Minyao Zhou

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 κ(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 κ(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 κ(T) in amorphous systems.

Numerical Simulation of Hopping Conductivity in Granular Metal Films

Abstract We numerically simulate the temperature dependence of thermally activated tunnelling conductance in granular metal films. By mapping the grains on to a simple-cubic lattice and considering hops up to third-nearest neighbours, it is shown that the hopping conductivity α follows - In σ α 1/T1/2 accurately over several orders of magnitudes. These results support the interpretation that this widely observed behaviour results from interpolating the high-temperature activation behaviour and the low-temperature - Inσ α 1/T1/4 behaviour.

Electrical properties of bitumen emulsions

The complex dielectric constant (permittivity) of water-in-bitumen emulsion was measured for emulsions with water volume fractions ranging from 0 to 0.5, in steps of 0.1. Significant departure from theoretical calculation was observed, and was attributed to the presence of polar components in the bitumen. Owing to the presence of salt in the water phase, the interfacial relaxation was found to be > 1 MHz. Low-frequency measurements can therefore be used to estimate the water volume fraction in laboratory emulsion flow experiments.

Virtual-Mode Excitations in Thin Metallic Films

By considering both the Drude and the interband components of the metal dielectric function, with the latter modeled as a sum of Lorentz oscillators, we show that there exist two virtual modes for thin metallic films which have distinct optical manifestations. Whereas one is absorptive in nature, the other is characterized by near-zero reflectivity. Calculated dispersion relations of the virtual modes are in good agreement with their measured optical manifestations in thin Ag films. The existence of both the absorption peak and the reflectivity minimum close to the optical plasmon frequency has been demonstrated experimentally for the first time.

Optical properties of finely structured metal‐insulator superlattice particulates

We investigate the optical properties of new ‘‘superlattice particles’’ that can be made of precisely controlled metal‐insulator layers fabricated by sputtering or evaporation techniques. We found that there are two limits to the reflectivity behavior corresponding to wide and narrow particles. In the narrow‐particle limit, a series of sharply defined absorption peaks at frequencies controlled by the width is observed. In the wide‐particle limit, most of the light incident on the particles will be absorbed. Materials of this type possess desirable infrared absorption characteristics and can be used for new polarization sensitive infrared absorbers and detectors.

Infrared optics of structured metal–insulator particulates

We show that rodlike particulates structured from interleaving layers of metal and insulator (or semiconductor) can exhibit novel infrared and far infrared characteristics that are tunable through the control of their geometric parameters. Results of scattering and absorption calculations are presented which demonstrate the validity of approximating the dielectric constant of the layered material by its effective medium value, thereby simplifying the prediction of the infrared properties. By clarifying the origin of the infrared absorption peaks, it is shown that the positions of the peaks are dependent on the transverse dimension of the particles as well as on the insulator dielectric constant and its relative fraction. The infrared characteristics of the structured particulates are contrasted with those of the metal, the insulator, and the semiconductor.

A Mean-Field Theory of Melting for Microcrystals

Publisher Summary The melting transition is one of the basic properties of a material. As it is inherently a collective phenomenon, one intuitively expects melting to be sensitive to the size of the sample, particularly in the realm of small molecular clusters. This chapter presents a mean-field theory for the calculation of some aspects of the melting transition in small molecular clusters. This theory predicts a systematic decrease in the melting temperature, in agreement with both experimental observations and results of molecular-dynamics simulations. A mean field theory for the melting behavior of molecular clusters yields results in reasonable agreement with both experimental observations and numerical simulations. However, because the present theory neglects kinetic effects, phenomenon such as the asymmetry observed between freezing and melting are outside the realm of the theoretical framework. The theory also is based on the assumption of two-body interactions. Thus, despite certain universal features, specific predictions based on this model may be inapplicable to those systems where the molecular interaction has a large many-body component.

Experimental observation of bending wave localization

Localization of bending waves has been observed for the first time for two‐dimensional (2D) acoustic wave propagation in an inhomogeneous composite system consisting of a steel plate decorated with two sets of randomly attached Lucite blocks. A significant experimental feature of the localized modes is an exponentially decay of the mode intensity from their peaked centers, with a decay length that increases as (f0−f)−1 when the mode frequency f approaches a quasimobility edge f0. The minimum attenuation length is of the order of a block diagonal and is about 40% of the banding wave’s wavelength. The experimental data, as well as results of finite‐element calculations, identify the source of the localization phenomenon as strong scattering of the bending wave by shear and flexural resonances of the Lucite blocks. This result supports theoretical predictions that resonant scattering enhances localization [cf. Scattering and Localization of Classical Waves in Random Media, edited by P. Sheng (World Scientifi...

Observation of acoustic wave localization.

Acoustic wave propagation has been studied in a composite elastic plate consisting of a periodic square array (horizontal period=7 in.) of 190 lucite blocks (3.5×3.5×3 in.) glued to a 1‐in.‐thick, 6×6‐ft steel plate. Elastic wave propagation is excited using a 0.2‐ms force transducer and the vertical response is measured by an accelometer (0–25.6 kHz). The significant feature of the data is a 20‐ to 30‐dB gap in the amplitude of the frequency response of the transfer function in the band between 2000 and 3800 Hz, which roughly corresponds to wavelengths coherently scattered by horizontal and diagonal arrays of lucite blocks, as well as to the first Brillouin zone of the square array. The decay length of the strongly localized bending wave mode is of the order of one unit cell ≊10 in. The decay length of the localized mode exponentially increases at the high‐frequency side of the gap with an inverse logarithmic derivative of 120 Hz. It is believed that the interaction between the propagating bending mode o...