Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

The Raman coupling function in v-GeO2 and v-SiO2: A new light and neutron scattering study

We report inelastic Raman and neutron scattering spectra for the network glass formers vitreous silica (v-SiO₂) and vitreous germania (v-GeO₂) measured at temperature from 10 to 300 K. The ability to determine the temperature dependence of the luminescence background in Raman scattering has allowed to obtain the Raman coupling function C(ω) and in particular, its low-frequency limit. This study indicates that C(ω) has a linear behavior near the Boson peak maximum and below.     

On the analysis of the vibrational Boson peak and low-energy excitations in glasses

Implications of reduction procedures applied to the low energy part of the vibrational density of states in glasses and supercooled liquids are considered by advancing a detailed comparison between the excess – over the Debye limit – vibrational density of states g(ω) and the frequency-reduced representation g(ω)/ω² usually referred to as the Boson peak. Analyzing representative experimental data from inelastic neutron and Raman scattering, we show that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency of the maximum and the intensity of the excess experience dramatic changes; the former is reduced while the latter increases. The frequency and the intensity of the Boson peak are also sensitive to the distribution of the excess. In the light of the critical appraisal between the two forms of the density of states (i.e. the excess and the frequency-reduced one) we discuss changes of the Boson peak spectral features that are induced under the presence of external stimuli such as temperature (quenching rate, annealing), pressure, and irradiation. The majority of the Boson peak changes induced by the presence of those stimuli can be reasonably traced back to simple and expected modifications of the excess density of states and can be quite satisfactorily accounted for by the Euclidean random matrix theory. Parallels to the heat capacity ‘Boson peak’ are also briefly discussed.     

Raman study of lattice dynamics in quasicrystals

We present the first Raman investigation of icosahedral quasicrystals. Broad structured bands in the energy range up to ~500 cm^− 1 have been observed in a series of AlCuFe, AlPdMn and AlPdRe systems. Original information on the vibrational density of states g(ω) was obtained for AlPdRe; for AlCuFe and AlPdMn estimated g(ω) shows a good agreement with the previous neutron results, but demonstrates finer structure. Strong increase in the parameter of electron-vibrational coupling for the low-energy vibrations and its correlation with changes in electronic conductivity has been observed in the series from AlCuFe to AlPdRe. This suggests the increase of the degree of localization for these vibrational excitations and involved electronic states.     

Studies on electrical and the dielectric properties in MS structures

In this study, we investigated frequency dependent electrical and dielectric properties of metal-semiconductor (MS) structures using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics in the frequency range 100 kHz-10 MHz in the room temperature. The dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C-V and G/ω-V measurements and plotted as a function of frequency. In general, ε′, ε″ and tan δ values decreased with increasing the frequency, while σ_ac increased with increasing frequency. Furthermore, the voltage and frequency dependence of series resistance were calculated from the C-V and G/ω-V measurements and plotted as functions of voltage and frequency. The distribution profile of R_S-V gives a peak in the depletion region at low frequencies and disappears with increasing frequencies. Also, series resistance values decreased with increasing frequency. The experimental results show that both frequency dependent electrical and dielectric parameters were strongly frequency and voltage dependent.     

Uncommon features in the dynamic structure factor of a molten transition metal

The collective S_C(Q, ω), single-particle S_S(Q, ω) dynamic structure factors as well as the spectra for transverse-current correlations C_T(Q, ω) for molten nickel close to melting have been calculated from a massive molecular dynamics simulation using an Embedded Atom Model potential. A number of features are found in the structure factors that are absent in the spectra of simpler liquids such as molten alkali metals. Estimates for the shear component of the generalized, wavevector-dependent viscosity are obtained from analysis of C_T(Q, ω). Such data together with those concerning the strength of the thermal conduction channels for the excitation decay enable us to subtract such contributions to form the linewidths of Brillouin peaks and thus to evaluate the Q-dependent bulk viscosity coefficient. The result shows that contrary to inferences based upon geophysical data, both bulk and shear-viscosity coefficients are of the same order of magnitude.     

