Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Evidence for an alkali-like conduction band in alkali graphite intercalation compounds

The valence bands of pure graphite and several alkali graphite intercalation compounds (AGIC's) were studied by UPS (hv = 21.2 eV). The most significant observation is an intensity peak at the Fermi energy E_F in the intecalation compounds. This peak is mainly due to alkali-like s-states. The density of states at E_F is enhanced by a factor of 30 compared to pure graphite. The alkali-like conduction bands in the first stage AGIC's are similar to those of pure alkali metals.     

Calculation of chemisorption energies of oxygen on narrow-gap semiconductors

Newns' self-consistent model for hydrogen chemisorption on transition metals is extended to calculate the chemisorption energy ΔE of oxygen on Si, Ge and III–V compounds. ΔE is found to depend on the widths of the energy gap and valence band, the crystal work function and the crystal plane, |ΔE₍₁₀₀₎|>|ΔE₍₁₁₁₎|. In general, larger ΔE's are associated with wider energy gaps.     

Dispersion relation of over-damped soft E-polariton in tetragonal BaTiO3

In tetragonal BaTiO₃, the over-damped E-polariton frequency of ωχ''(ω, Q), obtained from the near-forward Raman spectra, has been found to follow the usual dispersion relation. From the dispersion relation in a very small q region, ?_a has been determined with precision.     

High-resolution electroreflectance measurements of GaAs

High-resolution surface-barrier electroreflectance measurements are reported for GaAs, taken at 4.2 K in the Schottky barrier configuration. Interference effects between the bulk and space-charge regions are seen at the n = 1 exciton line for both E₀ and E₀ + Δ₀ transitions. We calculate the value μ_T = (0.055 ± 0.008) m_e for the transverse mass of the E₁ + Δ₁ transition from Franz-Keldysh oscillations observed at high fields. The E'₀ structure is resolved into separate critical points of Γ and Δ symmetry. Threshold and broadening energies are obtained at 4.2 K for the E₀,E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E'₀ (Γ and and E₂ (Σ and X) critical points.     

Optical response of saddle point excitons

The linear optical response of interband transitions near saddle points is studied within the framework of the effective mass approximation. The method is based on Stahl's configuration space theory. Unlike previous works the present treatment allows an unrestricted range of effective mass ratios, and it generates both real and imaginary part of the dielectric function. Spectra are calculated by numerical integration of a two dimensional second order partial differential equation. The results obtained for M₁ saddle points agree well with the observed lineshape of the E₁ peak in ZnTe.     

Exchange-correlation energy and the phase seperation of the electron- hole liquid in stressed semiconductors

Corrections to the exchange-correlation energy (E_xc) used by Kirczenow and Singwi to predict the phase seperation of the electron-hole liquid in <111>-stressed Ge are examined. Andryushin, Gel'fond and Silin (AGS) claimed that the corrections destroy this phase seperation. Combescot, Singwi and Vignale have claimed the opposite. The claims of AGS are shown to be unfounded. They stem from an inaccurate hole kinetic energy approximation. Kulakovskii and Kukushkin's finding that the corrections oppose the phase separation in Si conflicts with theory. Possible sources of the discrepancy are discussed. The effect of valence band structure on E_XC is considered.     

Effects of uniaxial stress on the cyclotron resonance in inversion layers on Si (100)

In electron inversion layers on Si (100) MOS capacitors uniaxial stress can cause simultaneous occupation of the two non-equivalent subbands E₀ and E₀'. Two distinct cyclotron resonance signals are observed and can be used to determine the effective masses and the relative occupation in the two subbands and their dependence on stress and electron density.     

Resonance photoelectron spectroscopy from autoionization states of CH3I

Resonance photoelectron spectra of methyl iodide have been taken by exciting individual autoionizing Rydberg states, and measuring the ejected electron energies by a time of flight technique. A number of spectra were obtained from autoionization states which are members of two series, converging respectively to the ²E_{$\text{1}\text{2}$} (ν = 0) and ²E_{$\text{1}\text{2}$} (ν₂ =1) states of the ion. In addition to observing vibrational bands not previously reported, we found that the peak intensities are consistent with Bardsley's formulation of the configuration interaction theory of autoionization.     

Chemisorption of NO and NH3 on cobalt: Studies by UPS,XPS, and work-function measurements

Chemisorption of NO and NH₃ on cobalt has been studied by UV and X-ray photoelectron spectroscopy (UPS and XPS) and work-function measurements. The 25°C data for NO are consistent with dissociative chemisorption below ～5 L exposure followed by molecular chemisorption at higher exposures; complete dissociation occurs after heating to 500°C. The UPS of molecularly chemisorbed NO exhibit peaks at ～2.7, 10.2 and 14 eV below E_F, corresponding to ionization of the 2π*, (1π + 5σ), and 4σ MO's. The UPS data for NH₃ exhibit peaks at 7.9 and 11.4 eV below E_F, consistent with ionization of the 3a₁ and 7e MO's of NH₃ and/or MO's of the fragments NH₂ and/or NH.     

