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  • Journal of Applied Physics
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  • 1
    Language: English
    In: Journal of Applied Physics, 15 July 2011, Vol.110(2)
    Description: Using a phenomenological approach, the light yield is derived for inorganic scintillators as a function of the rates of linear, bimolecular, and Auger processes occurring in the electron track initiated by an x ray or a γ -ray photon. A relation between the track length and incident energy is also derived. It is found that the nonproportionality in the light yield can be eliminated if either nonlinear processes of interaction among the excited electrons, holes, and excitons can be eliminated from occurring or the high density situation can be relieved by diffusion of carriers from the track at a faster rate than the rate of activation of nonlinear processes. The influence of the track length and radius on the yield nonproportionality is discussed in view of the known experimental results. Inventing new inorganic scintillating materials with high carrier mobility can lead to a class of proportional inorganic scintillators. Results agree qualitatively with experimental results for the dependence of light yield on the incident energy.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 2
    Language: English
    In: Journal of Applied Physics, 28 May 2013, Vol.113(20)
    Description: Calculations of the optical properties of GeTe in the cubic NaCl and rhombohedral ferroelectric structures are reported. The rhombohedral ferroelectric distortion increases the band gap from 0.11 eV to 0.38 eV. Remarkably, substantial changes in optical properties are found even at high energies up to 5 eV. The results are discussed in relation to the bonding of GeTe and to phase change materials based on it.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 3
    Language: English
    In: Journal of Applied Physics, 01 January 2012, Vol.112(8)
    Description: I use electronic structure calculations based on a newly developed functional that is designed to reliably predict band structure and band gaps of semiconductors to investigate the electronic structures and optical properties of TlPbI3, TlGeI3, and TlSnI3. The results indicate that TlPbI3 if activated may be a potentially useful low band gap scintillator. The possibility of using Sn as an activator is investigated.
    Keywords: Engineering ; Physics
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 4
    Language: English
    In: Journal of Applied Physics, 15 November 2012, Vol.112(10)
    Description: In this study, we examined the low substrate temperature ( T s ) growth mechanism of Ag thin films in the atomic shadowing regime ( T s ≪ melting point T m ). The Ag thin films were deposited using glancing angle deposition (GLAD) at different substrate temperatures varying from 320 K to 100 K. Interestingly, it is observed that on lowering the substrate temperature instead of showing a monotonic variation, the Ag film morphology changes from the ordered nanocolumns to random and distorted columns, and then to the columnar bunches of nanowires. These growth results suggest that this temperature regime of effective adatom shadowing does not hold a unique growth mechanism for the GLAD within the low temperature range from 320 K to 100 K and depending on the observed temperature dependent variation in morphological and structural properties of the Ag film, it can be sub-divided into three characteristic zones. The observed growth mechanism of the Ag film is explained in terms of the temperature dependent change in terrace diffusion and the interlayer diffusion of Ag adatoms which finally controls the formation of nucleation centers in initial stage and their evolution during the final growth. The understanding of low temperature growth mechanism along with the identification of appropriate temperature range for the growth of nanocolumnar metallic films during GLAD is the novelty in this work.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 5
    Language: English
    In: Journal of Applied Physics, 28 April 2013, Vol.113(16)
    Description: The optical properties and room temperature excitonic features of rf-sputtered ZnO films on Si substrates are studied using spectroscopic ellipsometry and multiphonon Raman scattering. The complex dielectric function ε (E) = ε 1 (E) + iε 2 (E) in the spectral region of 1.3 to 5.5 eV is measured using variable-angle spectroscopic ellipsometry and a Tauc-Lorentz multi-oscillator model is employed to fit the lineshape of the dielectric function. The values of the refractive index of the as-deposited and annealed samples are obtained. Excitonic features in the dielectric function are observed after thermal annealing of the films. Multiphonon Raman spectra up to 7 orders of the longitudinal-optic phonon mode are observed. The matching of the outgoing resonance condition with the exciton energy indicates a strong exciton-phonon coupling. The shape of the dielectric function as well as the intensities and widths of the multiphonon lines suggest the role of both discrete and continuum excitons in describing the optical properties. The observation of excitonic features and the values of the refractive index suggest the high optical quality of the films.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 6
    Language: English
    In: Journal of Applied Physics, 01 December 2012, Vol.112(11)
    Description: In this paper, we develop theoretical frameworks to explain the emergence of ferromagnetism in suspensions and agglomerates of superparamagnetic (SPM) nanoparticles. In the limit of strong anisotropy, the super moments can be treated as a collection of two-state Ising spins. When adequate in number, they interact via dipole-dipole coupling to produce a dipolar field and subsequently a permanent dipole moment. As a result, this effectual ferromagnet exhibits hysteresis on the application of an oscillating magnetic field yielding heat dissipation that is several orders of magnitude larger than in a paramagnet. Using our frameworks, we provide a design for a magnetite-blood suspension that yields heat dissipation in the mW range. Its important physical application is in remedial procedures for destroying tumor and cancer cells. We are also able to explain many experiments reporting manifestations of ferromagnetism in the form of hysteresis loops, return point memory and large heat dissipation in suspensions and aggregates of SPM nanoparticles. Our frameworks can be used to manipulate heat dissipation in variety of combinations of particles and their embedding mediums. They impart a basis to the often used ad-hoc methodologies in this subject.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 7
    Language: English
    In: Journal of Applied Physics, May 14, 2013, Vol.113(18), p.183904-1-183904-11
    Description: The magnetocaloric effect (MCE) in Fe-doped Ni-Mn-Ga alloys was investigated by a combination of first principles calculations and Monte Carlo simulations. The results revealed a near linear increase in inverse and conventional MCE as structural and magnetic transformation occurred.
