Absorption (logic)

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

Crossover from large to small polarons across the metal-insulator transition in manganites

Abstract: We report Mn $K$-edge extended x-ray absorption fine structure spectra on ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_{3}$ up to high momentum transfer across the metal-insulator $(M\ensuremath{-}I)$ transition. The data show compelling evidence for (i) large or intermediate Jahn-Teller polarons (IJTP), characterized by an anomalous longer Mn-O bond $(\ensuremath{\Delta}R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.09\AA{})$ in the metallic phase $(Tl170\mathrm{K})$, and (ii) appearance of small JT polarons (SJTP) at $Tg170\mathrm{K}$, characterized by a longer Mn-O bond $(\ensuremath{\Delta}R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.21\AA{})$, which coexist with the IJTP above the $M\ensuremath{-}I$ transition and has equal probability in the temperature range of colossal magnetoresistance.

Optical spectrum of a Hubbard chain

Abstract: The optical absorption of the one-dimensional Hubbard model is calculated by three different methods. Linear chains of five atoms and rings of five and seven atoms containing four electrons are treated numerically. The infinite-chain problem is solved first in the $t$-matrix approximation of Lyo and Holstein. It is shown that in this approximation, most of the high-frequency absorption is due to a bound state which lies above the band continuum. Finally, the absorption is evaluated in the memory-function formalism of G\"otze and W\"olfle, which reduces to ordinary perturbation theory at high frequency. The three approaches are in qualitative agreement, and the differences between them can be explained by the nature of the approximations involved. Applications to tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and the platinum salt ${\mathrm{K}}_{2}$Pt${(\mathrm{CN})}_{4}$${\mathrm{Br}}_{0.3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 3${\mathrm{H}}_{2}$O (KCP) are discussed.

Stark effect of one-dimensional Wannier excitons in polydiacetylene single crystals.

Abstract: Electric-field-modulated reflectivity of three different polydiacetylenes---PTS, poly[2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)]; PFBS, poly[2,4-hexadiyne-1,6-diol-bis(p-fluorobenzene sulfonate)]; and DCHD, poly[1,6-di(n-carbazolyl)-2,4-hexadiyne]---has been measured and is analyzed with respect to the underlying mechanism for the observed sensitivity of \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ transitions to electric fields. Excitons of high oscillator strength and their vibronic satellites respond to fields along the polymer backbone by a large quadratic Stark shift, revealing a large polarizability for this direction. About 0.5 eV above the excitonic absorption edge, in a region of relatively low absorption, electroreflectance signals of different origin are observed, which, contrary to the excitonic signals, vary strongly in size among different specimens of the same composition. Line-shape analysis and its dependence on the field strength identify this signal as the Franz-Keldysh effect of free-electron states, the continuum of the excitons, and exclude an assignment to forbidden exciton transitions. The large polarizability of the excitonic states, which results from unusually strong coupling to their continuum, is consistent with a Wannier exciton extending over about ten conjugated bonds and with a small reduced mass of the order 0.1${\mathit{m}}_{0}$. The large binding energy (0.5 eV) and oscillator strength (f\ensuremath{\approxeq}0.6) of the excitons and their extremely strong coupling to continuum states are attributed to the one-dimensional character of the electron states.

Sharp-Line Fluorescence, Electron Paramagnetic Resonance, and Thermoluminescence of Mn 4 + in α - Al 2 O 3

Abstract: Sharp-line fluorescence, paramagnetic resonance in the ground state, and optical absorption due to ${\mathrm{Mn}}^{4+}$ in $\ensuremath{\alpha}$-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ have been observed (analog of ${\mathrm{Cr}}^{3+}$). The ${\mathrm{Mn}}^{4+}$ valence state is obtained by charge compensation with ${\mathrm{Mg}}^{2+}$. The increased charge of ${\mathrm{Mn}}^{4+}$ compared to ${\mathrm{Cr}}^{3+}$ results in a stronger crystal field and greater covalency whose effects are clearly seen in both the optical and paramagnetic resonance results. The ground state splitting is 0.39 ${\mathrm{cm}}^{\ensuremath{-}1}$, almost the same as in ruby, while the metastable $^{2}E$ state splitting is 80 ${\mathrm{cm}}^{\ensuremath{-}1}$. When the crystals are irradiated with ultraviolet (3000 \AA{}), more than 50% of the ground state of ${\mathrm{Mn}}^{4+}$ is depopulated, as long-lived traps are filled which decay via the $^{2}E$ states through sharp-line thermoluminescence. The possible application to light masers is briefly discussed.

A Lower Bound on the Cosmic Baryon Density

Abstract: We derive analytic lower bounds on the cosmic baryon density by requiring that the high-redshift IGM contain enough neutral hydrogen to produce the observed \lya absorption in quasar spectra. The key theoretical assumption is that absorbing structures are no more extended in redshift space than in real space. This assumption might not hold if \lya clouds are highly overdense and thermally broadened, but it is likely to hold in the gravitational instability picture for the \lya forest suggested by cosmological simulations, independently of the details of the cosmological model. The other ingredients in these bounds are an estimate of the UV background from quasars, a temperature $T~10^4K$ for the "warm" photoionized IGM that produces most of the \lya absorption, a value of the Hubble constant, and observational estimates of the mean \lya flux decrement $\Dbar$ or, for a more restrictive bound, the distribution function $P(\tau)$ of \lya optical depths. With plausible parameter values, the mean decrement bound implies a baryon density parameter $\Omb \ga 0.0125/h^2$. With conservative values, the bound weakens to $\Omb \ga 0.005/h^2$, but the required clustering of the IGM is then incompatible with other properties of quasar spectra. A recent observational determination of $P(\tau)$ implies $\Omb \ga 0.0125/h^2$ even for a conservative estimate of the UV background, and $\Omb \ga 0.018/h^2$ for a more reasonable estimate. These bounds are consistent with recent low estimates of the primordial deuterium-to-hydrogen ratio $\dtoh$, which imply $\Omb \approx 0.025/h^2$, but the $P(\tau)$ bound can only be reconciled with high $\dtoh$ estimates by abandoning standard big bang nucleosynthesis or the gravitational instability picture for the origin of the \lya forest. (Shortened abstract.)