ALBERT

Notes: Using constrained local spin density functional theory it is possible to directly calculate the adiabatic magnon dispersion curve. Small amplitude magnons with wavelengths between 2 and 32 atomic layers were constructed and the energy change with respect to amplitude within the frozen potential approximation was calculated. The resulting dispersion curves give the spin wave stiffness for [111] face-centered-cubic Co and Ni in fair agreement with experiment. In Fe the stiffness was calculated along [100] and [110] directions. The two values of stiffness were found to be nearly equal to each other and roughly half the measured value. It was found that the calculated Fe stiffness did not change upon addition of gradient corrections to the local density approximation, indicating that the source of discrepancy is not likely a failure of the local density approximation. It is argued that the adiabatic approximation is more likely the source of discrepancy since the magnetic interactions in Fe are longer ranged than those in Co or Ni, thus making the dynamical retardation effect more important in Fe. © 1999 American Institute of Physics.

Notes: The simplest model for the contribution of pore surfaces to nuclear magnetic resonance (NMR) relaxation of a pore fluid gives R, the average relaxation rate minus the bulk rate, equal to a constant ρ, the velocity at which nuclear magnetization flows out of the pore fluid at the surfaces, times the pore-space surface-to-volume ratio S/V. Although ρ can vary widely, a great variety of porous media exhibit ρ values of the order of a few μm/s for longitudinal relaxation when S/V is measured by gas adsorption by the Brunauer, Emmett, and Teller (BET) method or high pressure mercury injection. For samples with wide distributions of relaxation rates it is of interest to find what functions of the relaxation data correlate best with S/V measurements and how different relaxation parameters relate to each other. Longitudinal relaxation data were taken for 77 sandstone samples of different origin, which had been cleaned and saturated with brine. After the NMR measurements the samples were dried and surface areas measured by BET. The samples have S/V from 1.5 to 150 (μm)−1, porosity from 3% to 28%, and permeability from less than 0.1 mD to more than 1 D. Longitudinal relaxation data were taken from 400 μs to 6 s and analyzed in many different ways, including stretched-exponential fits and multiexponential fits up to five components. S/V and ln(S/V) were correlated with various relaxation rates derived from these computed parameters.In principle, the relaxation parameter to use with a ρ value is the average rate, which is initial slope divided by initial amplitude, namely, R(0), where R(t)=(d/dt)ln S(t) at t=0 and S(t) is the relaxing signal. One can extrapolate an n component fit to t=0 to get Rn(0), but very good signal quality is required even to get small short components reliably for t well within the times covered by the data. Over half of the points have ρ's within a factor of 2 of the minimum value 0.9 μm/s when the average rate of a five-component fit to the data is used. There are numerous points with ρ up to 7 μm/s, but none of the high-ρ points are for samples with high S/V. All samples with high S/V have wide distributions of relaxation rates, but not vice versa. The best simple correlation with ln(S/V) was ln(S/V)≈1.81 ln(R33)−5.73, where R33 is the highest rate of a three-component fit without regard to the corresponding amplitude, and where S/V is in (μm)−1 and rate in s−1. This result was unexpected. This fit does not represent proportionality to a velocity ρ and does not correspond to any obvious physical model, but it can be of practical interest to estimate in a very simple and noninvasive manner S/V at the BET scale in sandstones. © 1996 American Institute of 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.

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.

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.

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.

Notes: The critical relationship between ion flux and substrate temperature which defines the threshold conditions for the formation of amorphous layers in GaAs at constant ion fluence is measured for the first time. At elevated temperatures, amorphous layers are formed in GaAs by a collapselike process when a critical free energy value is exceeded. The threshold conditions for amorphization are shown to be thermally activated, with an activation energy of 0.9±0.1 eV. However, specific defects and/or processes giving rise to this value are not yet known. © 1998 American Institute of Physics.