23 research outputs found

    High-pressure study of the Raman modes in YBa2(Cu0.96Ni0.04)4O8

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    URL:http://link.aps.org/doi/10.1103/PhysRevB.60.4363 DOI:10.1103/PhysRevB.60.4363We present a study of the Raman phonons in YBa2(Cu0.96Ni0.04)4O8 under hydrostatic pressure in the superconducting phase. A comparison with our earlier work on the undoped YBa2Cu4O8 shows that the pressure coefficients of two vibrational modes involving oxygen atoms differ significantly from those of the corresponding modes in the undoped material. These are the O(1) chain mode which shifts 33% faster and the O(2)-O(3) in-phase mode which shifts 23% slower than the undoped counterparts. The other Raman modes in the Ni-doped sample shift in a manner similar to the undoped material. The observed behavior of the O(1) chain and O(2)-O(3) in-phase modes in YBa2(Cu0.96Ni0.04)4O8 under pressure and the softening of the Cu(1) Ag mode frequency with increasing Ni doping suggest that the Ni atoms substitute for the Cu atoms in the chain, which in turn decreases the compressibility in the vicinity of the Cu(Ni) chain atom.This work was supported by U.S. Department of Energy Grant No. DE-FG02-90ER45427 through the Midwest Superconductivity Consortium. D.J.P. thanks the U.S. Department of Education for support through Grant No.P200A50259

    Towards a High Diffraction Efficiency of Photorefractive Multiple Quantum Wells

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    doi:10.1063/1.1994721 http://link.aip.org/link/?APCPCS/772/1579/1We propose a method to improve the diffraction efficiency of photorefractive multiple quantum well devices in the transverse-field geometry. Higher efficiencies have been achieved through systematic electrical modulation studies

    Pressure studies of impurity levels in AlxGa1-xAs

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    doi: 10.1088/0268-1242/4/4/033The authors present a study of the deep and shallow donor levels under hydrostatic pressure. The shallow levels follow the conduction bands, while the deep levels are strongly sublinear with pressure. The temperature dependence of the intensities and energies is used to obtain an energy level diagram of the deep levels at high pressures.This work was supported by theU S Army under grant number DAAL03-86K-0083, the US Department of Energy under grant number DE-AC02 84ER45048, and Amoco Corporation. M Chandrasekhar is a n A P Sloan Foundation Fellow

    Planarity of para Hexaphenyl

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    URL:http://link.aps.org/doi/10.1103/PhysRevLett.82.3625 DOI:10.1103/PhysRevLett.82.3625We present experimental and theoretical findings on the geometry of polycrystalline para hexaphenyl via Raman scattering. The planarity of the molecule is affected by hydrostatic pressure and temperature. Our studies indicate that the potential energy curve which governs the torsional motion between neighboring phenyl rings is “W” shaped. We determine the activation energy to promote the molecule from a nonplanar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. From the relative intensities of the 1280cm-1 to the 1220cm-1 Raman modes we show that high pressure planarizes the molecules, modifying the “W”-shaped potential energy curve to a “U”-shaped one.We acknowledge the financial support from U.S. Army Grant No. DAAL03-92-0381, University of Missouri Research Board and Österreichische Nationalbank (Project No. 6608)

    Functional Imaging Using InGaAs/GaAs Photorefractive Multiple Quantum Wells

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    doi:10.1063/1.1994722 http://link.aip.org/link/?APCPCS/772/1581/1We propose the use of an InGaAs/GaAs photorefractive quantum well (PRQW) as an adaptive beam combiner for holographic optical coherence imaging applications. Holograms have been observed by using a diode laser and an interferometer. A weaker quantum confined exciton leads to the saturation of electroabsorption and hence diffraction, under a high external electric field, in the InGaAs PRQW. A careful choice of external electric field modulation seems to reduce this effect. We examine several characteristics that govern the use of an InGaAs PRQW in a functional imaging system

    Hydrostatic pressure dependence of the luminescence and Raman frequencies in polyfluorene

