Absorption (logic)

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

Defect-induced Burstein-Moss shift in reduced V 2 O 5 nanostructures

Abstract: The main effects of oxygen vacancy defects on the electronic and optical properties of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ nanowires were studied through in situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic midgap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ from 1.95 to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as a result of defect formation. Further, in situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the threefold coordinated oxygen (O3).

Far-infrared absorption in thin superconducting lead films

Abstract: We describe measurements of the far-infrared conductivity ${\ensuremath{\sigma}}_{1}\ensuremath{-}i{\ensuremath{\sigma}}_{2}$ of thin superconducting lead films with resistances of about $\frac{200\ensuremath{\Omega}}{\mathrm{square}}$ in the normal state. The conductivity is inferred from measurements of the transmittance and reflectance of a thin film of lead which has been evaporatively deposited on a quartz crystal. We report results for the real part of the conductivity of lead over the frequency range 9 to 120 ${\mathrm{cm}}^{\ensuremath{-}1}$. From these we infer an energy gap for lead films at $T=0$ of 22.5\ifmmode\pm\else\textpm\fi{}0.5 ${\mathrm{cm}}^{\ensuremath{-}1}$, or $(4.5\ifmmode\pm\else\textpm\fi{}0.1)k{T}_{c}$, where $k$ is the Boltzmann constant and ${T}_{c}$ is the superconducting transition temperature. The measurements were made at 2.0, 4.3, and 5.5\ifmmode^\circ\else\textdegree\fi{}K, and the energy gap varies with temperature in the manner predicted by the Bardeen-Cooper-Schrieffer theory. The frequency dependence of the real part of the conductivity also is in good agreement with the theory. No evidence is found of the strong precursor absorption which had been previously reported in the energy gap in lead, but ${\ensuremath{\sigma}}_{2}$ is found to be anomalously low (by \ensuremath{\sim}25%) near and below the gap frequency. This is believed to result from the strong-coupling anomalies discussed by Nam. This anomalously low ${\ensuremath{\sigma}}_{2}$ would also account for the anomalously steep absorption edge observed in other experiments on lead.

Efficient phonon-assisted long-lifetime Nd 3 + fluorescence in Cs 2 NaNd Cl 6

Abstract: Spectroscopic measurements are reported on single crystals of the high-${\mathrm{Nd}}^{3+}$-concentration perovskite ${\mathrm{Cs}}_{2}$NaNd${\mathrm{Cl}}_{6}$ [$\ensuremath{\rho}(\mathrm{Nd})=3.2\ifmmode\times\else\texttimes\fi{}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$] and on ${\mathrm{Cs}}_{2}$Na${\mathrm{Nd}}_{0.01}$${\mathrm{Y}}_{0.99}$ ${\mathrm{Cl}}_{6}$.The room-temperature fluorescence lifetimes are 1.23 and 4.1 msec, respectively, which are the longest reported so far for ${\mathrm{Nd}}^{3+}$ in any environment. This compound shows weak concentration quenching of fluorescence comparable to that observed in Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ which has $\ensuremath{\rho}(\mathrm{Nd})=4\ifmmode\times\else\texttimes\fi{}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. This is related to the isolation of the ${\mathrm{Nd}}^{3+}$ ions which are located in discrete Nd$\mathrm{Cl}_{}^{3\ensuremath{-}}{}_{6}{}^{}$ octahedra and have a minimum separation of 0.77 nm. The long fluorescence lifetime results from the strict octahedral coordination of ${\mathrm{Nd}}^{3+}$ which discourages electric-dipole electronic transitions. Absorption and emission of the $^{4}\mathrm{F}_{3/2}$ - $^{4}\mathrm{I}_{9/2}$ and $^{4}\mathrm{F}_{3/2}$ - $^{4}\mathrm{I}_{11/2}$ levels are found to be dominated by phonon-assisted vibronic transitions. The ground state to the $^{4}\mathrm{F}_{3/2}$ no-phonon line is observed in absorption at 1.8 \ifmmode^\circ\else\textdegree\fi{}K, and the phonon-assisted spectra at shorter wavelengths are used to interpret the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{9/2}$ emission spectra at 1.8 \ifmmode^\circ\else\textdegree\fi{}K. The phononassisted spectra associated with the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{11/2}$ emission are not the same as observed for the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{9/2}$ emission but a tentative assignment has been made. Due to the reduction of the phonon populations at lower temperatures the emission cross sections are reduced and the lifetimes of the concentrated and dilute crystals increase to 5 and 11 msec, respectively, at 77 \ifmmode^\circ\else\textdegree\fi{}K. A comparison with YAG: Nd and Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ indicates that \ensuremath{\sim} 5% of absorbed photons are emitted into the $^{4}\mathrm{I}_{11/2}$ levels in ${\mathrm{Cs}}_{2}$NaNd${\mathrm{Cl}}_{6}$ at room temperature, and an approximate peak emission cross section of 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$ is calculated for the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{11/2}$ emission. This is \ensuremath{\sim} 100 times lower than found for Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ and indicates a low gain in pulsed laser operation. Because of the far longer lifetime, however, the gain per ion under cw operation is predicted to be \ensuremath{\sim} 10 times lower than for Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ and \ensuremath{\sim} 7 times lower if diluted samples of the two compounds are compared with each other.

Radiation Effects in Silica at Low Temperatures

Abstract: Optical absorption bands induced in fused silica and crystalline $\ensuremath{\alpha}$ quartz of low impurity content at 77\ifmmode^\circ\else\textdegree\fi{}K by fast electrons or x-rays bleach slowly at room temperature. The presence of O${\mathrm{H}}^{\ensuremath{-}}$ ions in fused silica inhibits the formation of such radiation-induced absorption. Comparison of the number of centers produced at 215 m\ensuremath{\mu} ($C$ band) in Corning 7943 fused silica (O${\mathrm{H}}^{\ensuremath{-}}$ free) for equal absorbed dose when irradiated with electrons and x-rays indicates that displacements are not involved in the initial formation of the color centers. A defect model requiring simple ionization seems adequate to explain most of the observed phenomena in this pure fused silica. No simple model can be proposed which adequately describes the data in the case of the Corning 7940 fused silica (O${\mathrm{H}}^{\ensuremath{-}}$ bearing).

Power dependence of electron-cyclotron current drive for low- and high-field absorption in tokamaks.

Abstract: A new relativistic, bounce-averaged Fokker-Planck-quasilinear code shows that for experimentally relevant rf power $\frac{{p}_{\mathrm{rf}}(\mathrm{W}/{\mathrm{cm}}^{3})}{{[{n}_{e}({10}^{13} {\mathrm{cm}}^{\ensuremath{-}3})]}^{2}}\ensuremath{\gtrsim}0.5$, electron-cyclotron current-drive efficiency (and absorptivity) is power dependent; at given plasma radius the current-drive efficiency increases above linear values for absorption of the rf on the low-field side of the cyclotron resonance layer, but decreases and even passes through zero for absorption on the high-field side. Thus, electron-cyclotron current drive by high-field launch will generally not be suitable.