Optical and structural properties of molecular-beam epitaxially grown Ga0.47In0.53As/Al0.48In0.52As superlattices,emitting at 1.55 μm at room temperature

We have combined photoluminescence and absorption measurements with double crystal x-ray diffraction in order to study the influence of growth conditions on optical and structural properties of Ga_0.47In_0.53As/Al_0.48In_0.52As superlattices of 10.5 nm well width grown by molecular beam epitaxy on InP<img src="/content/q6883jq605l85t4p/xxlarge8758.gif" alt="ratio" align="BASELINE" BORDER="0">Sn substrates. Superlattices with excellent structural properties can be grown even at low substrate temperatures of 500 °C, as indicated by narrow linewidths of the satellite peaks of the x-ray diffraction pattern of 20 to 25 s of arc (FWHM). However, elevated substrate temperatures are required to increase the intensity of the intrinsic luminescent subband transition at 77 K, to reduce the luminescence linewidth to 12 meV (FWHM), and to sharpen the absorption spectra. In addition to the absorption due to the steplike variation of the two-dimensional density of states, we observe excitonic transitions to the excitonic continuum at the subband edges.     

Sufficient and necessary conditions for limit laws on lag sums of I. I. D. R. V. S

Let {<i>X, Xi> _<i>ni> ;<i>ni>>-1} be a sequence of i.i.d.r.v.s with<i>EXi>=0 and<i>EXi> ²=σ²(0 < σ < ∞). <img src="/content/Q7W5G38169P30426/10114_2007_Article_BF02563891_TeX2GIFIE1.gif" alt=" $$S_n = \sum\limits_{i = 1}^n {X_i } $$ " align="middle" border="0"> we obtain some sufficient and necessary conditions foridth="100%">

On the relaxation ofμ + SR signal and diffusive neutron scattering in AF phase of HTSC

We consider the antiferromagnetic (AF) state of high-<i>Ti> _<i>ci> compounds and assume the existence of the localized magnetic two-level systems (TLS) with the relatively small energy<i>Ei> in the AF copper planes, which is compatible with a series of experimental data implying low-energy scale in these systems. It is shown in our previous paper that these TLS are formed, if one accepts Aharony et al. suggestion that the small doping results in the holes' localization on the oxygen ions in CuO₂ planes. Randomly distributed, these TLS cause partial disorder in the average values of AF copper spins. The manifestation of this effect in the<i><img src="/content/h128x1665j17k477/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">i> ⁺ SR experiments and the elastic neutron scattering is discussed. Our results are in a qualitative agreement with the temperature dependence and the magnitude of the line shift and the relaxation rate of<i><img src="/content/h128x1665j17k477/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">i> ⁺ SR signal. The elastic diffusive neutron scattering at the small wave-vectors near 2D AF Bragg point (1/2, 1/2,<i>li>) and near the point (0, 0,<i>li>) is predicted. However, the temperature dependence of this scattering, observed in the experiments with YBa₂Cu₃O_6+x at<i>xi>=0.38, isn't met by our formulae, obtained for a small doping. It is shown, that <img src="/content/h128x1665j17k477/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">SR data give stronger confidence to the <img src="/content/h128x1665j17k477/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">frustrated bond<img src="/content/h128x1665j17k477/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> rather and <img src="/content/h128x1665j17k477/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">frustrated plaquette<img src="/content/h128x1665j17k477/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> case of the hole localization.     

High-resolution selective reflection spectroscopy in intermediate magnetic fields

We have measured the modifications of frequency-modulated selective reflection spectra of the Cs<i>Di> ₂ line recorded with circularly polarized light by a longitudinal magnetic field in the range 120–280 G. The spectra recorded with<i><img src="/content/q5t84963152w0817/xxlarge963.gif" alt="sgr" align="BASELINE" BORDER="0">i> ⁺ and<i><img src="/content/q5t84963152w0817/xxlarge963.gif" alt="sgr" align="BASELINE" BORDER="0">i> ^– polarizations were found to be qualitatively different, but are well described by a theoretical model based on the diagonalization of the hyperfine-Zeeman Hamiltonian. The technique presented here is a simple way for investigating fully resolved complex Zeeman spectra in moderate magnetic fields and may find applications in the investigation of anisotropies in long-range atom-surface interactions.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic losses<i>Qi> ^–1 and of relative sound velocity change <img src="/content/q7474371x67v24j2/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"><i>v/vi> in YBa₂Cu₃O_{7–<img src="/content/q7474371x67v24j2/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} up to <img src="/content/q7474371x67v24j2/xxlarge8764.gif" alt="sim" align="MIDDLE" BORDER="0">60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculated<i>Qi> ^–1(<i>Ti>) and <img src="/content/q7474371x67v24j2/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"><i>v(T)/vi> dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_{7–<img src="/content/q7474371x67v24j2/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} due to the pseudo Jahn-Teller effect.     

Enhancement of the electron mobility with infrared photoexcitation in Si: Te at low temperatures

Hall measurements on Te-doped silicon (<i>Ni> _Te <img src="/content/q706r642m9pv213p/xxlarge8776.gif" alt="ap" align="MIDDLE" BORDER="0"> 10¹⁶ cm^–3) have been performed in the temperature range between 10 K and 300 K with infrared photoexcitation of electrons into the conduction band. The samples exhibit electron Hall mobilities which are increased by approximately 50% compared to measurements in the dark. The increased electron mobility can be correlated with an increased electron population of shallow donor levels by photoexcitation. Coulomb scattering due to charged shallow donor centers is converted into less efficient dipole scattering (Te-acceptor pairs) by the light-induced redistribution of electrons.     

