On the lattice scattering and effective mass of holes in natural diamond

Drift mobility and high-field drift velocity of holes in natural diamond are investigated with the time-of-flight technique. A theoretical interpretation based on Monte Carlo simulation suggests possible values for the deformation potential parameters and effective masses.     

The anisotropy of the hot-hole drift velocity in Ge

This paper presents a microscopical interpretation of the anisotropy of the hot-hole drift velocity in Ge at 77°K. The theory makes use of the Monte Carlo technique in solving the Boltzmann equation and is based on a single valence band model (the heavy mass one) warped and parabolic. The theoretical calculations carried out in a general form show that: (i) the anisotropy of the hot-hole drift velocity originates from the warping of the valence band structure; (ii) the strong interaction of holes with optical phonons via emission processes plays a fundamental role in the anisotropy supporting the reliability of the streaming motion model first suggested by Pinson and Bray.The comparison between theory and experiment at 77°K in Ge is satisfactory for the whole range of fields examined $\text{(50 ? E ? 10}{}^4\text{V}\text{cm}\text{)}$, confirming the basic validity of the present approach. The importance of optical and acoustic scattering mechanisms have been found comparable, the coupling strength ratio $\text{w}{}_{\mathrm{op}}\text{w}{}_{\mathrm{ac}}$ assuming the value 1.36.     

Microwave-induced transient parametric gratings

We propose a new method for the formation of light-induced transient gratings in microwave-biased semiconductor crystals that exhibit a negative differential resistivity. Nonuniform heating of the electron gas in alternating electric fields induces spatially periodic modulation of the refractive index with spacing that is tunable by the external field frequency. Numerical simulations performed on a bulk GaAs sample prove that transient parametric gratings of both free-carrier and electro-optic origin can be triggered by a spatially modulated light pattern or by uniform photoexcitation.     

Correlation MRI/ultrastructure in cerebral ischemic lesions: application to the interpretation of cortical layered areas

The origin and fate of cortical ischemic lesions, showing a stratified appearance at in vivo MRI-examination, was studied on rats in which a focal brain ischemia was induced by occlusion of the middle cerebral artery. One week after ischemia induction, six rats were selected in which three layers of different intensity were visible in the lesioned cortex. Two animals were sacrificed and studied by histology and electron microscopy. The external hyperintense layer was composed of pial and lesioned nervous tissue, the intermediate of degenerating nervous tissue in which an accumulation of macrophages was found, the deepest of edematous nerve tissue without a marked accumulation of macrophages. The remaining rats underwent further MRI examinations showing that, in the lesioned areas, cerebral blood volume was 14-69% lower than the contralateral healthy cortex. At histological and ultrastructural examination, a large part of the lesion was occupied by enlarged pial tissue and marginal glia. A dilatation of the ventricular cavity and cystic structures were also visible. In three animals an increase of the transverse diameter of the caudo-putamen ipsilateral to the lesion was found. The study suggests that the layered appearance is mainly due to an accumulation of macrophages in the intermediate layer and that several processes contribute to the occlusion of the space created by the removal of the necrotic tissue in stratified ischemic lesions (i.e. expansion of the pial tissue, thickening of the marginal glia; expansion of the caudo-putamen, enlargement of the ventricular cavity and development of cystic structures).