Atomic vibrational dynamics of amorphous Fe-Mg alloy thin films

The atomic vibrational dynamics of ⁵⁷Fe in 800-Å thick amorphous Fe_xMg_1−x alloy thin films (0.3≤x≤0.7) has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Mg in ultrahigh vacuum onto a substrate held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mössbauer spectroscopy. The ⁵⁷Fe-projected partial vibrational density of states, g(E), has been obtained from the measured NRIXS vibrational excitation probability, together with thermodynamic quantities such as the probability of recoilless absorption (f-factor), the average kinetic energy per Fe atom, the average force constant, and the vibrational entropy per Fe atom. A plot of g(E)/E² versus E proves the existence of non-Debye-like vibrational excitations with a peak at E_bp∼3-5 meV (boson peak). Both the boson peak height and E_bp were found to depend linearly on the composition x. Above the boson peak, g(E)/E² exhibits an exponential decrease.     

Indirect exciton dispersion in III–V semiconductors: “Camel's back” in GaP

A new perturbation approach to exciton dispersion in indirect gap semiconductors is developed. For GaP and AlSb existence of the “camel's back” in exciton dispersion is confirmed, and a precise value of the “camel's back” parameter for X^c₁-minima in GaP is reported: E(X^c₁)?E_min(Δ^c₁)=3.5±0.3 meV. At the X-point the 21.44 and 19.48 meV exciton binding energies in GaP are obtained. The corresponding valley-anisotropy splitting is 1.96 meV.     

Resonant Raman scattering in germanium and zincblende-type semiconductors temperature dependence

We have measured the resonance in the Raman scattering near the E₁ gaps of InAs and of a Ge_0.77 Si_0.23 alloy at 77, 300 and 594°K. In contrast to the E₁ gap determined in absorption and transmission measurements, the coresponding peak in the spectral dependence of the scattering cross section shifts very little with temperature; it occurs at all temperatures very near the energy of the absorption peak measured at low temperatures (∼ 77°K).     

Exciton transitions from spin-orbit split off valence bands in layer compound InSe

The absorption spectrum of E′₁ and E₁ exciton transitions in 3R(γ)-InSe has been measured at 1.8 K. Their photon energies, oscillator strengths and polarization dependences are fully explained in terms of Hopfield's quasicubic model. The analysis shows that the valence band structure reflects the crystalline field anisotropy characteristics of its layer structure.     

Absorption spectra of germanium nanocrystals

Absorption spectra of hydrogenated germanium nanocrystals are calculated in real space using ab initio pseudopotentials constructed within the local density approximation. Nanocrystals with over 800 atoms are considered. We find the calculated spectra are essentially bulk-like for systems with ∼250 atoms or more. The energy positions for the E₁ and E₂ reflectivity peaks are blue-shifted with respect to their bulk values owing to quantum confinement. Comparisons with available experimental data reveal good agreement for the energies of the E₁ peak, whereas the energy position of the E₂ peak is underestimated by a constant shift, which is independent of the nanocrystal size. This shift is consistent with local density calculations for bulk germanium.     

Optical and intervalley scattering in quantized inversion layers in semiconductors

The momentum relaxation time for scattering of electrons in quantized levels of an inversion layer on a semiconductor surface is calculated for interactions via optical and intervalley phonons. A selection rule is found which prohibits transitions between subbands belonging to the same valley or set of valleys, at least in the zero order to which these scattering processes may occur. Relaxation times for the zero-order interaction and the first-order interaction are obtained for intervalley phonons. The results are applied to the case of a (100)-silicon surface, with electrons in the three lowest subbands (with energy levels E₀, E₁, E'₀) of the two sets of valleys. Agreement with the experimental data of Fang and Fowler is good when the combined effects of intervalley and acoustic scattering are considered.     

Effects of superstrings in collisions

Superstring theory in d = 10 dimensions after Calabi—Yau compactification yields a minimum low-energy gauge group SU(3)_C × SU(2)_L × U(1)_Y × U(1)_E. The low-energy theory includes particles with the quantum numbers of 27 representations of E₆, each of which contains an extra neutrino ν^c conventionally called a “right-handed neutrino”. The contributions of ν and ν^c to through Z⁰ and Z_E mixing is calculated. Small contributions are found of the new right-handed neutrino and of the superstring boson Z_E to σ(e⁺e⁻ → γ + nothing).     

Directional dispersion of extraordinary phonons in layer compound InSe

The resonant Raman scattering of polar optical phonons has been measured in trigonal γ-InSe at the E'₁ exciton edge. The two-phonon scattering spectrum widely spreads over the frequency range of 50 cm^-1 with an extremely asymmetric lineshape. This spectrum has been interpreted in terms of the directional dispersion of extraordinary LO and TO phonons.     

Flicker noise of hot electrons in silicon at T = 78 K

From flicker-noise and current-voltage measurements performed on an n⁺nn⁺ silicon planar device at T = 78 K we calculated Hooge's parameter α as a function of the electric field strength, E₀. We found that α(E₀) = α(0)/[1 + (E₀/E_c)²]. E_c is a critical field where the drift velocity equals the sound velocity, indicating the connection of the observed effect with acoustical phonon scattering.     

Calculations of vacancy formation energies and divacancy binding energies for noble metals using pseudopotential formalism

Vacancy formation energies (E^f_1v) and divacancy binding energies (E^B_2v) for noble metals have been calculated using the Moriarty's pseudopotential. While the calculated E^f_1v are coincident with the experimental values except Cu, E^B_2v agree with them for all the metals tolerably.