AC conductivity in amorphous germanium

The ac conductivity in evaporated amorphous germanium films has been measured as a function of annealing and has been found to obey the ω^0.8 law, in accordance with the hopping model. The dc conductivity measurements on the same samples show a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ law behaviour. The densities of localized states near the Fermi level g(E_F), obtained from both experiments are in reasonable agreement with each other. Both the measurements show a reduction by about a factor of 2 in g(E_F) when a freshly prepared film is fully annealed. High-temperature substrate films also show the ω^0.8 behaviour. This suggests that the frequency dependence of the ac conductivity is not caused by voids alone. Other possible explanations of our results are also discussed.     

Experimental confirmation of oscillating properties of the complex conductivity: Dielectric study of polymerization/vitrification reaction

Clear evidence of the existence of fractional kinetics containing the complex power-law exponents were obtained by conductivity measurements of polymerization reaction of polyvinylpyrrolidone (PVP) performed inside a dielectric cell. We established the relationship between the Fourier image R(jω) of the complex memory function K(t) and the time-dependent mean square displacement 〈r²(t)〉. This relationship helps to understand the origin of the different power-law exponents appearing in the real part of complex conductivity Re[σ(ω)] and find a physical/geometrical meaning of the power-law exponents that can form the complex-conjugated values. The complex-conjugated values of the power-law exponents leading to oscillating behavior of conductivity follows from the fractional kinetics suggested by one of the authors (R.R.N.). The relationships [R(jω)⇔Re[σ(ω)]⇔〈r²(t)〉] are becoming very efficient in classification of different types of collective motions belonging to light and heavy carriers involved in the relaxation/transfer process. The conductivity data obtained for Re[σ(ω)] during the whole polymerization process of the PVP at different temperatures (80, 90, 100 °C) are very well described by the fitting function that follows from the suggested theory. Original fitting procedure based on the application of the eigen-coordinates (ECs) method helps to provide a reliable fitting procedure in two stages and use the well-developed and statistically stable linear least square method (LLSM) for obtaining the correct values of the fitting parameters that describe the behavior of Re[σ(ω, T_r)] in the available frequency range for the current time of the chemical reaction T_r measured during the whole process of polymerization. The suggested theory gives a unique possibility to classify the basic types of motions that take place during the whole polymerization process.     

Simulation and scaling of ac conductivities in ion-conducting glasses

Simulation is made about ac conductivity, σ(ω), in ion-conducting glass by recurring to one of the Maxwell’s equations. It is concluded that (1) σ(ω) is made of conduction and conduction-related polarization of the mobile ions, both governed by the same relaxation time, τ, and that (2) high frequency dispersion in σ(ω), high frequency dispersion in the electric modulus peak, and distortion of a semicircle in the complex impedance plane, all arise from a distribution in τ, caused by the disordered structure of glass. Finally, a procedure of scaling ac data measured at various temperatures into a single master curve is given.     

Atomic dynamics in liquid KxSb1−x alloys

The atomic dynamics of liquid K_xSb_1−x alloys are studied using cold neutron inelastic scattering. Dynamical properties extracted from the experimental spectra, like the dispersions obtained from the maxima of the longitudinal current correlation function, J_l(Q, ω), and the vibrational spectra, show a strong dependence on concentration. With increasing content of Sb, higher frequency modes are observed in the vibrational spectrum. For the Sb-rich compositions the dispersion is split into two branches. This indicates the de-coupling of the dynamics of the two components, due to the difference in atomic mass, already around the position of the principal peak of the static structure factor, S(Q). The observations are consistent with results from earlier experiments on similar systems and from ab initio molecular dynamics simulations.     

Study on the influence of isotope exchange of hydrogen with deuterium on the vibrational spectrum of lysozyme by inelastic neutron scattering

The influence of isotope exchange of hydrogen with deuterium on the lysozyme dynamics was studied by incoherent inelastic neutron scattering. The generalized vibrational densities of states G(ω) were constructed from experimental results for protonated and deuterated protein samples at 200, 280, and 311 K. The major isotope effect was observed in G(ω) in the frequency region higher than 100 cm^?1. At all temperatures, both the Debye mode and the region of G(ω), whose spectral dimension corresponds to the fracton mode, are observed in the low-frequency region of the densities of states of both protonated and deuterated lysozyme. The influence of the hydrogen isotope exchange on the low-frequency region of G(ω) is insignificant.     

