Refractive index spectral dependence, Raman and transmission spectra of high-purity 28^{28}Si, 29^{29}Si, 30^{30}Si, and nat^{nat}Si single crystals

Precise measurement of the refractive index of stable silicon isotopes $^{28}$Si, $^{29}$Si, $^{30}$Si single crystals with enrichments above 99.9 at.% and a silicon single crystal $^{nat}$Si of natural isotopic composition is performed with the Fourier-transform interference refractometry method from 1.06 to more than 80 mkm with 0.1 cm$^{-1}$ resolution and accuracy of $2 \times 10^{-5} ... 1 \times 10^{-4}$. The oxygen and carbon concentrations in all crystals are within $5 \times 10^{15}$ cm$^{-3}$ and the content of metal impurities is $10^{-5} ... 10^{-6}$ at.%. The peculiar changes of the refractive index in the phonon absorption region of all silicon single crystals are shown. The coefficients of generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also studied

Isotope dependence of band-gap energy

The results of the quantitative investigations of the renormalization of the absorption edge of different compounds by the isotope effect are described.Comment: 7 pages, 3 figure

Electron-Phonon Interaction in Tetrahedral Semiconductors

Effects of electron-phonon interactions on the band structure can be experimentally investigated in detail by measuring the temperature dependence of energy gaps or critical points (van Hove singularities) of the optical excitation spectra. These studies have been complemented in recent years by observing the dependence of such spectra on isotopic mass whenever different stable isotopes of a given atom are available at affordable prices. In crystals composed of different atoms, the effect of the vibration of each separate atom can thus be investigated by isotopic substitution. Because of the zero-point vibrations, such effects are present even at zero temperature (T = 0). In this paper we discuss state-of-the-art calculations of the dielectric function spectra and compare them with experimental results, with emphasis on the differences introduced by the electron-phonon interaction. The temperature dependence of various optical parameters will be described by means of one or two (in a few cases three) Einstein oscillators, except at the lowest temperatures where the T4 law (contrary to the Varshini T2 result) will be shown to apply. Increasing an isotopic mass increases the energy gaps, except in the case of monovalent Cu (e.g., CuCl) and possibly Ag (e.g., AgGaS2). It will be shown that the gaps of tetrahedral materials containing an element of the first row of the periodic table (C,N,O) are strongly affected by the electron-phonon interaction. It will be conjectured that this effect is related to the superconductivity recently observed in heavily boron-doped carbon.Comment: 17 pages, 17 fifure

C-axis I-V characteristics in highly c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes

We have measured c-axis I-V characteristics of highly c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes (Bi2223/Ag) in the field H 0.5 T near Tc (80 to 110 K). The Josephson junction effect is observed. In each I-V curve a voltage jump V0 is clearly found which may correspond to the transition from states with Cooper tunneling to states with quasi-particle tunneling. This implies the Josephson coupling between the grains or between the CuO2 tri-layers in Bi2223/Ag tapes along the c-direction. The temperature dependence of the superconducting gap (T) can be successfully scaled to that obtained from BCS theory. The gap does not depend much on a magnetic field of H 0.5 T for H c. The voltage jumps observed in the experiments are not as steep as in a single SIS junction, which may originate from the variety of junctions in Bi2223/Ag tapes or the thermal fluctuation

Continuous-wave operation up to 350K of optically-pumped antimony-based midinfrared VCSELs

International audienceWe report on the growth by MBE and characterization of optically-pumped mid-infrared Vertical Cavity Surface Emitting Lasers (VCSELs), where the optical cavity is formed by a semiconductor Sb-based Bragg mirror, an air gap and a high reflectivity dielectric concave mirror. These lasers operate between 2 µm and 2.5 µm in continuous wave regime at room temperature with a circular TEM00 beam. Two different fabrication processes are tested and the properties of the corresponding devices are compared