Effect of densification on the density of vibrational states of glasses

We studied the effect of densification on the vibrational dynamics of a Na(2)FeSi(3)O(8) glass. The density of vibrational states (DOS) has been measured using nuclear inelastic scattering. The corresponding changes in the microscopic, intermediate-range, and macroscopic properties have also been investigated. The results reveal that, in the absence of local structure transformations, the Debye level and the glass-specific excess of vibrational states above it have the same dependence on density, and the evolution of the DOS is fully described by the transformation of the elastic medium.     

Short-time vibrational dynamics of metaphosphate glasses

In this paper we present the picosecond vibrational dynamics of a series of binary metaphosphate glasses, namely Na₂O–P₂O₅, MO–P₂O₅ (M=Ba, Sr, Ca, Mg) and Al₂O₃–3P₂O₅ by means of Raman spectroscopy. We studied the vibrational dephasing and vibrational frequency modulation by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. The fitting method used enables one to model the real line profiles intermediate between Lorentzian and Gaussian by an analytical function, which has an analytical counterpart in the time domain. The symmetric stretching modes ν_s(PO₂^?) and ν_s(P–O–P) of the PO₂^? entity of PØ₂O₂^? units and of P–O–P bridges in metaphosphate arrangements have been investigated by Raman spectroscopy and we used them as probes of the dynamics of these glasses. The vibrational time correlation functions of both modes studied are rather adequately interpreted within the assumption of exponential modulation function in the context of Kubo–Rothschield theory and indicate that the system experiences an intermediate dynamical regime that gets only slower with an increase in the ionic radius of the cation-modifier. We found that the vibrational correlation functions of all glasses studied comply with the Rothschild approach assuming that the environmental modulation is described by a stretched exponential decay. The evolution of the dispersion parameter α with increasing ionic radius of the cation indicates the deviation from the model simple liquid indicating the reduction of the coherence decay in the perturbation potential as a result of local short lived aggregates. The results are discussed in the framework of the current phenomenological status of the field.     

Low-lying vibrational states of145,147,149Nd

The fragmented neutron states in^145,147,149Nd detected through¹⁴⁴Nd(d, p) and^148,150Nd(d, t) reactions can be accounted for in terms of quasiparticles coupled with anharmonic vibrator model. The wave functions, obtained from diagonalisation of the Hamiltonian matrices are utilised to calculate B(E2), B(M1) and branching ratios in^145,147Nd. The calculated results are discussed in the light of the recent experimental findings.     

Nature of soft vibrational modes in metallic glasses

A general theory of vibrational excitations in metallic glasses is presented, based on the disclination model of the structure of solids with local icosahedral symmetry. The representation of nonorthogonal plane waves is used, which allows one to separate the diagonal terms in the Hamiltonian, and also the terms describing scattering of these waves by fluctuations of the density and the force constants, and by topological structure fluctuations. We have derived and solved the Dyson equation for the Green’s function. We have obtained an analytical expression for the density of vibrational states of the glass. The nature of the soft vibrational modes is discussed. It is shown that these modes are resonant modes, whose presence is due to the fluctuations of the density and the force constants. Special attention is given to the role of forward scattering and the nature of the spatial variation in the amplitude of the given modes. Fiz. Tverd. Tela (St. Petersburg) 41, 372–379 (March 1999)     

Tunneling states in ferromagnetic glasses

We study the interaction between structural defects, represented by two level systems, and spin waves in a ferromagnetic glass. The damping and energy shift of the spin wave due to this interaction have a “resonant” and a “relaxation” contribution in analogy with the case of phonons. Ferromagnetic resonance in amorphous ferromagnets can provide useful information on structural relaxation in these materials.     

Heat capacity and low-frequency vibrational density of states. Inferences for the boson peak of silica and alkali silicate glasses

A simple method is used to determine in an absolute way the low-frequency part of the vibrational density of states, g(ω), from inversion of low-temperature heat capacities of glasses. Calculations have been made from adiabatic measurements performed from 10 to 300 K for vitreous SiO₂ and a series of Li, Na and K silicate glasses with up to 50 mol% metal oxide. In all cases, the calculated low-frequency feature is made up of two bands whose maxima lie between 100 and 140 cm⁻¹ and near 400 cm⁻¹. Both the frequency and intensity of the first peak of g(ω) increase linearly with metal oxide content from pure SiO₂ to the metasilicate stoichiometry, reflecting weaker bonding with nonbridging oxygens in the order Li, Na, K, i.e., in order of increasing cationic radius and mass. As represented by the peaks found below 50 cm⁻¹ in plots of g(ω)/ω² vs. ω and below 20 K in plots of C_p/T³ vs. T, the boson peak varies in a different way with composition. Its intensity increases with increasing polymerization for Li and Na silicates whereas the converse holds true for K silicates. These variations agree with the enhancement of the contribution of floppy modes and transverse acoustic modes to the low-frequency vibrational density of states that has previously been reported, but they also point to the importance of localized vibrations in which network modifier cations participate. For either the first peak of g(ω) or the boson peak, however, the size of the alkali cation exerts a stronger influence than the degree of polymerization of the anionic framework. Finally, the universal phenomenology of the boson peak is borne out by the collapse of the calorimetric data on a single master curve when one plots the measurements in a reduced form by using the intensity and position of the peaks as scaling parameters.     

