Interface Roughness Parameters Research Articles

High-frequency bottom backscattering: Volume scattering from gassy mud

Bottom backscattering data at 40 kHz were collected from a stationary tripod deployed in a muddy embayment characterized by subsurface methane gas. Near-surface sediment geoacoustic, physical, and roughness properties were measured concurrently with the acoustic data in order to compare model predictions with measured backscatter strength as a function of grazing angle. Insitu values for sediment compressional wave velocity and attenuation, sediment density values from diver-collected cores, and values for interface roughness parameters from underwater photogrammetry were used to calculate the parameters for the composite roughness model. The model prediction for scattering strength from the sediment–water interface fell well below the measured value, indicating that volume scattering from within the sediment was the dominant process. Data obtained by other investigators reveal a layer of free methane located approximately 1 m below the sediment–water interface. These gas bubbles were evidently responsible for the high level of backscattering observed. A fit of the volume scattering component of the composite-roughness model to the data provides a measure of the contribution of these bubbles to acoustic backscatter.

A model/data comparison for low-frequency bottom backscatter

This article describes a model for bottom backscattering strength applicable over the frequency range 100–1000 Hz. The model includes backscatter due to both a rough water/sediment interface and internal sediment inhomogeneities—i.e., sediment-volume scattering within an upward-refracting sediment. The net result is predictions for bottom backscattering strength that depend on the following: frequency, sound speed of the water near the water/sediment interface, density ratio, sound absorption within the sediment, near-surface sound-speed gradient within the sediment, two interfacial roughness parameters, and a parameter proportional to the scattering cross section of a unit volume of sediment. The sediment sound-speed profile is specified in terms of near-surface values for the sound-speed ratio and gradient. Salient features of the modeled backscatter strength include marked departures from Lambert’s law for sediments with sound speed ratios greater than 1; 1–5-dB oscillations as a function of grazing angle, due to the complex structure of the sound field in the sediment; the dominance of sediment-volume scattering over rough surface scattering for most of the grazing angles, frequencies, and sediment types considered; and, for soft sediments, a decrease by about 8–10 dB in the overall level of backscatter as frequency increases from 100 Hz to almost 1000 Hz.

Resonant tunneling of double-barrier quantum wells affected by interface roughness.

Resonant tunneling of double-barrier quantum wells (DBQW's) affected by interface roughness has been investigated. Our results show that interface roughness induces oscillation resonant structure around the principal resonant peak. EA'ects of interface roughness on the resonant bias voltage, peak-to-valley current ratio, and the width of the principal resonant peak are also investigated. Temperature eAect is discussed. The results obtained here may be used to explain the oscillation or intrinsic instability observed in DBQW resonant-tunneling structures. With recent developments in semiconductor artificial structures, such as quantum wells (QW's) and superlattices (SL's), many important physical properties have been explored. In fundamental physics, quantum-well and superlattice structures have been used to explore the physical properties of low-dimensional systems and quantum efFects. Many novel phenomena in the quantum regime have been discovered, such as resonant tunneling of double-barrier quantum wells with negative differential resistance. ' Applications of these quantum wells and superlattices include high-speed electronics, photoelectronics, and photonic devices such as quantum-well lasers, modulation-doped field-effect transistors (MODFET's), photodetectors, etc. Double-barrier quantum-well (DBQW) tunneling structures have recently drawn a great deal of attention because tunneling is the fastest charge-transport phenomenon in semiconductors. Many important properties of resonant tunneling of electrons in DBQW's have been observed, such as negative differential resistance. Although many important properties of DBQW tunneling structures have been discovered, there are still many important questions to be answered, such as those concerning oscillation or instability in DBQW resonanttunneling structures. Although molecular-beam-epitaxy (MBE) can be used to fabricate the quantum-well or superlattice structures on an atomic scale, interface roughness cannot be cornpletely eliminated. It is important to understand how this interface roughness afFects the transport properties of DBQW structures. In some cases the presence of interface roughness simply acts as a small perturbation on effects that already exist in the absence of interface roughness. In other cases it can lead to quantitative changes in phenomena that exist in the absence of interface roughness. An example of this would be the roughness-induced splitting of the surface-plasmon dispersion curve observed experimentally, and discussed theoretically. Overall, roughness can have a significant influence on effects occurring at interfaces. The effects of interface roughness on the optical properties of quantum wells have been studied previously. The dominant effects observed due to interface roughness were the exciton linewidth broadening and the Stokes shift of the emission: the emission of the lowest heavyhole exciton generally is slightly shifted to lower energy (typically a few millielectron-volts) with respect to the absorption- or excitation-spectrum maximum. Recently, dynamic processes of excitonic transitions affected by interface roughness have also been studied. The excitontransition peak shifting towards lower energy with increasing delay time caused by interface roughness has been observed. In this paper, effects of interface roughness on the resonant tunneling in DBQW structures have been investigated. We show that interface roughness can cause multiple resonance peaks in DBQW's, and thus strongly modify the I- V characteristic. The I- V characteristic for different interface-roughness parameters as well as for different DBQW structures are studied. The effects on multiple resonance peaks are discussed. From our results, it is possible that the previously observed oscillations between the low- and high-current states in the DBQW s (Ref. 5) or intrinsic instability is induced by interface roughness.

The classification of binary eutectics

Values of the interface roughness parameter (α) have been calculated for the primary phases of the 12 simple binary eutectic systems for which sufficient thermodynamic data are available; namely, Ag−Bi, Ag−Cu, Ag−Pb, Al−Ge, Al−Sn, Bi−Cd, Bi−Sn, Cd−Pb, Cd−Zn, In−Zn, Pb−Sb and Sn−Zn. It is shown that in the presence of solute the values of α can be significantly different from those of the pure component elements. The calculated, values of α are correlated with eutectic morphologies on the basis of the Hunt and Jackson classification. If the broken lamellar structure is also in the faceted/nonfaceted group then the results are in agreement with Hunt and Jackson’s classification. The analysis is discussed in the light of recent interpretations of the theories of solid-liquid interfaces and resultant growth morphologies. It is concluded that thermodynamic calculations are useful in the prediction of eutectic structures even though they may not necessarily resolve arguments concerning the mechanisms of eutectic solidification.