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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Insertable planar gradient coils offer the potential for significant performance increases in magnetic resonance imaging through higher gradient strength and shorter rise times. Using variational methods to minimize inductance, and thereby to optimize switching speeds, we have analyzed and constructed a biplanar y-gradient coil for insertion into a solenoidal magnet system where z is the magnet axis. We have also analyzed biplanar x-gradient and z-gradient coil designs using the same methods. These biplanar coils offer an advantage over a cylindrical coil of comparable diameter in that they achieve high gradient strengths with relatively short rise times while maintaining patient access. Although the requirement that the currents for the x gradient lie in the same plane as for the y and z gradients increases the stored energy by a factor of 3 with respect to the other two gradients, this stored energy is still smaller by a factor of 2 than that of a comparably constrained x-gradient cylindrical coil. The biplanar coil design offers improved linearity over its single planar coil alternative. The particular designs we have investigated are generally limited to small-volume imaging.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects of Si ion fluence and oxygen concentration on secondary defect formation and gettering of metallic impurities in MeV self-implanted silicon have been studied for Czochralski (Cz) and float zone (FZ) silicon by means of deep level transient spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and optical microscopy/chemical etching. We found that the density, depth distribution, and number of extended defects is strongly dependent upon both the Si ion fluence and the oxygen concentration. Effective gettering of iron to below 1010 cm−3 can be achieved in both FZ and Cz wafers at implantation doses of 1015 cm−2. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report experimental and theoretical evidence for electrostatic lateral confinement induced by a nanoprobe. The lateral confinement is manifest as oscillations of the differential conductance of a near-surface resonant-tunneling heterostructure in air at room temperature. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 91-92 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Single-frequency integrated external cavity distributed Bragg reflector lasers emitting near 1.5 μm have been fabricated. The chirp and cw linewidth measurements of these devices show that lasers with long external cavity exhibit lower chirp and lower cw linewidth compared to lasers with short external cavity. The experimental data are explained by using theoretical calculations.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 2459-2465 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Impurities in MeV-implanted and annealed silicon may be trapped at interstitial defects near the projected ion range, Rp, and also at vacancy-related defects at approximately Rp/2. We have investigated the temperature dependence of impurity trapping at these secondary defects, which were preformed by annealing at 900 °C. The binding energies of Fe, Ni, and Cu are greater at the vacancy-related defects than at extrinsic dislocation loops. During subsequent processing at temperatures up to 900 °C, the amount of these impurities trapped at Rp/2 increases with decreasing temperature while the amount trapped at Rp decreases, with most of the trapped metals located at Rp/2 in samples processed at temperatures (approximately-less-than) 700 °C. However, intrinsic oxygen is trapped at both types of defects; this appears to have little effect on the trapping of metallic impurities at extrinsic dislocations, but may inhibit or completely suppress the trapping at vacancy-related defects. © 1998 American Institute of Physics.
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  • 6
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We consider the in-plane electrical conductivity of a spin valve, specifically, a system consisting of a copper slab between two cobalt slabs. We calculate the dependence of the resistivity on the canting angle, where the canting angle θ is defined as the angle between the magnetization vectors of two ferromagnetic slabs in a spin valve. To calculate the electronic structure, we utilize the layer-KKR formalism. Electron scattering by impurities, phonons, magnons, etc. is modeled using a layer and spin-dependent complex self-energy. Scattering rates are chosen to match Cu and Co resistivities. We assume a spin asymmetry scattering rate factor of 7 in Co, matching the Fermi-level minority-to-majority density-of-states ratio. No additional interfacial scattering is included. The nonlocal layer dependent conductivity is calculated using the Kubo–Greenwood formula for systems consisting of 3 and 7 monolayer fcc (111) Cu slabs in Co. We find electron channeling in Cu dominates the conductivity and at θ=π/2 the resistivity is increased by 4.9% and 2.3% from a linear 1−cos θ dependence. We find giant magnetoresistive GMR values of 64% and 36% for 3 and 7 monolayers of Cu, respectively. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 7681-7683 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The formation of amorphous layers in GaAs during ion bombardment at elevated temperatures, where dynamic annealing of radiation-induced defects is substantial, is shown to be extremely sensitive to the implantation temperature. For example, we have found that a temperature change of only 6 °C can change the residual damage from small clusters barely visible by conventional transmission electron microscopy and Rutherford backscattering to a thick amorphous layer. The temperature at which this occurs is strongly dependent upon the ion flux. © 1997 American Institute of Physics.
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  • 8
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of applying an electric field across a quasi-phase-matched frequency doubling lithium tantalate waveguide has been investigated. The waveguide was fabricated by a two-stage ion exchange process in pyrophosphoric acid. An electric field of 2 kV/mm was found to shift the phase matching wavelength by 0.05 nm. It is estimated that more optimized waveguides could produce wavelength shifts of ±4 nm for an applied electric field of ±20 kV/mm and could compensate for temperature variations of ±67 °C. © 1996 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 660-666 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An investigation was made of the defect characteristics of single-crystal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal transport methods. Only Cz-grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated current (TSC) studies in conjunction with temperature-dependent optical-absorption measurements and room-temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy 〈0.7 eV are a factor of 103 smaller in hydrothermal than in Cz BSO. At least six different defects were identified in the TSC measurements. Temperature-dependent optical-absorption measurements indicate two Urbach band tails for Cz-grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity band tail which can be observed through temperature dependent optical absorption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the BiSi defect responsible for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in Cz material must proceed via a trap-hopping conduction mechanism that is missing in the intrinsic material.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6019-6027 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The emission intensity, wavelength, and lifetime of the fluorophore nitrobenzoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE) are sensitive to the local environmental structure when this species is present as a component of an amphiphilic membrane. Alterations of the physical and electrostatic structure of a membrane can result in changes in the fluorescence signal owing to changes in the extent of self-quenching of the probe. To investigate self-quenching, NBD-PE was incorporated into monolayers and vesicles composed of Egg phosphatidylcholine at concentrations of 0.1 to 50 mol %. Monolayer samples were dipcast onto glass slides at a pressure of 35 mN m−1. Both the integrated intensity per fluorophore (quantum yield) from vesicles and dipcast monolayers, and the mean fluorescence lifetime from vesicles decreased as the concentration of fluorophore in the membranes was increased. At all concentrations studied the decay of NBD-PE fluorescence was fitted to two discrete exponentials, and both lifetime components were observed to change with concentration. The complexity of the fluorescence decay did not permit the use of standard theoretical models such as the Klafter–Blumen or Stern–Volmer equations which are normally employed to describe changes in fluorescence lifetime with changes in quencher concentration. Instead, a phenomenological approach was used to develop an empirical model of fluorescence self-quenching which could describe the observed alterations in the fluorescence lifetime and intensity. The model was based on a combination of Perrin quenching and Förster energy transfer. The fluorescence data was fit by a model wherein NBD-PE formed nonemissive trap sites with a critical radius of Rc=1.0±0.1 nm (Perrin quenching), with Förster energy transfer occurring to the trap sites with an R0 value of 2.55±0.10 nm as determined from spectral overlap integrals.
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