ALBERT

Notizen: Transport measurements on large single crystals of Cd0.8Zn0.2Te:Cl indicate that Cl donors form DX centers in CdZnTe. We have observed persistent photoconductivity (PPC) with an annealing temperature Ta≈130 K. Hall-effect experiments indicate that the PPC arises from a persistent increase in the density of charge carriers; the saturation density is Nsat=6×1016 cm−3. The deep binding energy of the DX center is Ed=0.22 eV. © 1995 American Institute of Physics.

Notizen: High resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) were used to study lateral interactions in the adsorbate layer of the CO/Rh(111) system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C–O vibrations is observed over the whole coverage range. The frequency of the metal CO (M–CO) vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M–CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of 12CO and 13CO were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C–O stretching vibration at total CO coverage of 0.33 ML in the ((square root of)3×(square root of)3)R30° structure arise purely from dipole–dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole–dipole coupling. Therefore, experiments with a small amount of 13CO as a tracer to monitor the frequency shifts in the 12CO adlayer were carried out over the entire coverage range of 12CO. The results demonstrate that dipole–dipole coupling causes the frequency shifts at low coverage (〈0.5 ML), whereas chemical effects set in at higher coverage (0.5–0.75 ML), connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules. © 2001 American Institute of Physics.

Notizen: We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the "DX" center. Crystals containing these defects have the characteristics of local photorefractive materials in that their refractive index is modified in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the DX deep trap states into the conduction band, is persistent at low temperatures due to a capture barrier, Ecap, which limits reformation of the DX centers. The effect is reversed by heating above an annealing temperature, which scales with Ecap and varies with the crystal host and active dopant. A number of DX materials have now been identified with long-term persistence temperatures ranging from 50 to 180 K. In this paper, we briefly review the physics of the DX center and present theoretical estimates of several important optical properties of these materials based on a simple model. We calculate spatial resolution, maximum refractive index shift, and sensitivity, and compare our predictions with measurements on one member of the DX family, AlGaAs:Te. In a 345 μm thick sample of this material doped at 9×1017 cm−3, we find a refractive index shift, Δn, of 2×10−3 and an exposure sensitivity, S, of 0.012 cm3/J. Our expectation that the maximum refractive index change scales linearly with the doping density is consistent with our previous measurement of Δn=1.1×10−2 obtained for a sample of AlGaAs:Si doped at 4×1018 cm−3. The measured values of Δn and S, are, respectively, two and three orders of magnitude larger than corresponding values for the photorefractive material LiNbO3, and are shown here to be independent of exposing irradiance from 10−3 to 108 W/cm2. At the latter irradiance, the refractive index shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exhibits high sensitivity, large refractive index change, and fast write time, all desirable properties of an optical holographic storage medium. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% for grating periods from 0.13 to 15 μm. No degradation of sensitivity is observed after large numbers of exposure–erasure cycles. Experiments with multiple-hologram exposures show that the DX materials require no exposure schedule: equal strength holograms are obtained using equal exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system parameters are estimated for a 1 Tbyte holographic storage device based on angle multiplexing in a DX material. © 1998 American Institute of Physics.

Notizen: We have optically written persistent but erasable metallic features in insulating epilayers of AlxGa1−xAs doped with Si and Se, which form DX centers. The photocarriers, which remain in the AlxGa1−xAs layer, move freely in the conduction band but are confined to the exposed regions. We demonstrate this confinement by optical excitation in a striped pattern; the resulting modulation of the free carrier density is evinced by an anisotropy of the sample conductance parallel and perpendicular to the stripes. The anisotropy, like the photoconductivity itself, is persistent at low temperatures. Erasure is achieved by thermal annealing. We estimate that features can be written with better than 1000 A(ring) resolution. © 1994 American Institue of Physics.

Notizen: Zusammenfassung Es wird aus dem Viskositätstemperaturverhalten der Schmieröle geschlossen, daß sie Assoziate enthalten, die thermisch dissoziieren. Die hieraus fließenden Folgerungen für den Schmiervorgang werden besprochen.

Notizen: Zusammenfassung Die aus dem Viskositäts-Temperaturverhalten verdünnter acetonischer Lösungen technischer Collodiumwollen erhaltenen Platzweohselenergien erweisen sich als eine lineare Funktion desDP. Auf Grund dieses Verhaltens kann die Staudinger-KonstanteK m einer physikalischen Deutung unterzogen werden. Der Zuwachs der Platzwechselenergie pro Grundmolekül ergibt sich für Zellulosenitrat in dem Stickstoffbereich 10,9–12,2% N zu 8,41 cal/Grundmolekül/Mol.