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We present a detailed investigation of the terahertz vibrational dynamics of 3-methylpentane performed by means of high-resolution inelastic x-ray scattering (IXS). We probe the dynamics in a large temperature range, which includes the glass, the supercooled liquid, and the liquid phases. The characteristic frequency of the excitations follows a well-defined dispersion curve extending beyond 8 nm(-1) at all the investigated temperatures, indicating the persistence of a solidlike behavior also in the liquid phase. This implies the existence of a pseudo-Brillouin zone whose size compares surprisingly well with the periodicity inferred from the first sharp diffraction peak in the static structure factor. We show that, in the investigated temperature range, both sizes undergo a variation of about 15%-20%, comparable to that of the average intermolecular distance. We finally show that the IXS sound velocity coincides with the infinite frequency sound velocity previously inferred from visible and ultraviolet Brillouin spectroscopy data. This analysis confirms the role of the shear relaxation processes in determining the variation with frequency of the apparent sound velocity.

Absorption coefficient measurements near the optical band edge of liquid iodine along the melting curve up to 1.52 GPa and 683 K are reported. An externally heated diamond anvil cell has been used to pressurize the sample. The spectra are dominated in the 4000\char21{}12000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range by exponential tails of the Urbach type with strongly temperature-dependent slopes and amplitudes. The possibility of connecting the evolution of this spectral feature with the conductivity jumps recently observed in liquid iodine around 3\char21{}4 GPa is discussed.

Neutron-diffraction measurements have been performed on supercritical fluid He4 in the 1485 K temperature range along an isochore passing close to the lower point. The quantum nature of the particle pair distribution functions is analyzed by comparing our results with classical molecular-dynamics simulations. Deviations from classical behavior are seen to persist up to T=85 K although they are much larger at lower temperatures. A measure of these deviations is obtained through an empirical parameter describing quantum delocalization around the classical positions. Limiting low-temperature behavior of this parameter is discussed and compared with direct experimental evidence of low-temperature quantum delocalization obtained from the particle momentum distribution. © 1994 The American Physical Society.

Neutron-diffraction data on liquid iodine at three temperatures on the coexistence curve are presented. The data show that the strong orientational correlations between neighboring molecules, already found at the melting point, persist up to the vicinity of the critical point. The A2 effective pair potential model is found to describe satisfactorily the structure of the liquid along the entire coexistence curve.

We report an extensive study of strained $\text{Ge}/{\text{Si}}_{0.2}{\text{Ge}}_{0.8}$ multiquantum wells grown by ultrahigh-vacuum chemical-vapor deposition. The microstructural properties of the samples were characterized by transmission electron microscopy and Raman spectroscopy. Their electronic properties have been investigated by means of infrared absorption measurements. Both interband and intersubband transitions were analyzed. Intersubband absorption energies were found in the 20--50 meV range, depending on the quantum well width. Interband and intersubband transition energies have been successfully described by means of both a $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ approach and a tight-binding model. In particular, we found a conduction-band offset between the L edges of 124 meV, well suited for the development of optoelectronic devices operating in the terahertz range. We also found that the energy difference between the ${\ensuremath{\Delta}}_{2}$ minima in the barrier and the L minima in the well is only $\ensuremath{\sim}40\text{ }\text{meV}$. This explains the observed ineffectiveness of the transfer doping in the strained heterostructures considered.

We report ultraviolet Brillouin light-scattering experimental data on $v\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ in an unexplored frequency region, performed with a newly available spectrometer, up to exchanged wave vector $q$ values of $0.075\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$, at different temperatures. The measured attenuation follows a ${q}^{2}$ law and is temperature dependent. Such temperature dependence is found to be in good agreement with that measured at lower $q$, suggesting that the broadening of the Brillouin peak is mainly due to a dynamic attenuation mechanism. The comparison of the present data with those obtained by inelastic x-ray and visible scattering indicates the existence of a crossover among different attenuation mechanisms, whose nature is briefly discussed.

The evolution of the dynamic structure factor $S(q,\ensuremath{\omega})$ of liquid Ne with exchanged momentum q and frequency $\ensuremath{\omega}$ has been studied by inelastic x-ray scattering in the q range extending from 0.3 to $16{\mathrm{\AA{}}}^{\ensuremath{-}1},$ i.e., covering the whole transition region from the collective regime to the single-particle one. The values of the first three spectral moments are extracted from the experiment either by direct line shape integration of the spectra or as a result of line shape best fitting procedures. The q dependence of different ratios constructed using the measured spectral moments clearly shows the transition from a classical regime, characteristic of the low-q macroscopic limit, to a quantum one, reached in the high-q microscopic limit. Moreover, direct information on the values of physically relevant parameters such as the mean atomic kinetic energy and the kinetic structure function is derived.

Inelastic-neutron-scattering measurements at high exchanged momentum have been performed in fluid $^{4}\mathrm{He}$ along a supercritical isochore in the 4.2--50 K temperature range. The data have been analyzed in the plane-wave impulse approximation to obtain the root-mean-square values of the particle momentum. Deviations from classical behavior have been interpreted as arising from the quantum nature of the particle oscillations in the framework of a cell model for the fluid.

Inelastic x-ray scattering (IXS) is used to measure the vibrational spectrum of liquid nitrogen at a temperature of 76 K and a pressure of 1 bar as a function of wave vector q in the $1.45\char21{}11.8 {\mathrm{A}}^{\ensuremath{-}1}$ range. The absolute IXS cross section for the vibrational excitation is determined in the explored q range. Its comparison with a theoretical calculation is very satisfactory, and leads to a precise determination of the intramolecular distance and the vibrational root-mean-square displacement. This demonstrates that the IXS technique can provide accurate information on the vibrational dynamics in molecular systems. The peculiarities of IXS with respect to other vibrational spectroscopies are briefly discussed.