Absorption (logic)

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

Excitons and fundamental absorption in quantum wells

Abstract: We present an accurate theory for the excitonic absorption in quantum-well structures which yields very good agreement with a wide range of experimental data. Our approach is based on an expansion of the exciton wave function in terms of multicomponent envelope functions of electron and hole states. It is rather general with respect to band-structure effects and potential profiles. In momentum space the two-particle Schr\"odinger equation becomes a set of coupled integral equations for the expansion coefficients which we solve by means of a modified quadrature method. Our calculations reproduce all the essential details of a great variety of experimental spectra from high-quality GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We demonstrate that this requires one to take into account both band-structure effects (HH-LH coupling and nonparabolicity) and the coupling of different electron-hole subband pairs caused by the Coulomb interaction. These effects strongly modify the peaks in the exciton continuum which correspond to Fano resonances. Moreover, the Coulomb coupling of subband pairs is often so substantial that it is not possible to attribute excitons to a single subband pair. The free electron-hole absorption coefficient can have pronounced Van Hove singularities which disappear in the excitonic absorption spectrum. Thus, in contrast to bulk semiconductors, the Coulomb interaction can decrease the absorption coefficient at the fundamental absorption edge.

Chandra X-ray analysis of the massive high-redshift galaxy clusters ClJ1113.1-2615 and ClJ0152.7-1357

Abstract: We present an analysis of Chandra observations of two high-redshift clusters of galaxies, ClJ1113.1-2615 at z=0.725 and ClJ0152.7-1357 at z=0.833. We find ClJ1113 to be relaxed with kT=4.3^{+0.5}_{-0.4}keV and a mass (within the virial radius) of 4.3^{+0.8}_{-0.7}*10^{14}Msol. ClJ0152, by contrast, is resolved into a northern and southern subcluster, each massive and X-ray luminous, in the process of merging. The temperatures of the subclusters are found to be 5.5^{+0.9}_{-0.8}keV and 5.2^{+1.1}_{-0.9}keV respectively, and their respective masses are 6.1^{+1.7}_{-1.5}*10^{14}Msol and 5.2^{+1.8}_{-1.4}*10^{14}Msol within the virial radii. 2D modelling of the X-ray surface brightness reveals excess emission between the subclusters; suggestive, but not conclusive evidence of a shock front. We make a first attempt at measuring the cluster M-T relation at z~0.8, and find no evolution in its normalisation, supporting the previous assumption of an unevolving M-T relation. We also find little or no evolution in the L-T relation, the gas fraction-T relation, the beta-T relation or the metallicity. These results suggest that, in at least some massive clusters, the hot gas was in place, and containing its metals, at z~0.8. We also highlight the need to correct for the degradation of the Chandra ACIS low energy quantum efficiency in high-redshift cluster studies when the low energy absorption is often assumed to be the Galactic value, rather than measured.

Photon statistics of a bistable absorber

Abstract: We discuss the quantum-statistical properties of a system of absorbing atoms in a ring cavity under the action of a resonant classical field. At a semiclassical level, this system exhibits a bistable behavior in the sense that the transmitted light ${E}_{T}$ is a discontinuous function of the incident light ${E}_{I}$ with a hysteresis cycle, which resembles a first-order phase transition. Here we specify this analogy stressing the nonthermodynamic properties of this coherently driven system. The quantized field ${E}_{T}$ is described by a Glauber $P$ function, which obeys to a Fokker-Planck equation. This equation is derived from a very general master equation in the case of a good quality cavity by neglecting derivatives of order higher than second. A remarkable feature of this equation is that the diffusion coefficient is a nonlinear function of the field intensity. The steady-state solution of the Fokker-Planck equation has the following features. The $P$ function can be single or double peaked according to the value of ${E}_{I}$. In fact, our Fokker-Planck equation is like the one of a Brownian particle moving in a potential which presents one minimum or minima separated by a barrier, depending on the value of ${E}_{I}$. The peaks of the $P$ function correspond to the minima of the potential. In the double-peaked case, the peaks have different widths. The maxima of the $p$ function occur at values of ${E}_{T}$, which coincide with the classical stationary solutions that are stable according to the linear stability analysis. These semiclassical solutions give a discontinuous two-valued function of ${E}_{T}$ vs ${E}_{I}$ (hysteresis cycle). On the contrary, the mean value $〈{E}_{T}〉$ is a continuous single-valued function of ${E}_{I}$, which behaves similarly to the Maxwell construction of a first-order phase transition: $〈{E}_{T}〉$ jumps from low to high values in a very sharp transition region of ${E}_{I}$, which lies between the discontinuity points of the hysteresis cycle. However, the Maxwell rule that we obtain is quite different from that of equilibrium thermodynamics. The mean-square fluctuations of ${E}_{T}$ are always very small except in the transition region where they are quite remarkable. We also present an exact expression of the stationary $P$ function, which takes into account all higher-order derivatives, and we show that the Fokker-Planck approximation is very good for calculating mean values and mean-square fluctuations. Finally, we discuss the connections which link bistable absorption to the laser with injected signal and to the laser with a saturable absorber.

Evidence for a radiatively driven disc-wind in PDS 456?

Abstract: We present a newly discovered correlation between the wind outflow velocity and the X-ray luminosity in the luminous ($L_{\rm bol}\sim10^{47}\,\rm erg\,s^{-1}$) nearby ($z=0.184$) quasar PDS\,456. All the contemporary XMM-Newton, NuSTAR and Suzaku observations from 2001--2014 were revisited and we find that the centroid energy of the blueshifted Fe\,K absorption profile increases with luminosity. This translates into a correlation between the wind outflow velocity and the hard X-ray luminosity (between 7--30\,keV) where we find that $v_{\rm w}/c \propto L_{7-30}^{\gamma}$ where $\gamma=0.22\pm0.04$. We also show that this is consistent with a wind that is predominately radiatively driven, possibly resulting from the high Eddington ratio of PDS\,456.

Optical properties of the alkali-metal-doped superconducting fullerenes: K 3 C 60 and Rb 3 C 60

Abstract: We have measured the optical reflectivity over a broad spectral range of single-phase ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. We identify various features in the excitation spectrum of the normal state: a Drude contribution at low frequency, a midinfrared absorption, and several interband transitions. In the superconducting state our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit. We also evaluate several intrinsic parameters characterizing both the normal and superconducting state, such as the plasma frequency, the relaxation scattering rate, and the mean free path, besides the superconducting gaps and the penetration depth. We compare these quantities with similar ones from other experiments and find satisfactory agreement. Moreover, we present our calculation of the electrodynamic response above and below ${\mathit{T}}_{\mathit{c}}$ with the standard Eliashberg electron-phonon theory of superconductivity, which strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes.