General Electric

About: General Electric is a company organization based out in Boston, Massachusetts, United States. It is known for research contribution in the topics: Turbine & Rotor (electric). The organization has 76365 authors who have published 110557 publications receiving 1885108 citations. The organization is also known as: General Electric Company & GE.

Optical Absorption of TetrahedralFe2+(3d6) in Cubic ZnS, CdTe, and MgAl2O4

Abstract: The optical absorption spectrum of substitutional ${\mathrm{Fe}}^{2+}$ ions at concentrations from 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ to 4\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ has been studied for cubic ZnS, CdTe, and Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$ single crystals from 3 to 300\ifmmode^\circ\else\textdegree\fi{}K. The ${\mathrm{Fe}}^{2+}$ ions show a single broad absorption band at 300\ifmmode^\circ\else\textdegree\fi{}K in the infrared region between 1500 and 7500 ${\mathrm{cm}}^{\ensuremath{-}1}$ (1.3 to 6.7 \ensuremath{\mu}) that arises from the $^{5}E\ensuremath{\rightarrow}^{5}T_{2}$ transition. At low temperatures this band shows many distinct lines which are identified as resulting from zero-phonon and phonon-assisted transitions between the spin-orbit levels of $^{5}E$ and $^{5}T_{2}$ in tetrahedral symmetry. The levels of $^{5}E$ are found to be described well by crystal-field theory: There are five uniformly spaced levels split in second order by spin-orbit interactions, with an interval given by 15, 10, and 13 ${\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{Fe}}^{2+}$ in ZnS, CdTe, and Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$, respectively. The levels of $^{5}T_{2}$ do not fit the predictions of crystal-field theory; they can, however, be understood if a moderately strong Jahn-Teller effect occurs in the $^{5}T_{2}$ state, so that the first-order spin-orbit splitting of $^{5}T_{2}$ is quenched to a small fraction of its crystal-field value. Values for this Jahn-Teller energy of 535, 255, and 945 ${\mathrm{cm}}^{\ensuremath{-}1}$ are derived from the data for ZnS, CdTe, and Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$, respectively. A phenomenological Hamiltonian is found which describes the dynamical Jahn-Teller effects in ZnS very well, and which may also be appropriate for Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$ but does not suffice for CdTe. An alternative interpretation of the spectrum for ${\mathrm{Fe}}^{2+}$ in ZnS, not requiring so strong a Jahn-Teller effect, more nearly accords with the predictions of crystal-field theory, but at the expense of assuming that some of the observed zero-phonon lines arise from ${\mathrm{Fe}}^{2+}$ associated with some other defect common to all samples, or from a mixture of cubic and hexagonal regions within the crystals. Values of the cubic-field parameter $\mathrm{Dq}$ for ${\mathrm{Fe}}^{2+}$ in these crystals are -340, -248, and -447 ${\mathrm{cm}}^{\ensuremath{-}1}$ for ZnS, CdTe, and Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$, respectively. The phonon-assisted transitions yield values for the transverse acoustic, longitudinal acoustic, transverse optic, and longitudinal optic phonons in ZnS and CdTe which are 115, 184, 296, 331 ${\mathrm{cm}}^{\ensuremath{-}1}$ and 65, 105, 140, 180 ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively.

Noise in Oscillators

Abstract: Noise affects the behavior of oscillators in at least two important ways. During sustained oscillation, noise creates undesired perturbations or modulation in both the amplitude and the phase of the wave. The amplitude perturbations produce a continuous spectrum which in typical situations is quite weak and broader than the bandwidth of the resonator. The phase perturbations disperse the nominal frequency into a continuous distribution which is of the same form but much stronger and narrower than for the amplitude perturbations. During the initiation of oscillation, noise constitutes the starting voltage and therefore affects the time required for the wave to reach some pre-established amplitude. The resulting jitter in the starting time of pulsed oscillators is objectionable because it degrades the signal-to-noise ratio in systems employing super-regenerative receivers or pulse-time modulation. The time and spectral distributions of noise effects in typical oscillators are derived and discussed in the following sections.