Interaction of atomic hydrogen with the graphite single-crystal surface

We have studied the chemisorption of atomic hydrogen on the basal plane of natural graphite single crystals. LEED and angle-resolved photoemission were used to characterize the clean surface. The adsorption of H saturates at rather low exposures, accompanied by a decrease of the work function by <img src="/content/v126m3u5340k8x73/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"><img src="/content/v126m3u5340k8x73/xxlarge966.gif" alt="phgr" align="MIDDLE" BORDER="0">=(100±20) meV. The photoemission spectra indicate a clear carbon-hydrogen interaction, leading to shifts of substrate bands by up to about 200 meV. No detectable etching of the surface occurs at room temperature, in agreement with earlier work. Our results are qualitatively consistent with theoretical considerations about a strong H(1<i>si>)-C(2<i>pi> _z) chemical bond.     

Chemical shifts of atomic core levels and structure of K1−xTi1−xSbxOPO4, x=0-0.23, solid solutions

Antimony-doped K_1&minus;xTi_1&minus;xSb_xOPO₄, x=0.23, crystals have been prepared by spontaneous nucleation from the flux in the quaternary system K₂O-TiO₂-P₂O₅-Sb₂O₅. Crystal structure observation with TEM method reveals the presence of superstructure ordering. Core level electronic parameters have been studied by X-ray photoelectron spectroscopy. Strong effect of Sb doping has been detected for inner shells of Ti⁴⁺ ions. Prominent decreasing of the binding energy difference Δ(O 1s-Ti 2p_3/2) correlates with the shortening of mean oxide bond length L(Ti&minus;O) at x=0.23 that suggests increased ionicity of Ti&minus;O bonds in K_1&minus;xTi_1&minus;xSb_xOPO₄ solid solutions.     

Reactions of the Complexes ReNCl2(PMe2Ph)3, [ReNCl4]—, [OsO3N]—, and Cl3VNCl with Metal Halides

The nitrido complexes ReNCl₂(PMe₂Ph)₃ and [OsO₃N]^— have strong basic terminal nitrido ligands which can react with Lewis acidic metal halides to form nitrido bridges. The synthesis and structure of complexes with ReNCl₂(PMe₂Ph)₃ and nitrido bridges Re≡N‐M (M = B, Ga, Sn, Ti, Zr, V, Nb, Ta, Mo, Re, Pd, Au, and Zn) as well as of complexes with [OsO₃N]^— and nitrido bridges Os≡N‐M (M = Pd and Pt) are reported. Strong Lewis acids can also remove phosphine or chloro ligands from ReNCl₂(PMe₂Ph)₃. The resulting complex fragments subsequently combine to yield oligomeric complexes with nitrido bridges Re≡N‐Re. If the reaction with strong Lewis acids is carried out in a chlorinated solvent the solvent can be decomposed to form HCl which then protonates the nitrido ligand affording an imido complex. [ReNCl₄]^— is able to form nitrido bridges to electrophilic halides if a donor ligand is coordinated in <i>transi> position to the nitrido ligand to enhance its basicity sufficiently. The synthesis and structure of examples with nitrido bridges Re≡N‐M (M = Pd, Pt, Ta) are reported. The chloro imido complex Cl₃V≡N‐Cl can act as a nitride ion transfer reagent. Its reaction with MoCl₅ yields Mo₂NCl₈ whereas with MoCl₃ the nitride chlorides Mo₃N₂Cl₁₁ and MoNCl₃ are obtained. Cl₃VNCl can also act as an reactive intermediate by the reaction of VN with a halide as was shown by the reaction of MoCl₅ with VN yielding Mo₂NCl₇. The structures of these molybdenum nitride chlorides are discussed.     

Gasphasenreaktionen von H3PF2: Darstellung der Monohalogenphosphane H2PF und H2PCl

Gas Phase Reactions of H₃PF₂: Synthesis of the Monohalogenophosphanes H₂PF and H₂PCl Gas phase reactions of H₃PF₂ at ≤ 10 mbar with hydrides (B₂H₆, HSi(CH₃)₃, HSn(CH₃)₃), amines (H₃N, N(CH₃)₃) and electrophilic halides of B, Si and Ti in the beam of an IR interferometer have been investigated with a time delay of 15 sec. between successive measurements. Gas/solid phase reactions of H₃PF₂ with KF and AlCl₃ and reactions of H₃PF₃ and HPF₄ have been studied similarly. With H₃N, N(CH₃)₃ and electrophilic halides, H₃PF₂ was converted into its dehydrofluorination product H₂PF. This eliminates further HF in the presence of H₃N or N(CH₃)₃, while BCl₃ or TiCl₄ yield the hitherto unknown H₂PCl. The IR spectra of the short-lived species H₂PF and H₂PCl as well as their deuterated isotopomers, which reveal at room temperature and ≤ 10 mbar a halflife time of ≤ 12 min. and ≤ 50 sec. respectively, were recorded and analysed in order to characterize the novel species.