Evidence for a crossover in the frequency dependence of the acoustic attenuation in vitreous silica

We report measurements of the sound attenuation coefficient in vitreous silica, for sound waves of wavelength between 50 and 80 nm, performed with the new inelastic UV light scattering technique. These data indicate that in silica glass a crossover between a temperature-dependent (at low frequency) and a temperature-independent (at high frequency) acoustic attenuation mechanism occurs at Q approximately equal to 0.15 nm(-1). The absence of any signature in the static structure factor at this Q value suggests that the observed crossover should be associated with local elastic constant fluctuations.     

High frequency dynamics in a monatomic glass

The high frequency dynamics of glassy selenium has been studied by inelastic x-ray scattering at beam line BL35XU (SPring-8). The high quality of the data allows one to pinpoint the existence of a dispersing acoustic mode for wave vectors (Q) of 1.5相似文献   

Ultrasonic properties of strontium titanate at the 105°K transition

The temperature dependence of the sound velocity and the ultrasonic attenuation has been measured by the pulse-echo method in single crystals of strontium titanate in the vicinity of the 105°K structural transformation. Special attention is paid to the anisotropy of these measured quantities. The behaviour of the attenuation in the cubic phase can nearly quantitatively be interpreted as interaction with critical fluctuations belonging to the R-corner phonon mode. A numerical estimate leads to the conclusion that also in the tetragonal phase this fluctuation scattering contributes most to the attenuation, in comparison to resonant interaction (of the Landau-Khalatnikov type) and domain effects, which under the present experimental conditions dominate the sound velocity behaviour.     

Anomalous phonon behavior: blueshift of the surface boson peak in silica glass with increasing temperature

We present helium atom scattering measurements of the boson peak at the surface of vitreous silica between 127.0 and 368.5 K. The most probable energy shows a strong temperature dependence and increases linearly with temperature in the measured range. The observed blueshift of the surface boson peak (shift rate 0.008+/-0.002 meV/K) is a factor of 4 to 10 times stronger than shift rates measured in the bulk by inelastic neutron and Raman scattering. We suggest that the anomalous shift direction of the boson peak to higher energies with increasing temperature has the same origin as the unusual temperature dependence of the bulk modulus of silica glass.     

非晶态快离子导体(AgI)x(Ag4P2O7)1-x的声学性质

用液氮骤冷方法制备了(AgI)_x(Ag₄P₂O₇)_1-x系列非晶态快离子导体。对AgI摩尔浓度x=0.50,0.60,0.67,0.75,0.80的样品,在77—300K温度范围及2,5,10,15MHz的频率上测量了纵波和横波的超声衰减和声速。发现在200—240K附近存在一个异常强的弛豫型超声吸收峰,随AgI含量的增加,该峰的位置向低温方向移动,且峰的高度增大。在实验的温度范围内,观察到纵波和 关键词：     

Temperature dependence of electron mean free path in molybdenum from ultrasonic measurements

The temperature dependence of the electronic mean free path in molybdenum has been obtained from ultrasonic attenuation measurements. For temperatures up to 30 K a T^?2 law is followed suggesting the importance of electron-electron scattering in the attenuation mechanism.     

Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-1). This, enabling S(Q,E) measurements as a function of Q at constant E transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with Q, to a high-Q regime, where it is no longer possible to observe a dominant excitation in the S(Q,E).     

Localized excitation in the hybridization gap in YbAl3

The intermediate valence compound YbAl3 exhibits a broad magnetic excitation in the inelastic neutron scattering spectrum with characteristic energy E1 approximately 50 meV, equal to the Kondo energy (T(K) approximately 600-700 K). In the low temperature (T < T(coh) approximately 40 K) Fermi liquid state, however, a new peak in the scattering occurs at E2 approximately 33 meV, which lies in the hybridization gap that exists in this compound. We report inelastic neutron scattering results for a single-crystal sample. The scattering at energies near E1 qualitatively has the momentum (Q) dependence expected for interband scattering across the indirect gap. The scattering near E2 has a very different Q dependence: it is a weak function of Q over a large fraction of the Brillouin zone and is smallest near (1/2,1/2, 1/2). A possibility is that the peak at E2 arises from a spatially localized excitation in the hybridization gap.     