    Keywords: Electron-electron Interactions -- Analysis ; Gallium Compounds -- Magnetic Properties ; Gallium Compounds -- Thermal Properties ; Manganese Alloys -- Magnetic Properties ; Manganese Alloys -- Thermal Properties ; Monte Carlo Methods -- Usage
    ISSN: 0021-8979
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: Journal of Applied Physics, 15 June 2012, Vol.111(12)
    Description: We report the results of magnetization and electron paramagnetic resonance (EPR) studies on nanoparticles (average diameter ∼ 30 nm) of Bi 0.25 Ca 0.75 MnO 3 (BCMO) and compare them with the results on bulk BCMO. The nanoparticles were prepared using the nonaqueous sol-gel technique and characterized by XRD and TEM analysis. Magnetization measurements were carried out with a commercial physical property measurement system (PPMS). While the bulk BCMO exhibits a charge ordering transition at ∼230 K and an antiferromagnetic (AFM) transition at ∼130 K, in the nanoparticles, the CO phase is seen to have disappeared and a transition to a ferromagnetic (FM) state is observed at T c  ∼ 120 K. However, interestingly, the exchange bias effect observed in other nanomanganite ferromagnets is absent in BCMO nanoparticles. EPR measurements were carried out in the X-band between 8 and 300 K. Lineshape fitting to a Lorentzian with two terms (accounting for both the clockwise and anticlockwise rotations of the microwave field) was employed to obtain the relevant EPR parameters as functions of temperature. The results confirm the occurrence of ferromagnetism in the nanoparticles of BCMO.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 9
    Language: English
    In: Journal of Applied Physics, Jan 15, 2012, Vol.111(2), p.024314-1-024314-5
    Description: Various studies are carried out for examining the perovskite phase transformation demonstrated by the 0.65Pb([[Mg.sub.1/3][Nb.sub.2/3])[O.sub.3]-0.35PbTi[O.sub.3] nanoparticles derived by sol-gel. The different mechanisms involved in the transformation and the implications of the same are all analyzed.
    Keywords: Nanoparticles -- Usage ; Transmission Electron Microscopes -- Technology Application ; X-ray Diffraction -- Methods ; Ceramics -- Electric Properties ; Ceramics -- Thermal Properties ; Crystallization -- Analysis ; Lead Compounds -- Electric Properties ; Lead Compounds -- Thermal Properties
    ISSN: 0021-8979
    Source: Cengage Learning, Inc.
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  • 10
    Language: English
    In: Journal of Applied Physics, 15 January 2011, Vol.109(2)
    Description: Results on two series of lead-free piezoelectric solid solutions of 1 − x − y ( Bi 0.5 Na 0.5 ) TiO 3 − x ( Bi 0.5 K 0.5 TiO 3 ) − y ( Na 0.5 K 0.5 ) NbO 3 (BNT-BKT-KNN) ceramics, synthesized in the vicinity of ferroelectric rhombohedral ( x = 0.17 ) and ferroelectric tetragonal ( x = 0.20 ) phase boundary of 1 − x ( BNT ) – xBKT are presented. Orthorhombic KNN in stoichiometric proportion as third component with 0.0 ≤ y ≤ 0.06 is added to prepare these two series (i) with x = 0.17 and (ii) with x = 0.20 . Addition of KNN up to a critical y is observed to improve tetragonality in both series and solid solubility of KNN was verified at least up to y = 0.1 in this system. Depolarization temperature T d of these stoichiometric BNT-BKT-KNN series were found to be considerably lower than that of their parent BNT-BKT compositions. As a result of this, coexistence of polar and nonpolar phases for compositions at morphotropic phase boundary (MPB) were indicated at room temperature, resulting in pinched polarization versus electric field ( P - E ) loops and large strain (strain of ∼ 0.80 % was observed for y = 0.01 sample in ×20 series and strain of 0.50% was seen for y = 0.04 sample in ×17 series) The MPB of ×17 and ×20 series found to be at y = 0.04 and y = 0.01 , respectively.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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