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  • 1
    Electronic Resource
    Electronic Resource
    Inherent temperature effects in magnetic tunnel junctions (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7567-7569 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357126
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  • 2
    Electronic Resource
    Electronic Resource
    An octopole electrodynamic balance for three-dimensional microparticle control (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 3380-3385 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new octopole electrode design improves the particle trapping stability of an electrodynamic balance (EDB) and allows one to make three-dimensional force measurements on a trapped particle. A conventional double-ring EDB was modified by splitting each ring into four equal segments that are electrically independent. Three dc sources were combined such that eight potentials were applied to the eight segments of the electrodes. An additional ac voltage was superimposed on each ring segment as in a conventional double-ring EDB. The resulting electric field has dc components in the x, y, and z directions, which can be controlled independently by the three dc supplies. The z component is used to balance and measure vertical forces such as gravity, radiation pressure, or phoretic forces. The x and y fields can be used to suppress lateral oscillations of the trapped particles, and lateral forces on the particle can be measured in terms of the x and y dc voltages. The apparatus and the electric fields are described herein, and the operation of the device is demonstrated for spheres and aggregated particles. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1384453
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  • 3
    Electronic Resource
    Electronic Resource
    Local crystal analysis using near-field optical second harmonic microscopy: Application to thin ferroelectric films (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 206-211 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Application of near-field optical second harmonic microscopy to nanometer scale crystal and strain analysis of thin polycrystalline ferroelectric films is described. The near-field signal is shown to be sensitive to local crystal orientation by experimental and theoretical studies of polarization properties of second harmonic generation from a poled single crystal of BaTiO3. Spatial resolution of the order of 80 nm has been demonstrated by recovery of the local poling direction of individual ferroelectric domains in polycrystalline thin Pb(ZrxTi1−x)O3 films. Near-field strain induced second harmonic generation from thin BaxSr1−xTiO3 films has also been studied, and an agreement between theory and experiment has been established. Thus, the technique described provides a tool for nanometer scale crystal and strain analysis of polycrystalline samples. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1331342
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  • 4
    Electronic Resource
    Electronic Resource
    Defect modes in coaxial photonic crystals (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 2642-2649 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One-dimensional (1D) photonic crystals have been constructed by connecting segments of coaxial cable of differing characteristic impedance. Impurities have been introduced into these crystals by inserting cable segments to break the crystal symmetry. This system provides a simple way to study 1D photonic band structure effects with complete control over impurities in the lattice. We have studied the effects of the size, number, and location of defects in the lattice. We have also measured directly the concentration of energy in the steady-state electromagnetic fields within doped crystals, and observed the influence of the defects on the phase (dispersion). A modified dielectric stack model was developed to describe this system, with the results in excellent agreement with our measurements. Our findings compare favorably to previously published measurements of transmission and phase change in three-dimensional photonic crystals. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1391220
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  • 5
    Electronic Resource
    Electronic Resource
    Electronic relaxation dynamics of carbon cluster anions: Excitation of the C˜ 2Πg←X˜ 2Πu transition in C6− (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 11185-11192 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Anion femtosecond photoelectron spectroscopy (FPES) has been used to monitor intramolecular electronic relaxation dynamics following the excitation of the C˜ 2Πg←X˜ 2Πu 000 electronic transition in C6−. The time-dependent photoelectron spectra provide a detailed picture of the relaxation dynamics in which the initially excited C˜ 2Πg (v=0) level evolves into highly vibrationally excited C6− in its ground electronic state. The spectra show evidence for a two-step relaxation mechanism: internal conversion (IC) to vibrationally excited B˜ 2Σu+ and A˜ 2Σg+ states, occurring on a time scale of 730±50 fs, followed by IC from these intermediate states to highly vibrationally excited levels in the X˜ 2Πu ground state with a time constant of 3.0±0.1 ps. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1421378
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  • 6
    Electronic Resource
    Electronic Resource
    Probing nuclear quadrupole interactions in the rotationally resolved S1←S0 electronic spectrum of 2-chloronaphthalene (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 225-235 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally resolved S1←S0 fluorescence excitation spectra of 2-chloronaphthalene (2ClN) are measured using a newly constructed ultraviolet (UV) laser/molecular beam spectrometer. More than 1000 well-resolved rotational lines are recorded at a resolution of ∼3 MHz for the two overlapping band origins of the 35Cl and 37Cl isotopomers (Δνorigin∼3 GHz) over a 3.5 cm−1 spectral region at a rotational temperature of 10 K. Both spectra are hybrid bands, having 18% a-/82% b-type character in accordance with 1La←1A1 excitations observed for other 2-substituted naphthalenes. Additionally, Cl nuclear quadrupole splittings are observed as broadened asymmetric line shapes with linewidths from 5 to 20 MHz. Least-squares fits of these line shapes coupled with high-precision Fourier-transform microwave measurements of the ground state have provided accurate S1 quadrupole coupling constants. Specified relative to the principal quadrupole tensor orientation of S0, the S0 and S1 values of eQqzz are −71.198(5) MHz and −67.8(30) MHz, and the asymmetry parameters, η=(eQqyy−eQqxx)/eQqzz, are 0.076 38(5) and 0.13(5), respectively. The experimental line shapes are fit to Voigt functions, yielding 4.9(3) MHz homogeneous (lifetime) and 3.0(2) MHz inhomogeneous (Doppler) contributions. All of the observed structure are well reproduced by more than 20 000 transitions predicted for the nuclear-quadrupole-split rotational spectra of the two hybrid-band origins. Furthermore, the homogeneous linewidths show no dependence on either the upper state energy or angular momentum quantum numbers. The absence of additional level structure at the resolution of 1 MHz or more, from heavy-atom-enhanced intersystem crossing mechanisms, is discussed within the framework of radiationless transition theory. The quadrupole tensor components are used in simple theoretical models to investigate the orbital character of the S1 state and its radiationless decay mechanisms. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1376629