Effect of electric field on the longitudinal ultrasonic waves in Rochelle salt

The temperature dependence of velocity and attenuation for the longitudinal ultrasonic waves propagating along the (100) direction has been measured in the vicinity of the upper transition temperature under the various electric conditions in Rochelle salt.     

Rayleigh scattering cross sections of combustion species at 266, 355, and 532 nm for thermometry applications

Rayleigh scattering cross sections are measured for nine combustion species (Ar, N2, O2, CO2, CO, H2, H2O, CH4, and C3H8) at wavelengths of 266, 355, and 532 nm and at temperatures ranging from 295 to 1525 K. Experimental results show that, as laser wavelengths become shorter, polarization effects become important and the depolarization ratio of the combustion species must be accounted for in the calculation of the Rayleigh scattering cross section. Temperature effects on the scattering cross section are also measured. Only a small temperature dependence is measured for cross sections at 355 nm, resulting in a 2-8% increase in cross section at temperatures of 1500 K. This temperature dependence increases slightly for measurements at 266 nm, resulting in a 5-11% increase in cross sections at temperatures of 1450 K.     

Fast sound in expanded fluid Hg accompanying the metal-nonmetal transition

The dynamic structure factor S(Q,omega) of expanded fluid Hg has been measured up to the metal-nonmetal transition region at 9.0 g cm(-3) (1723 K and 1940 bars) using high-resolution inelastic x-ray scattering, at momentum transfers, Q, from 0.2 to 4.8 A(-1). Analysis in the framework of generalized hydrodynamics reveals that the frequencies of the collective excitations increase faster with Q than estimated from the macroscopic speed of sound. The effective sound velocity at 9.0 g cm(-3) estimated from the dispersion relation is triple the ultrasonic sound velocity. The present result suggests the existence of fast sound in expanded fluid Hg accompanying the metal-nonmetal transition.     

Temperature dependent ultrasonic characterization of biological media

Quantitative ultrasound (QUS) is an imaging technique that can be used to quantify tissue microstructure giving rise to scattered ultrasound. Other ultrasonic properties, e.g., sound speed and attenuation, of tissues have been estimated versus temperature elevation and found to have a dependence with temperature. Therefore, it is hypothesized that QUS parameters may be sensitive to changes in tissue microstructure due to temperature elevation. Ultrasonic backscatter experiments were performed on tissue-mimicking phantoms and freshly excised rabbit and beef liver samples. The phantoms were made of agar and contained either mouse mammary carcinoma cells (4T1) or chinese hamster ovary cells (CHO) as scatterers. All scatterers were uniformly distributed spatially at random throughout the phantoms. All the samples were scanned using a 20-MHz single-element f/3 transducer. Quantitative ultrasound parameters were estimated from the samples versus increases in temperature from 37 °C to 50 °C in 1 °C increments. Two QUS parameters were estimated from the backscatter coefficient [effective scatterer diameter (ESD) and effective acoustic concentration (EAC)] using a spherical Gaussian scattering model. Significant increases in ESD and decreases in EAC of 20%-40% were observed in the samples over the range of temperatures examined. The results of this study indicate that QUS parameters are sensitive to changes in temperature.     

Relaxation and resonance ultrasound attenuation by Jahn-Teller centers in a GaAs:Cu crystal

The interaction of ultrasound with Cu_Ga4As in a GaAs:Cu crystal has been experimentally studied. The temperature dependences of the attenuation of all normal ultrasonic modes propagating in the ??110?? direction both in doped copper and in nominally pure gallium arsenide crystals have been measured. In the GaAs:Cu crystal, the attenuation peak has been revealed for a transverse wave polarized along the ??110?? axis whose elastic shifts correspond to the symmetry of the tetragonal mode of the Jahn-Teller effect. The temperature dependence of the attenuation of this wave indicates that two types of attenuation??relaxation and resonance??occur. The constructed temperature dependence of the relaxation time indicates that tunneling through the potential barrier between the minima of the adiabatic potential energy is the main relaxation mechanism at temperatures below 10 K. Tunneling splitting estimated from experimental data is in good agreement with the theoretical estimate.     

