Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

Simple Rules for Discontinuities in Finite Temperature Field Theory

Abstract: The discontinuity, or imaginary part, of the self-energy function at $T\ensuremath{ e}=0$ is found to be of the form $\ensuremath{\Gamma}={\ensuremath{\Gamma}}_{d}\ensuremath{\mp}{\ensuremath{\Gamma}}_{i}$ for bosons and fermions, respectively. The generalized decay rate ${\ensuremath{\Gamma}}_{d}$ and inverse decay rate ${\ensuremath{\Gamma}}_{i}$ are recognizable as integrals over phase space of amplitudes squared, weighted with certain statistical factors that account for the possibility of particle absorption from the medium or particle emission into the medium. Nonequilibrium statistical mechanics shows that $\ensuremath{\Gamma}$ gives precisely the rate at which the single-particle distribution function approaches the equilibrium form.

Ferromagnetism induced by clustered Co in Co-doped anatase TiO2 thin films.

Abstract: We investigated ferromagnetism of a newly discovered ferromagnetic semiconductor Co-doped anatase ${\mathrm{T}\mathrm{i}\mathrm{O}}_{2}$ thin film, using the magnetic circular dichroism (MCD) at the Co ${L}_{2,3}$ absorption edges. The magnetic moment was observed to be $\ensuremath{\sim}0.1{\ensuremath{\mu}}_{B}/\mathrm{C}\mathrm{o}$ in the measurements, but the MCD spectral line shape is nearly identical to that of Co metal, showing that the ferromagnetism is induced by a small amount of clustered Co. With thermal treatments at $\ensuremath{\sim}400\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the MCD signal increases, and the moment reaches up to $\ensuremath{\sim}1.55{\ensuremath{\mu}}_{B}/\mathrm{C}\mathrm{o}$, which is $\ensuremath{\sim}90%$ of the moment in Co metal. In the latter case, the cluster size was observed to be 20--60 nm.

GaAs lower conduction-band minima: Ordering and properties

Abstract: Synchrotron-radiation Schottky-barrier electroreflectance spectra from the $\mathrm{Ga} 3{d}^{V}$ core levels to the lower $s{p}^{3}$ conduction band have shown that the ${L}_{6}^{C}$ lower conduction-band minima are located 170 \ifmmode\pm\else\textpm\fi{} 30 meV in energy below the ${X}_{6}^{C}$ minima in GaAs. Here, we investigate the implications of this ordering, which is opposite to that commonly accepted as correct. We find that, without exception, the results of previous experiments that apparently supported the opposite ordering can be reinterpreted within the ${\ensuremath{\Gamma}}_{6}^{C}\ensuremath{-}{L}_{6}^{C}\ensuremath{-}{X}_{6}^{C}$ model. By performing a line-shape analysis, we resolve an apparent discrepancy between intraconduction band absorption measurements of the ${X}_{6}^{C}\ensuremath{-}{\ensuremath{\Gamma}}_{6}^{C}$ energy separation. By comparing these optical results with other modulation spectroscopic ($s{p}^{3}$ valence-conduction-band electroreflectance, high-precision reflectance) data, combining these with the results of photoemission, transport (high pressure and high temperature), semiconductor alloy, and luminescence measurements, nonlocal pseudopotential calculations $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ theory, the rigid-valence-band hypothesis, and using the systematics of other tetrahedrally bonded semiconductors with temperature and pressure, we obtain a set of consistent parameters describing the ${\ensuremath{\Gamma}}_{6}^{C}$, ${L}_{6}^{C}$, and ${X}_{6}^{C}$ lower conduction-band minima of GaAs. This model resolves the former contradictions in the apparent indirect threshold energy as determined previously by photoemission, transport, and optical measurements. Previous photoemission data for cesiated GaAs show clearly after structure reassignment that hot electrons thermalize in the ${L}_{6}^{C}$ minima. This implies that Gunn oscillator operation in GaAs involves the ${L}_{6}^{C}$, and not ${X}_{6}^{C}$, conduction-band minima. We obtain the variation of these minima with composition, $x$, in the $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ alloy series, and show that the increase in binding energy of the N isoelectronic trap with increasing As fraction in this series is in qualitative agreement with the prediction of a two-level model wherein a Koster-Slater isoelectronic trap potential interacts with the densities of states of both ${L}_{6}^{C}$ and ${X}_{6}^{C}$. These results have clear implications for the theory of operation of light-emitting diodes of GaAs and its alloys.

