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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 30 (1989), S. 1570-1578 
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: A unified theory based on a homogeneous fiber bundle Q(M,G/H) is discussed in detail. In spite of the fact that the theory retains the full G-gauge invariance, the physical gauge group K is shown to be K=H*/(H∩H*), where H* is the centralizer of H in G. A principal fiber bundle P(M,G,H) is also constructed by introducing an additional left action H on P(M,G) that commutes with the right action G, and a unified theory based on P(M,G,H) is discussed. It is shown that the theory based on Q(M,G/H) is, in fact, the H projection of the Einstein–Hilbert action from P(M,G,H), with the identification Q(M,G/H)=P(M,G,H)/H.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Bi4Ti3O12 thin films have been grown by laser ablation on SrTiO3(100) and SrTiO3(110) substrates. Substrate surface orientation is found to be an important growth parameter which determines crystal axis orientation, grain growth behavior, and electro-optic properties of the Bi4Ti3O12 thin films. The films grown on SrTiO3(110) shows a ferroelectric phase transition near 720 °C and a large quadratic electro-optic effect with the effective coefficient 1.1×10−16 m2/V 2.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical and magnetic properties of the superconducting oxides with nominal composition (Bi0.7 Pb0.3 )2Sr2Ca2 Cu3 Ox prepared from polymeric precursors have been investigated. Superconducting transition with zero-resistance temperature at T=108 K was achieved for a sample sintered at 840 °C under a low oxygen pressure. The critical current density at 77 K in a zero magnetic field is ∼400 A/cm2 , which is much higher than that of the Pb-free sample. Variations of the superconducting transition temperature, the critical current density, and magnetic properties with heat treatment time are discussed in connection with the structural properties.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the unique properties of SE in GaN thin films at high temperatures could potentially be utilized in optoelectronic devices. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The Korea Multipurpose Accelerator Complex (KOMAC) project has been initiated to develop and build a high current proton/H− linear accelerator capable of delivering an 1 GeV cw proton beam with an intensity of 20 mA in the final stage. The major proton beam will be used for nuclear-waste transmutation, energy production, and nuclear physics experiments while utilizing the minor negative hydrogen beam for basic science research and medical therapy. A Duoplasmatron proton source for the KOMAC linear accelerator has been built at Korea Atomic Energy Research Institute. The hydrogen beam currents of up to 50 mA at the extraction voltage of 50 kV are routinely obtained. A low normalized rms emittance of 0.2 π mm mrad and a proton fraction of over 80% are obtained in this source. With the 100% duty factor of ion source operation, the arc filaments of the source have survived over 40 h. Except for the filament lifetime, the achieved parameters of the proton beam source are satisfying most requirements of the KOMAC ion source. The ion sources can extract the 10 mA nitrogen beam and 3 mA argon beam for the industrial surface modification processes. © 2000 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 7909-7913 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The chemical, morphological, and structural characteristics of nitrogen plasma treated c-plane sapphire substrate surfaces were studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The plasma treatment was carried out by exposing sapphire substrates (Ts=700 °C) to a flow of 35 sccm of activated nitrogen species generated by a constricted-plasma source for 5–60 min. The emergence of the N 1s peak in XPS indicates nitrogen incorporation in sapphire as soon as after 5 min of nitridation. AFM images show that the sapphire contained a high density of islands after 1 h of nitridation. A thin polycrystalline AlN layer was observed on the nitridated sapphire surface by TEM. Both the thickness of the AlN layer and the N 1s photoelectron peak intensity increase nonlinearly with respect to nitridation time. The nonlinear relationship between the thickness of the nitridated layer and the reaction time suggests the growth of the AlN layer follows a diffusion limited growth mechanism. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 1887-1889 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gallium nitride (GaN) thin films grown on sapphire substrates were successfully bonded and transferred onto Si substrates using a Pd–In metallic bond. After bonding, a single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire followed by a low-temperature heat treatment to remove the substrate. Channeling-Rutherford backscattering measurements revealed the thickness of the defective interfacial region to be approximately 350 nm. The full width at half maximum, low-temperature (4 K), donor-bound exciton photoluminescence (PL) peak was larger by 25% on the exposed interfacial layer compared to the original GaN surface. Ion milling of the exposed interface to a depth of 400 nm was found to remove the interfacial layer and associated defects. The minimum channeling yield and PL linewidths from the exposed interface were found to be comparable to those obtained from the original GaN surface after ion milling. © 1999 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 2242-2244 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Laser action was observed in GaN pyramids under strong optical pumping at room temperature. The pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal–organic chemical vapor deposition. Each pyramid had a 15-μm-wide hexagonal base and was on average 15 μm in height. The pyramids were individually pumped, imaged, and spectrally analyzed through a high-magnification telescope system using a high-density pulsed excitation source. Under high levels of optical pumping, multimode laser at room temperature was observed. The integrated emission intensity for both spontaneous and lasing peaks was studied as a function of excitation power density. The effects of pyramid geometry and short-pulse excitation on the multimode nature of laser oscillations inside of the pyramids is discussed. This study suggests that GaN microstructures could potentially be used as pixel elements and high-density two-dimensional laser arrays. © 1998 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 1623-1625 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stimulated emission (SE) in optically pumped InGaN/GaN multiquantum well (MQW) structures grown by metalorganic chemical vapor deposition was experimentally studied in the temperature range of 175–575 K. The GaN barriers were intentionally doped with a different Si concentration ranging from 1×1017 to 3×1019 cm−3 and the effects of Si doping of GaN barriers on the optical properties of InGaN/GaN MQWs were investigated. The SE threshold was measured as a function of temperature and compared with bulk GaN. We observed that the SE threshold had a low value and a weak temperature dependence: for example, ∼25 kW/cm2 at 175 K, ∼55 kW/cm2 at 300 K, and ∼300 kW/cm2 at 575 K for one of the samples. Low SE thresholds are attributed to the high-quantum efficiency of the MQWs, possibly associated with the large localization of excitons. The characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for development of laser diodes that can operate well above room temperature. © 1998 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 31 (1990), S. 695-698 
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: Exact time-independent spherically symmetric black hole solutions of the (4+3)-dimensional Kaluza–Klein theory are obtained by imposing the isometry group E2 on the unified metric. The solutions include the five-dimensional Dobiasch–Maison–Lee solutions as a special case. The physical implication of the solutions is discussed.
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