G. A. Baraff

Superconductivity in alkali intercalated C60

Abstract Superconductivity observed in alkali intercalated C60 solid can be explained on the basis of conventional BCS theory. Intra-molecular Jahn-Teller type vibrations with high frequencies couple to conduction electrons in C60π -orbitals with strength V. The density of these states (N) is determined by the relatively weak intermolecular coupling. This results in a real space factorization of the coupling parameter λ = NV which has several experimental consequences. We present detailed calculations that lead to this picture and compare with existing experiments.

Hydrogen chemisorption on the 100 (2 × 1) surfaces of Si and Ge

Abstract This paper combines a theoretical study of the Si(100) surface having a monolayer of atomic hydrogen chemisorbed to it with an experimental study of the analogous Ge(100) and Ge(110) surfaces. In the theoretical work the underlying (100) silicon surface is taken to be reconstructed according to the Schlier-Farnsworth-Levine pairing model with the hydrogen located on the unfilled tetrahedral bonds of this structure. Self-consistent calculations of the electronic potential, charge density, spectrum, and occupied surface density of states are carried out. The force on the hydrogen atoms is then calculated using the Hellman-Feynman theorem. This force is found to be close to zero, confirming that the hydrogen atoms are indeed at the equilibrium position for the chosen silicon geometry. Features in the calculated photoemission spectrum for the Si(100) 2 × 1 : H surface are discussed in terms of related features in the photoemission spectrum of Si(111) : H, but are found not to agree with the previously measured photoemission spectrum of Si(100) 2 × 1 : H. Measured photoemission and ion-neutralization spectra for Ge(100) 2 × 1 : H agree in their major features with what is calculated for Si(100) 2 × 1 : H, however, suggesting that the Ge(100) 2 × 1 : H surface is reconstricted according to the pairing model. Similarly, measured spectra for clean Ge(100) 2 × 1 agree with calculations for the row dimerized Si(100) surface.

Electronic structure at an abrupt GaAs–Ge interface

The potential, charge density, and interface states have been calculated for the ideal interface between intrinsic GaAs, terminated on a (100) Ga plane, and intrinsic Ge. The conduction band is found to be nearly continuous across the interface and only a small interface dipole moment is found. Even though the two crystals, Ge and GaAs, have the same crystal structure and lattice parameters (a situation which might be thought to produce an interface containing no states in the forbidden gap at the Fermi level), a simple quantum‐mechanical counting argument shows that there must be interface states at the Fermi energy for the unreconstructed interface. These states are the band‐picture equivalent of unsaturated bonds. We find that fractional occupancy of the interface bonds (each of which contains 1.75 rather than two electrons) arises via a single partially occupied band of interface states. Consideration of the electronic energy suggests that the actual interface will be reconstructed.

Superconductivity in alkali-intercalated C60

Abstract Superconductivity observed in alkali-intercalated, solid C60 can be explained on the basis of conventional Bardeen-Cooper-Schrieffer theory. Intramolecular Jahn-Teller-type vibrations with high frequencies couple to conduction electrons in C60 π orbitals with strength V . The density of these states ( N ) is determined by the relatively weak intermolecular coupling. This results in a real space factorization of the coupling parameter λ = NV , which has several experimental consequences. We present detailed calculations that lead to this picture and compare these with existing experiments.

Modeling of gain for InGaAsP-based lasers

Experimental and theoretical results for gain in bulk and multiquantum well active layer 1.3 micrometers InGaAsP based lasers are reported. Gain, loss, transparency energy, and carrier density have been measured in the subthreshold regime at room temperature and elevated temperatures. Gain has been calculated using an eight band k(DOT)p model for the electronic structure and a conventional density matrix formulation. The calculated and experimental results for the gain spectra, the gain versus density, the chemical potential (quasifermi level separation) versus density, and the gain versus chemical potential are compared at room temperature and 85 C. There is aagreement at several points, but the model substantially underestimates the temperature sensitivity of the gain which has been found in the experiments to be an important factor in the overall temperature sensitivity of threshold current.

Theoretical study of the GaAs (100) surface

We present the results of self‐consistent calculations of the potential, wave functions, spectrum, and charge density of the Ga (100) surface of GaAs. The spectrum contains two bands of gap surface staes whose spatial form is similar to those of (100) silicon. In contrast to Si, these bands are separated by a 1/2‐V energy gap, a gap which is caused by the heteopolar potential. The lower energy band is 3/4 full, a situation which is predicted by a simple quantum‐mechanical counting argument and which leads to a conclusion that back bonds are very similar to bulk bonds. Its Fermi surface lies in a region of high state density and has an almost perfect diamond shape. By studying the change in ionization potential with position of the surface plane, we have calculated an effective charge of 0.2 electrons for surface Ga atoms.

Modeling of optical spectra for characterization of multiquantum well InGaAsP-based lasers

The features observed in luminescence and photoreflectance spectra are interpreted by detailed modeling of the electronic states, absorption and luminescence of the active region. The electronic states for the full active region (quantum wells together with separate confinement layers) are calculated using an eight band k (DOT) p model. The axial approximation, tested to be sufficiently accurate, is used to reduce the computational burden. The Poisson equation is included self consistently for the optically pumped case. Analysis of the photoreflectance spectra includes incorporation of an electric field across the active region. Good agreement for the positions of the features and their trends with compositional variables verifies the accuracy of the model. Higher lying transitions involve electron levels above the barrier energy which can be confined to the region of the wells by the self consistent field for pumped material.

The effective g-factor of holes in bismuth

Measurements have been made of the Shubnikov-de Haas effect in bismuth at 1.5°K and fields up to 88 kG. The effect consists of oscillations in the resistance as a function of magnetic field. The oscillations result from the quantization of the transverse energy of the carriers in a magnetic field. From the positions of the oscillations, one can obtain information concerning the band structure. In the present investigation, we have obtained values of the g-factors for holes in bismuth and have determined the variation of Fermi energy as a function of magnetic field. First an outline of the theory will be presented and then the results.