Fonction Dielectrique Complete D'un Cristal Moleculaire: Polarisabilite Moleculaire et Absorption Infrarouge

In the special case of a molecular crystal, it has been shown that the background of the infrared absorption spectra is not only a function of the dielectric constant at high frequency: the pertinent formulation introduces the entity (ε) _fc in which the contribution of all the optical modes have been taken into account. An analytical expression for (ε) _fc is suggested and make possible the determination of the oscillators strengths with a good accuracy. Calculations have been performed for polycrystalline samples of H.M.T and H.M.T.D. This new parameter has no effect upon the damping constants and slightly affects the static dielectric constant. On the other hand, its influence is clearly established for the infrared reflexion spectra.

The relation between microscopic molecular polarizability α(ω) and macroscopic dielectric function ε(ω) has been given. From the analytical expression of ε(ω), both real and imaginary parts of α(ω) have been expressed in terms of frequency. All the characteristic points of them have been calculated for spectral ranges corresponding to molecular crystal infrared absorption. Application to the F₂ vi-brational modes of solid H.M.T has been given. In other respects the contribution of the nuclei to the static molecular polarizability has been estimated with a good accuracy.     

Stochastic dynamics in liquid potassium as explored by polarized neutron scattering

The coherent S_c(Q, ω) and single particle S_s(Q, ω) dynamic structure factors which contribute to the low-energy spectrum of molten potassium are separated by means of neutron polarization analysis. The linewidth and amplitude of the single-particle spectra follow an apparent Fickian behavior with a diffusion coefficient well below the value found by macroscopic means. Once this is accounted for, the results are found to conform with predictions made from kinetic theory. In contrast, the available theoretical recipes to account for the coherent quasielastic intensity are seen to be semiquantitative, at best.     

Electrical conductivity and dielectric relaxation in non-crystalline films of tungsten trioxide

Amorphous tungsten trioxide (a-WO₃) thin films were prepared by thermal evaporation technique. The electrical conductivity and dielectric properties of the prepared films have been investigated in the frequency range from 100 Hz to 100 kHz and in the temperature range 293–393 K. In spite of the absence of the dielectric loss peaks, application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The frequency dependence of σ(ω) follows the Jonscher’s universal dynamic law with the relation σ(ω) = σ_dc + Aω^s, where s is the frequency exponent. The conductivity in the direct regime, σ_dc, is described by the small polaron model. The electrical conductivity and dielectric properties show that Hunt’s model is well adapted to a-WO₃ films.     

Optical properties of fluid sulfur up to supercritical conditions

The optical constants ?₁(ω) and ?₂(ω) of fluid sulfur have been derived for the first time over the entire liquid-vapour range at temperatures between 120°C and 1100°C and at pressures between 20 bar and 500 bar. The data have been obtained from reflectivity measurements by use of a modified Kramers-Kronig analysis.     

Comparison of analysis of dielectric spectra of PCL in the ε1 and the M1 formalism

The complex dielectric modulus, M¹(ω, T), for poly(ε-caprolactone) has been obtained by inverting the complex dielectric permittivity data, ε¹(ω, T) recorded in a wide temperature and frequency range. The analysis of the dielectric loss modulus spectra M″(ω, T) have been performed by using a simulated annealing Monte Carlo procedure in order to extract the relaxation parameters for each relaxation mode. Besides the local modes γ and β, the segmental α mode associated to the glass transition, the conductivity and the interfacial polarization peaks could be analyzed with a better precision than by using the conventional non-linear least squares method in ε″ domain. The merging of the α and β modes at high temperatures is much better defined due to the less significant contribution of the ionic conductivity in M¹ formalism.     

Effect of proton irradiation on electrical properties of a-As2S3

This paper reports the effect of proton irradiation on the electrical properties of a-As₂S₃ in the temperature range of 323–418 K and frequency range 0.1–100 kHz. The variation of transport property is studied with proton irradiation dose (1 × 10¹³ ions/cm² and 1 × 10¹⁵ ions/cm²). It has been observed that proton irradiation changes the dc conductivity (σ_dc), dc activation energy (ΔE_dc) and ac conductivity (σ_ac(ω)). The σ_dc and σ_ac(ω) increases with dose of proton irradiation. The value of frequency exponent (s) decreases with the temperature and irradiation dose. These results are explained in terms of change in density of defect states in these glasses.     