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  • 7
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9352-9360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has been investigated via ultrafast infrared pump–probe experiments. Exciting at 2690 cm−1, only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions ≤0.025, a 2.15 ps single exponential decay is observed. At mole fractions ≥0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and (very-much-greater-than)300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415447
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  • 8
    Electronic Resource
    Electronic Resource
    Simple method for estimating linear attenuation coefficients from x-ray diffraction tomography data (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 1918-1920 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article, we demonstrate how the spatial distribution of x-ray linear attenuation coefficients within an object can be estimated from x-ray diffraction tomography data. The experimental arrangement to achieve this exploits a position sensitive detector and an analyzer crystal. The quality of reconstructed maps of linear attenuation coefficients is comparable to results obtained from conventional transmission computed tomography. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344171
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  • 9
    Electronic Resource
    Electronic Resource
    An efficient tabulated collisional radiative equilibrium radiation transport model suitable for multidimensional hydrodynamics calculations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3480-3489 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A computationally efficient method for transporting radiation in multidimensional plasmas has been developed and evaluated. The basis of this method is a uniform plasma approximation that allows one to utilize existing escape probability techniques that are successfully used in one-dimensional (1D) calculations to approximately solve the multidimensional radiation transport problem. This method is superior to diffusion methods because (1) the probability of escape technique insures that the plasma goes to the correct optically thin and thick limits, (2) the effects of line absorption due to photoexcitations are modeled, and (3) this method uses source functions that are based on a self-consistent nonlocal thermodynamic equilibrium calculation, not an ad hoc assumption that the source functions are Planckian. This method is highly efficient because equation of state information from 1D calculations is tabulated as a function of plasma internal energy, ion density, and the line probability of escape from a uniform plasma, and then used in multidimensional calculations. Given the internal energy and ion density, and by calculating the line probability of escape from a zone of the multidimensional plasma, the equation of state, including emissivities and absorption coefficients, of the zone is determined from the table. Total radiative power, K-shell radiative power, total radiative yield, K-shell radiative yield, and plasma density and temperature profiles obtained from 1D Z-pinch calculations employing this method are in good agreement with the same powers, yields, and profiles calculated using a full radiation transport model. This method has been implemented in the 2D plasma radiating imploding source model code [F. L. Cochran et al., Phys. Plasmas 2, 2765 (1995)] to determine the influence of radiation transport in argon Z-pinch experiments performed on the Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1373415
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  • 10
    Electronic Resource
    Electronic Resource
    Trends in plasma conditions inferred from an analysis of x-ray data from high wire-number, Z-pinch load implosions (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3708-3717 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analysis of x-ray data from two series of Z-pinch shots taken on the short current-risetime Saturn accelerator at Sandia National Laboratories [Proceedings of 6th International IEEE Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] is presented. In one series, the array radius was held constant and the array mass was varied; in the other series, the array mass was held constant and its radius varied. In both sets of experiments, large wire-number loads (N≥93) of aluminum were used in contrast to earlier small wire-number aluminum array experiments on Saturn where N≤42. Average electron temperatures and ion densities were inferred from the data. In addition, from the measured size of the emission region of K-shell x rays and from the inferred ion density, a fraction of the total array mass that participated in the K-shell emission was inferred and found to be directly correlated to the K-shell yields that were measured. This paper also demonstrates that the yields varied as a function of array mass and radius in much closer agreement with predictions [J. Appl. Phys. 67, 1725 (1990)] than had been observed in the earlier small wire-number experiments. Thus, a serious misperception that the reason for the early disagreement was in the calculations and not in the experiments is corrected. These predictions were made using one-dimensional (1D) magnetohydrodynamics calculations. The density and temperature trends inferred from the data analysis are well-behaved and consistent with the 1D calculations. This data analysis confirms the importance of achieving uniform plasma initial conditions and implosion symmetry when comparing computer code calculations with experiment. When the wire number of an array load is increased, a more uniform shell of plasma is calculated initially as the wires explode and, as the plasma stagnates on axis, the x-ray powers and yields are found experimentally to approach the powers and yields predicted by 1D calculations. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1380695
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