Theory of defects in vitreous silicon dioxide

Abstract: Models for defects in Si${\mathrm{O}}_{2}$ fall into the two basic categories of "vacancy-bridge" and valence-alternation models. We have calculated the local electronic structure of the main defects in each model, using the tight-binding and recursion methods. The localization of each level is found and compared to that measured by ESR for the paramagnetic centers. The silicon dangling bond, the neutral oxygen vacancy, and the positively charged oxygen vacancy (${E}^{\ensuremath{'}}$ center) all give deep states near mid-gap. The Si---Si bond gives a bonding state in the lower gap and an antibonding state near the conduction-band minimum. The positive, threefold-coordinated oxygen site $\mathrm{O}_{3}^{}{}_{}{}^{+}$(${\mathrm{Si}}_{3}$) gives a state bound only by its Coulombic field. In general, all positively charged centers possess a "shallow" bound state 1-2 eV below the conduction-band minimum. Such shallow states account for the prevalence of optical absorption around 7.6 eV in Si${\mathrm{O}}_{2}$. The nonbridging oxygen introduces states just above the valence-band maximum. The peroxyl bridge and radical give states both at mid-gap and in the lower part of the gap. A broad absorption band around 5-6 eV is associated with the peroxyl radical, for the first time. It is suggested that valence-alternation defects must still be present in $v\ensuremath{-}\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}$, but at a much lower concentration, of order ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, than previously supposed, due to a higher valence-alternation creation energy in Si${\mathrm{O}}_{2}$ than in $a\ensuremath{-}\mathrm{S}\mathrm{e}$ or $a\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Se}}_{3}$.

Relativistic Band Calculation and the Optical Properties of Gold

Abstract: The energy band structure of gold is calculated by the relativistic augmented-plane-wave (RAPW) method. A nonrelativistic calculation is also presented, and a comparison between this and the RAPW results demonstrates that the shifts and splittings due to relativistic effects are of the same order of magnitude as the gaps (approximately 1 eV). Various integrated functions, density of states, joint density of states, and energy distributions of joint density of states are derived from the RAPW calculation. These functions are used in an interpretation of photoemission and static reflectance measurements. It is shown that the photoemission results are extremely well described in terms of a model assuming all transitions to be direct whereas a nondirect model fails. The ${\ensuremath{\epsilon}}_{2}$ profile calculated in a crude model assuming constant matrix elements matches well the corresponding experimental results. The calculated interband edge ($\ensuremath{\hbar}{\ensuremath{\omega}}_{i}=2.38$ eV) agrees with experimental values, and the absorption tail below the interband edge which is found in experimental traces is also contained in the theoretical curve. By means of a calculation of the Fermi surface and the constant-energy-difference surfaces it has been possible to trace out the regions in $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space where the edge and tail transitions occur. It is demonstrated that structure in the static reflection curves are not related to critical points in the band structure. The arguments are supported by calculations of temperature shifts of the critical-point energies and comparison to the observed temperature shifts of the elements of structure in the experimental ${\ensuremath{\epsilon}}_{2}$ function. Such structure may originate in extended rather than localized regions of $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space. In contrast, critical-point transitions show up clearly in modulated reflectance spectra, and all elements of structure are fully accounted for by our band model. The temperature and strain responses in the band structure are determined by performing the RAPW calculation with two lattice constants and estimating the effects of the lattice vibrations by means of an OPW-LCAO (linear combination of atomic orbitals) scheme with pseudopotential Fourier constants reduced by the appropriate Debye-Waller factors. The phonon spectrum has been calculated for the latter purpose.