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    DOI: 10.1103/PhysRevB.68.115203 http://link.aps.org/doi/10.1103/PhysRevB.68.115203We present studies of the photoluminescence (PL), absorption, and Raman scattering spectra from poly[2,7-(9,9′-bis(2-ethylhexyl))fluorene] under hydrostatic pressures of 0-100 kbar at room temperature. The well-defined PL and associated vibronics that are observed at atmospheric pressure change dramatically around 20 kbar in the bulk sample and at around 35 kbar for the thin-film sample. Beyond these pressures the PL emission from the backbone is swamped by strong peaks due to aggregates and keto defects in the 2.1-2.6 eV region. The Raman peaks shift to higher energies and exhibit unexpected antiresonance line shapes at higher pressures, indicating a strong electron-phonon interaction.S.G. acknowledges the donors of the American Chemical Society Petroleum Research Fund No. 38193-B7! for partial support of this research. U.S. thanks SONY International Europe, Stuttgart, and the Deutsche Forschungsgemeinschaft (DFG) for financial support

    Temperature dependence of strain in ZnSe(epilayer)/GaAs(epilayer)

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    doi:10.1063/1.360477A study of biaxial strain as a function of temperature in a ZnSe epilayer grown on a GaAs substrate is presented. The strains are determined by measuring the heavy‐ and light‐hole related excitonic transitions via photomodulated spectroscopy. The strain is found to increase with increasing temperature. The data are compared with a calculation using a previously determined elastic constant and thermal expansion coefficients. The temperature dependence determined here allows a comparison of various other optical measurements performed at different temperatures.The work by H. R. C. was supported in part by the U. S. Department of Energy under Contract No. DE-FG02-89ER45402. M. C. acknowledges the support from the U.S. Army Research Office DAAL-03-92-G0381. A. K.. R. acknowledges support from the National Science Foundation (Materials Research Group No. DMR89-13706) and R. L. G. from AFOSR-89-0438; both A. K. R. and R. L. G. also acknowledge support from DARPA-URI Grant No. 218-25015. We thank Lok C. Lew Yan Voon and L. R. Ram-Mohan for many stimulating discussions

    Photoluminescence of short-period GaAs/AlAs superlattices: A hydrostatic pressure and temperature study

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    URL:http://link.aps.org/doi/10.1103/PhysRevB.58.7222 DOI:10.1103/PhysRevB.58.7222The temperature and pressure dependence of type-I and -II transitions from photoluminescence (PL) spectra in a series of (GaAs)m/(AlAs)m superlattices show that the temperature dependence of energy bands can be described very well with a Bose-Einstein-type equation. From these measurements the parameters that describe the temperature dependence of excitonic transition energies and the corresponding broadening of the PL line are deduced. The pressure dependence of the PL linewidths of the type-I exciton as a function of pressure and temperature yield the intervalley deformation potential. Beyond the type-I-type-II crossover, the PL linewidth increases as a function of both pressure and temperature. The electron-phonon deformation potential for Γ-X scattering is found to be temperature dependent.We thank S. Satpathy and S. Zollner for valuable discussions. One of us ~H.R.C.! acknowledges support by the NSF under Grant No. DMR-9633107. M.C. thanks the U.S. Army for support through Grant No. DAAL03-92-0381. The work at Purdue University was supported by the National Science Foundation: Materials Research Science and Engineering Center Grant Nos. DMR 94-00415 and DMR 93-03186

    Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

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    doi:10.1063/1.373643ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.This work was supported in part by University of Missouri Research Board under Grant No. RB95-061, the U.S. Army Research Office ~Research Triangle Park, N.C.! under Grant No. DAAH04-94-G-0305, and the Office of Naval Research-Electronics Division, Contract No. N00014-99-1-0288

    Pressure tuning of strain in CdTe/InSb epilayer: A photoluminescence and photomodulated reflectivity study

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    doi:10.1063/1.354415The heavy‐hole and light‐hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch‐induced splitting between the light‐hole and heavy‐hole related transitions increases in a continuous and reversible manner because of the additional pressure‐induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer undergoing a structural phase transition at approximately 30 kbar. For pressures between 20 and 30 kbar, the behavior of the CdTe epilayer is similar to that of the bulk sample; the strain appears to have been relaxed due to the structural phase transition which has taken place in InSb. Values of the first‐ and second‐order pressure coefficients for bulk CdTe and for the CdTe epilayer as well as values of the hydrostatic and shear deformation potentials are obtained at 14 and 80 K and compared with previously quoted values.The work by H.R.C. was supported in part by the U.S. Department of Energy under Contract No. DE-FG02-89ER45402. M.C. acknowledges partial support from the Research Corporation and the U.S. Army Grant No. DAAL-03-92-G-038 1. M.S.B. acknowledges partial support by the G. Ellsworth Huggins Fellowship. A.K.R. and R.L.G. acknowledge support from the National Science Foundation (Materials Research Group No. DMR89-13706)
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