Raman scattering and the low-frequency vibrational spectrum of glasses

We present a theory of low-frequency Raman scattering in glasses, based on the concept that light couples to the elastic strains via spatially fluctuating elasto-optic (Pockels) constants. We show that the Raman intensity is not proportional to the vibrational density of states (as was widely believed), but to a convolution of Pockels constant correlation functions with the dynamic strain susceptibilities of the glass. Using the dynamic susceptibilities of a system with fluctuating elastic constants we are able for the first time to describe the Raman intensity and the anomalous vibration spectrum of a glass on the same footing. Good agreement between the theory and experiment for the Raman spectrum, the density of states, and the specific heat is demonstrated at the example of glassy As(2)S(3).     

Exotic states in long-range spin glasses

We consider Ising spin glasses onZ ^d with couplingsJ _xy =c _y–x Z _xy , where thec _y 's are nonrandom real coefficients and theZ _xy 's are independent, identically distributed random variables withE[Z _xy ]=0 andE[Z _xy ² ]=1. We prove that if _y |c _y |= while _y |c _y |²=, then (with probability one) there are uncountably many (infinite volume) ground states , each of which has the following property: forany temperatureT<, there is a Gibbs state supported entirely on (infinite volume) spin configurations which differ from only atfinitely many sites. This and related results are examples of the bizarre effects that can occur in disordered systems with coupling-dependent boundary conditions.Research supported in part by NSF Grants DMS-9196086 and 9209053Research supported in part by U.S. Department of Energy Grant No. DE-FG03-93ER25155     

Coexistence of aging states in spin glasses

The influence of temperature variations on the aging process of a Cu(Mn) spin glass has been investigated. It is found that the dynamics of the aging process is not very susceptible to small temperature variations around the measurement temperature. The astonishing possibility for the spin glass system to house two coexisting aging states is demonstrated.     

Rotational and vibrational states in 115Te

High-spin states of the ¹¹⁵Te were studied by in-beam spectroscopy with the ⁸⁹Y (²⁹Si, p2n) fusion-evaporation reaction at a beam energy of 108 MeV. γ?γ coincidence and γ?γ angular correlation analyses were employed for determining the level scheme of ¹¹⁵Te. We have identified two vibrational-like bands built on the νh_11/2 and νg_7/2 quasiparticle states and the noncollective oblate states from the full alignment of quasiparticle configurations. In addition, a regular ΔI = 2 sequence with positive-parity was found for the first time in odd-A Te nuclei. This sequence is interpreted as a deformed structure resulting from three-quasiparticle alignment having the [π(g_7/2, h _11/2) ? ν(h _11/2)] configuration. Calculations of total Routhian surfaces and cranked shell model were performed and were used for assigning quasiparticle configurations to the states in ¹¹⁵Te.     

Density of vibrational states of a hyperquenched glass

The vibrational density of states of a hyperquenched and an annealed glass has been measured using nuclear inelastic scattering. The hyperquenched sample shows a higher number of vibrational states in the low-energy region with respect to the annealed glass. It reveals, however, lower density and sound velocity and, therefore, smaller Debye energy. After rescaling the energy axes in Debye energy units and area renormalization, the density of states of both samples becomes identical. Thus, the effect of quenching is described by the transformation of the continuous medium.     

Numerical studies of the vibrational isocoordinate rule in chalcogenide glasses

Many properties of alloyed chalcogenide glasses can be closely correlated with the average coordination of these compounds. This is the case, for example, of the ultrasonic constants, dilatometric softening temperature and the vibrational densities of states. What is striking, however, is that, at a given average coordination, these properties are nevertheless almost independent of the elemental composition. Here, we report on some numerical verification of this experimental rule as applied to the vibrational density of states. We find that this rule is not exact but holds qualitatively well over a wide range of compositions and local chemical correlations. Received 25 April 2000     

Anomalous vibrational states of a monatomic surface film

Vibrational states of a one-dimentional chain of atoms which models peculiarities of a monatomic surface film are considered. A set of nontrivial equilibrium states of the system is determined in whose vicinity almost linear band vibrations are possible. Peculiarities of anomalous branches of vibrational states are discussed. It is shown that the low-frequency gap of vibrational states disappears under certain conditions. This determines the possibility of appearance of low-frequency sound vibrations which are not possible for a trivial equilibrium distribution of atoms in the film.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika No. 7, pp. 22–25, July, 1982.     

Submillimeter-wave spectroscopy of HCN in excited vibrational states

Measurements of the rotational spectrum of HCN in excited vibrational states have been extended to higher-J values. The transitions reach J=8←7 around 710 GHz for most vibrational states studied in this investigation and J=22←21 near 2 THz for the (020) and (030) vibrational states. Using a pure sample of gaseous HCN at 350 K, selected states up to one quantum in the C–H stretching vibration at 3311.5 cm⁻¹ have been investigated. Even transitions having two quanta in the C–H stretch could be studied employing a glow discharge in a gas mixture of CH₄ and N₂. Molecular constants in 13 vibrational states have been obtained, several of which have been studied for the first time by rotational spectroscopy. The vibrational temperature in the discharge system is found to be about 1500 K for the stretching vibrational modes and about 600 K for the bending states.     

Direct observation of self-trapped vibrational states in alpha-helices

Femtosecond infrared pump-probe spectroscopy of the N-H mode of a stable alpha-helix reveals two excited-state absorption bands, which disappear upon unfolding of the helix. A quantitative comparison with polaron theory shows that these two bands reflect two types of two-vibron bound states connected to the trapping of two vibrons at the same site and at nearest neighbor sites, respectively. The latter states originate from an acoustic phonon in the helix, which correlates adjacent sites.