous Ni62Nb38 and a model vibrational density of state of Ni44Nb56 were compared with the experimental ones. The former gives a better agreement than the latter.

Resonant guided elastic waves in an adsorbed slab: theoretical analysis of the density of states

An isolated elastic slab presents an infinite number of guided vibrational modes. Upon its adsorption on a semi-infinite substrate some of them become resonant with the bulk modes of the substrate. Such resonances were initially studied by Brillouin and by inelastic helium-atom scattering. We present here an exact method for obtaining the total vibrational density of states of the adsorbed slab. This method is then applied to isotropic elastic media and gives a semi-analytical expression for the vibrational density of states. Detailed analysis for an Al slab on a W substrate and vice versa shows that the resonant modes appear in general as well defined peaks in the total density of states. The position of these peaks enables us to study the speed of the resonant modes as a function of the thickness of the slab or of the propagation vector parallel to the surface.

Dynamic structure factor and vibrational properties of SiO2 glass.

Molecular-dynamics simulations are performed to determine dynamic correlations in ${\mathrm{SiO}}_{2}$ glass. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalizing the dynamical matrix. Dynamic structure factors, partial and total vibrational density of states (DOS), and participation ratios are calculated. The neutron-weighted dynamic structure factor, ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}), exhibits all the important features observed in the inelastic-neutron-scattering experiments on ${\mathrm{SiO}}_{2}$ glass. As a function of \ensuremath{\omega}, ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}) has two regions separated by a gap near 120 meV. The dominant features in ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}) are the peaks around 10--20 meV, an almost monotonic decrease between 20 and 100 meV, the gap near 120 meV, and a broad peak between 130 and 160 meV. The dynamic structure factor oscillates with variations in q. The total phonon DOS show two well-delineated bands, a broad band between 5 and 110 meV and a narrow band between 120 and 180 meV. The modes below 100 meV are spatially extended, whereas the high-frequency modes are localized. Calculations of the generalized neutron-weighted effective density of states are compared with neutron-scattering experiments.

Dynamical Structure Factor and Vibrational Normal Modes of SiO2 Glass

ABSTRACTWe study the atomic vibrational dynamics in silica glass (a-SiO2) using molecular-dynamics (MD) simulations and classical lattice dynamics method. The SiO2 glasses were generated by molecular-dynamics and steepest-descent quench (SDQ) using an effective interatomic potential consisting of two-body and three-body interactions. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalization of the dynamical matrix. The partial and total vibrational density of states (DOS), bond-projected DOS, participation ratio (PR), and neutron-weighted dynamic structure factor are calculated. The results are compared with inelastic neutron scattering experiments on SiO2 glass.

Stimulated-emission pumping spectroscopy of highly excited states of CH_3O(X˜E2): zero-order vibrational states at 3000 cm^−1 ≤ 9000 cm^−1

By the technique of stimulated-emission pumping, CH3O(X˜E2) radicals were excited to high-lying rotation–vibration states at energies of 3000 cm−1 ≤ Eυ ≤ 9000 cm−1. Over this range the total vibrational density of states increases from 0.05 states/cm−1 ≤ ρυ ≤ 4 states/cm−1. Stimulated-emission pumping spectra were recorded at a resolution of Δν ≈ 2 cm−1 with the pump laser tuned to the A˜←X˜,301, pP11(2.5, +1) transition. The spectra were found to be dominated by distinctive vibrational band progressions. The main progression was assigned to the ν3(C–O stretch) zero-order states with 3 ≤ υ3 ≤ 9. The spin–orbit splittings of the excited ν3 states were resolved. Higher-resolution spectra show that Franck–Condon intensity is carried by only few bright zero-order vibrational states, which, with higher excitation, are increasingly coupled to a manifold of dark background states. At energies of Eυ ≳ 6500 cm−1, above the adiabatic H–CH2O dissociation energy, the spectra show an underlying continuum, which can be attributed to effective intramolecular vibrational redistribution, promoted by the strong anharmonicity of the potential energy surface in the vicinity of the CH3O dissociation and isomerization barriers, and coupling of CH3O resonance states to the H + H2CO continuum.