Structural studies of nano silica employing on-line ultrasonic studies

Monodispersed, nano silica particles have been prepared by sol-gel hydrolysis and condensation of the metal alkoxide using pH buffer. The prepared particles are characterised by XRD, FTIR, BET, SEM, TEM measurements. The measurements reveal that the size and shape of silica particles depend on concentration of water. In addition, the ultrasonic longitudinal velocity and attenuation of the nano silica particles have been measured at a frequency of 5?MHz over a wide range of temperatures from 300?K to 1150?K in nano silica. The different structural transitions, such as monoclinic, orthorhombic, orthorhombic with a non-integral super lattice, stable orthorhombic and hexagonal, which exist in silica are explained based on on-line high-temperature ultrasonic velocity and attenuation measurements.     

Temperature dependence of hypersonic attenuation in NaCl and KCl crystals

The usual theories of ultrasonic attenuation (Landau-Rumer and Akhiezer) are in disagreement with experiments involving hypersonic phonons (Brillouin scattering). This is shown here by measurements on NaCl and KCl. We observe a Tⁿ dependence with n approximately equal to 2.8, and an attenuation increasing with temperature in the low and high temperature range respectively. An extension of the Landau-Rumer and Akhiezer theories to hypersonic frequencies is proposed. Our theoretical expressions give a good account of the temperature dependence of the attenuation in the whole temperature range.     

Crystal-like high frequency phonons in the amorphous phases of solid water

The high frequency dynamics of low-density (LDA) and high-density (HDA) amorphous ice and of cubic ice ( I(c)) has been measured by inelastic x-ray scattering in the 1-15 nm(-1) momentum transfer ( Q) range. Sharp phononlike excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8 nm(-1) in LDA and I(c) and up to 5 nm(-1) in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to soundlike excitations which is interpreted as a sign of low local disorder.     

Thermal conductance of thin silicon nanowires

The thermal conductance of individual single crystalline silicon nanowires with diameters less than 30 nm has been measured from 20 to 100 K. The observed thermal conductance shows unusual linear temperature dependence at low temperatures, as opposed to the T3 dependence predicted by the conventional phonon transport model. In contrast to previous models, the present study suggests that phonon-boundary scattering is highly frequency dependent, and ranges from nearly ballistic to completely diffusive, which can explain the unexpected linear temperature dependence.     

Electron relaxation and transport in nanostructured and bulk silica

Processes of ballistic and hot electron relaxation in extended bulk as well as nanostructured silica have been analyzed by means of a phonon-based scattering model and respective Monte-Carlo computer simulation. Optical as well as acoustic phonons are taken into account. Trajectories of electrons and their energy attenuation in nanostructured silica are additionally affected by scattering processes at the grain boundaries between the nanoparticles, i.e. by surface phonon as well as potential scattering. Moreover, a flatter conduction band and a higher effective electron mass have been taken into account too. According to these calculations, electrons with an initial energy of several eV, but still below the valence band ionization threshold, were thermalized in 50–300 fs increasing with the silica grain size from 1 nm up to bulk material. The electron emission probabilities over the surface barrier into vacuum are extended up to depths of 60–100 nm, respectively, increasing with enhancement by an electric field.     

Attenuation of micro-wave ultrasonics in the film superconducting Pb

The ultrasonic attenuation of superconducting Pb film in the magnetic field was measured at frequency 9.4 GHz. The field dependence of the attenuation coefficient of the thin film Pb has been compared to theoretical expression.     

A study of ultrasonic velocity and attenuation on nanocystalline MgCuZn ferrites

The nanocrystalline MgCuZn ferrites with particle size (∼30 nm) have been synthesized by microwave-hydrothermal (M-H) method at 160 °C/45 min. The powders were densified at 750-900 °C/30 min using microwave sintering method. The sintered samples were characterized using X-ray diffraction and scanning electron microscope. The grain sizes of the sintered samples are in the range of 60-80 nm. The ultrasonic velocities have been measured on MgCuZn ferrites using the pulse transmission method at 1 MHz. The ultrasonic velocity is found to decrease with an increase of temperature. A small anomaly is observed around the Curie temperature, 520 K. The anomaly observed in the thermal variation of longitudinal velocity and attenuation is explained with the help of magneto-crystalline anisotropy constant.