Thermal conductivity and specific heat of glasses

Universality of ultrasonic attenuation coefficient of amorphous systems at low temperatures

The competition between unretarded dispersion interactions between molecules prevailing at medium range order length scales and their phonon induced coupling at larger scales leads to appearance of nano-scale sub structures in amorphous systems. The complexity of intermolecular interactions gives rise to randomization of their operators. Based on a random matrix modelling of the Hamiltonian and its linear response to an external strain field, we show that the ultrasonic attenuation coefficient can be expressed as a ratio of two crucial length-scales related to molecular dynamics. A nearly constant value of the ratio for a wide range of materials then provides a theoretical explanation of the experimentally observed qualitative universality of the ultrasonic attenuation coefficient at low temperatures.

Why Phonon Scattering in Glasses is Universally Small at Low Temperatures

We present a novel view of the standard model of tunneling two level systems (TLSs) to explain the puzzling universal value of a quantity, C∼3×10^{-4}, that characterizes phonon scattering in glasses below 1 K as reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound velocity. Physical considerations lead to a broad distribution of phonon-TLS couplings that (1) exponentially renormalize tunneling matrix elements, and (2) reduce the TLS density of states through TLS-TLS interactions. We find good agreement between theory and experiment for a variety of individual glasses.

Aging effects on thermal conductivity of glass-forming liquids

Thermal conductivity of a model glass-forming system in the liquid and glass states is studied using extensive numerical simulations. We show that near the glass transition temperature, where the structural relaxation time becomes very long, the measured thermal conductivity decreases with increasing age. Second, the thermal conductivity of the disordered solid obtained at low temperatures is found to depend on the cooling rate with which it was prepared. For the cooling rates accessible in simulations, lower cooling rates lead to lower thermal conductivity. Our analysis links this decrease of the thermal conductivity with increased exploration of lower-energy inherent structures of the underlying potential energy landscape. Further, we show that the lowering of conductivity for lower-energy inherent structures is related to the high-frequency harmonic modes associated with the inherent structure being less extended. Possible effects of considering relatively small systems and fast cooling rates in the simulations are discussed.

Boson peak, heterogeneity and intermediate-range order in binary SiO2-Al2O3 glasses

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.

Unusual specific heat of almost dry L-cysteine and L-cystine amino acids

A detailed quantitative analysis of the specific heat in the 0.5- to 200-K temperature range for almost dry L-cysteine and its dimer, L-cystine, amino acids is presented. We report the occurrence of a sharp first-order transition at ∼76 K for L-cysteine associated with the thiol group ordering which was successfully modeled with the two-dimensional Ising model. We demonstrated that quantum rotors, two-level systems (TLS), Einstein oscillators, and acoustic phonons (the Debye model) are essential ingredients to correctly describe the overall experimental data. Our analysis pointed out the absence of the TLS contribution to the low temperature specific heat of L-cysteine. This result was similar to that found in other noncrystalline amorphous materials, e.g., amorphous silicon, low density amorphous water, and ultrastable glasses. L-cystine presented an unusual nonlinear acoustic dispersion relation ω(q)=vq0.95 and a Maxwell-Boltzmann-type distribution of tunneling barriers. The presence of Einstein oscillators with ΘE∼70 K was common in both systems and adequately modeled the boson peak contributions.

Glassy thermal conductivity in the two-phase Cu(x)Ag(3-x)SbSeTe(2) alloy and high temperature thermoelectric behavior

We have measured the thermal transport properties over the temperature range 1.8 K<T<700 K of a two-phase alloy synthesized by reacting AgSbTe(2) and Ag(2)Se in a 1:1 molar ratio. Typical electrical resistivity values at 700 K are in the range ∼4 mΩ cm≤ρ≤20 mΩ cm, while low thermal conductivity values (κ<1 W m(-1) K(-1)) were obtained. We find that the thermal conductivity of this crystalline alloy has a temperature dependence strikingly similar to those of amorphous solids. In addition the thermal conductivity, thermopower, and electrical resistivity decouple. This result makes it possible to optimize thermoelectric performance by minimizing the electrical resistivity. It is therefore envisaged that this system has potential as a high performance bulk thermoelectric.

Observation of the onset of strong scattering on high frequency acoustic phonons in densified silica glass

The linewidth of longitudinal acoustic waves in densified silica glass is obtained by inelastic x-ray scattering. It increases with a high power alpha of the frequency up to a crossover where the waves experience strong scattering. We find that alpha is at least 4, and probably larger. Resonance and hybridization of acoustic waves with the boson-peak modes seems to be a more likely explanation for these findings than Rayleigh scattering from disorder.

Heat Conductivity of Amorphous Solids: Simulation Results on Model Structures

Through numerical simulation and consideration of phonon scattering by two-level states, the heat conductivity kappa(T), where T is temperature, has been calculated on model structures. The values obtained are in good quantitative agreement with measured data on polymethylmethacrylate, epoxy, amorphous selenium, and amorphous silicon dioxide over the temperature range 0.1 to 100 K. The calculated results reproduce the plateau feature, in the range of 5 to 20 K, that is generic to the heat conductivity of amorphous solids. Two model parameters, one characterizing the degree of structural disorder and the other related to the relaxational absorption of two-level states, are identified as being responsible for the behavior of kappa(T) at T >/= 5 K. The simulation results indicate the existence of a frequency-independent phonon diffusion regime that is consistent with the minimum phonon mean-free-path hypothesis. The magnitude of the phonon diffusion constant in this regime is shown to give a reasonable quantitative account of high-temperature kappa(T) in amorphous systems.