Absorption (logic)

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

Nonzero orbital moment in high coercivity ε-Fe2O 3 and low-temperature collapse of the magnetocrystalline anisotropy

Abstract: The magnetic properties of $ϵ{\text{-Fe}}_{2}{\text{O}}_{3}$ nanoparticles are investigated by x-ray magnetic circular dichroism. Sum rules relating the orbital and spin moment in the $\text{Fe}\text{ }3d$ band to the $\text{Fe}\text{ }{L}_{2,3}$ absorption cross sections show that the Fe orbital moment $({m}_{\text{orb}})$ is considerably high, explaining the origin of the large coercivity of this material at room temperature. Moreover, at $T\ensuremath{\sim}110\text{ }\text{K}$, the collapse of the coercivity $({H}_{c})$ and the magnetocrystalline anisotropy coincides with a strong reduction of the spin-orbit coupling evidenced by a drastic drop of ${m}_{\text{orb}}$. The decrease in ${m}_{\text{orb}}$ originates from changes in the electron transfer between Fe and O ions accompanied by significant modifications of some of the Fe-O bond distances. Similarly, the recovery of ${m}_{\text{orb}}$ at lower temperatures mimics the behavior of the Fe-O bond lengths.

Optical Absorption in Single-Domain Ferroelectric Barium Titanate

Abstract: The optical-absorption coefficients for polarized light have been measured for single-domain crystals of $n$-type BaTi${\mathrm{O}}_{3}$. The measurements were made over a wavelength range from approximately 0.4 to 12 \ensuremath{\mu}, and over a temperature range from 26 to 130\ifmmode^\circ\else\textdegree\fi{}C. This temperature range includes both the cubic and the ferroelectric tetragonal states. The most significant feature of the measurements is a broad peak in the optical-absorption coefficient $\ensuremath{\alpha}$ centered near a photon energy of 0.6 eV; the absorption exhibits a threshold at a photon energy of approximately 0.2 eV. In addition, a large anisotropy, comparable to that observed in the transport behavior, is noted in the infrared values of $\ensuremath{\alpha}$ in the tetragonal state. It is shown that a consistent explanation of the optical-absorption properties is that most of the absorption in the visible and near infrared is the result of optical excitation of electrons in impurity states approximately 0.2 to 0.3 eV below the conduction-band minimum, superimposed on a free-carrier and lattice-absorption background. The deep impurity states are probably associated with oxygen vacancies.

Pressure-Induced Metal-Semiconductor Transition and 4 f Electron Delocalization in Sm Te

Abstract: The pressure variation of resistivity and optical absorption in SmTe has been studied. A continuous pressure-induced semiconductor-to-metal transition is observed, which we ascribe to the promotion of electrons from the $4f$ level into the conduction band as the gap between them shrinks with pressure and finally vanishes. The gap deduced from the saturation resistivity ratio $\frac{\ensuremath{\rho}{(P)}_{\mathrm{sat}}}{\ensuremath{\rho}(0)}$ is in good agreement with the gap of 0.62 \ifmmode\pm\else\textpm\fi{} 0.02 eV obtained from infrared absorption data.

Theory of single photon detection by 'dirty' current-carrying superconducting strip based on the kinetic equation approach

Abstract: Using kinetic equation approach we study dynamics of electrons and phonons in current-carrying superconducting nanostrips after absorption of single photon of near-infrared or optical range. We find that the larger the ratio $C_e/C_{ph}|_{T_c}$ ($T_c$ is a critical temperature of superconductor, $C_e$ and $C_{ph}$ are specific heat capacities of electrons and phonons, respectively) the larger part of photon's energy goes to electrons, they become stronger heated and, hence, could thermalize faster during initial stage of hot spot formation. Thermalization time $\tau_{th}$ could be less than one picoseconds for superconductors with $C_e/C_{ph}|_{T_c}\gg 1$ and small diffusion coefficient $D\simeq 0.5 cm^2/s$ when thermalization occurs mainly due to electron-phonon and phonon-electron scattering in relatively small volume $\sim \xi^2d$ ($\xi$ is a superconducting coherence length, $d<\xi$ is a thickness of the strip). At larger times due to diffusion of hot electrons effective temperature inside the hot spot decreases, the size of hot spot increases, superconducting state becomes unstable and normal domain spreads in the strip at current larger than so-called detection current. We find dependence of detection current on the photon's energy, place of its absorption in the strip, width of the strip, magnetic field and compare it with existing experiments. Our results demonstrate that materials with $C_e/C_{ph}|_{T_c} \ll 1$ are bad candidates for single photon detectors due to small transfer of photon's energy to electronic system and large $\tau_{th}$. We also predict that even several microns wide dirty superconducting bridge is able to detect single near-infrared or optical photon if its critical current exceeds 70 $\%$ of depairing current and $C_e/C_{ph}|_{T_c} \gtrsim 1$.

Photoinduced two-photon absorption and second-harmonic generation in As 2 Te 3 -CaCl 2 -PbCl 2 glasses

Abstract: Photoinduced nonlinear optical phenomena in amorphous ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}{\ensuremath{-}\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{l}}_{2}{\ensuremath{-}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_{2}$ glasses have been studied using experimental and theoretical quantum chemical and molecular-dynamics methods. Especially photoinduced two-photon absorption (TPA) and second-harmonic generation (SHG) were measured in the IR region from 5.5--21 \ensuremath{\mu}m. CO laser (\ensuremath{\lambda}=5.5 \ensuremath{\mu}m) and parametrically generated wavelengths (5.2--13.7 \ensuremath{\mu}m) were used as a source of pumping light. We have found that with an increase of photoinducing power, the SHG for probe ${\mathrm{CO}}_{2}$ laser (for the double frequency \ensuremath{\lambda}=5.3 \ensuremath{\mu}m) signal increases and achieves its maximum value at photoinducing power 1.45 ${\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ per pulse. The absolute values of the SHG were more than one order less comparing to ${\ensuremath{\chi}}_{222}$ tensor for ${\mathrm{Ag}}_{3}{\mathrm{AsSe}}_{3}$ single crystals. With decreasing temperature, the SHG signal strongly increases within the 16--24 K temperature range. Femtosecond probe-pump measurements indicate on an existence of SHG maximum at pump-probe time delay about 25 ps. Spectral positions of the TPA maxima are strongly dependent on the pump power. Contrary to the SHG behavior, for the TPA we observe at least two time delayed maxima: at 20--27 and 65 ps. We explain these dependencies within a framework of the quantum chemical approach taken into account with photoinduced anharmonic electron-vibration interaction. We have revealed that As-Te tetrahedra play a key role in the observed photoinduced nonlinear optics effects. The obtained results show that the mentioned effects can be used as a powerful tool for investigations of picosecond IR nonlinear optics processes. Simultaneously the investigated glasses are promising materials for IR femtosecond quantum electronics.