Observing the twinkling fractal nature of the glass transition

A fundamental understanding of the nature and structure of the glass transition in amorphous materials is currently seen as a major unsolved problem in solid-state physics. A new conceptual approach to understanding the glass transition temperature (T_g) of glass-forming liquids called the twinkling fractal theory (TFT) has been proposed in order to solve this problem. The main idea underlying the TFT is the development of dynamic rigid percolating solid fractal structures near T_g, which are said to be in dynamic equilibrium with the surrounding liquid. This idea is coupled with the concept of the Boltzmann population of excited vibrational states in the anharmonic intermolecular potential between atoms in the energy landscape. Solid and liquid clusters interchange or “twinkle” at a cluster size dependent frequency ω_TF, which is controlled by the population of intermolecular oscillators in excited energy levels. The solid-to-liquid cluster transitions are in accord with the Orbach vibrational density of states for a particular fractal cluster g(ω) ~ ω^df − 1, where the fracton dimension d_f = 4/3. To an observer, these clusters would appear to be “twinkling.” In this paper, experimental evidence supporting the TFT is presented. The twinkling fractal characteristics of amorphous, atactic polystyrene have been captured via atomic force microscopy (AFM). Successive two-dimensional height AFM images reveal that the percolated solid fractal clusters exist for longer time scales at lower temperatures and have lifetimes that are cluster size dependent. The computed fractal dimensions, ≈ 1.88, are shown to be in excellent agreement with the theory of the fractal nature of percolating clusters in accord with the TFT. The twinkling dynamics of polystyrene within its glass transition region are captured with time-lapse one-dimensional AFM phase images. The autocorrelation cluster relaxation function was found to behave as C(t) ~ t^− 4/3 and the cluster lifetimes τ versus width R were found to be in excellent agreement with the TFT via τ ~ R^1.42. This paper provides compelling new experimental evidence for the twinkling fractal nature of the glass transition.     

High frequency acoustic attenuation of vitreous silica: New insight from inelastic x-ray scattering

We present new experimental results on the propagation and damping of the high frequency acoustic-like modes in vitreous silica. The new data are measured by means of the inelastic x-ray scattering technique down to an exchanged wavevector Q ∼ 0.9 nm ^− 1, at the limit of the instrument capabilities. Thanks to the continuous development of the technique, the new spectra are characterized by a very high signal to noise ratio when compared to previous experiments. The higher data quality finally allows for a reliable determination of the position and width of the inelastic excitations. The new data show that the sound damping Γ is marked by a frequency dependence compatible with the Rayleigh law, Γ ∼ ν⁴, for frequencies below the position of the excess vibrational modes at the boson peak. We show that the new data are in good agreement with estimates of the acoustic mean free path from the thermal conductivity, which take into account the peculiar plateau at a few Kelvin. The connection between the boson peak and the Rayleigh law is further confirmed by a comparison of the present data with literature data for the sound attenuation in a permanently densified silica sample.     

Space charge capacitance spectroscopy in amorphous silicon Schottky diodes: Theory,modeling, and experiments

Numerical modeling of capacitance spectroscopy of hydrogenated amorphous silicon Schottky diodes is carried out. We test the accuracy of the determination of the density of states at the Fermi level, g(E_F), from an analytical treatment of the temperature (T) and frequency (f) dependence of the capacitance (C). Assessment of the position of the Fermi level and the attempt-to-escape frequency of states at E_F is also addressed. It is shown that the precision and reliability of the determination of g(E_F) is strongly dependent on the position of the Fermi level and the shape of the DOS and that the attempt-to-escape frequency is overestimated. Numerical calculations are then used to fit experimental capacitance data. Material parameters that provide the best fits are found in quite good agreement with independent modulated photocurrent and constant photocurrent measurements. Again, the attempt-to-escape frequency deduced from the simplified analytical treatment of capacitance data is found to be overestimated.