Benjamin Segall

Band-structure analysis of the conduction-band mass anisotropy in 6H and 4H SiC.

The band structures of 6H SiC and 4H SiC calculated by means of the full-potential linear-muffin-tin-orbital method are used to determine the effective mass tensors for their conduction-band minima. The results are shown to be consistent with recent optically detected cyclotron resonance measurements, which find the ratio of cyclotron masses for B\ensuremath{\perp}c to B\ensuremath{\parallel}c to be larger (smaller) than unity for the 6H (4H) polytype. However, contrary to previous suggestions, appreciable anisotropies in the c plane are found. For 6H SiC, a strong dependency on band filling is predicted because of the occurrence of a double-well minimum along the ML axis. The calculated mass tensors for 3C and 2H are also reported.

Electronic structure and total energy of diamond/BeO interfaces

Electronic structure calculations are used to study the bonding at diamond/BeO interfaces. The {110} interface between zinc blende BeO and diamond is used as a representative model for general reconstructed interfaces characterized by an equal amount of Be–C and O–C bonds. The interface energy is calculated to be 2 J/m 2 and combined with the estimated free surface energies to obtain an estimate of the adhesion energy. It is found to be close to the adhesion of BeO to itself, but somewhat lower than that of diamond to itself. The effects of the 7% lattice mismatch on the total energy and the band structure for a biaxially strained pseudomorphic diamond film are investigated. The effect of misfit dislocations, expected to occur for thicker films, on the adhesion energy is estimated to be lower than 10%. The bulk properties, such as equilibrium lattice constant, bulk modulus, cohesive energy, and band gap of BeO are shown to be in good agreement with experimental values and previous calculations. The valence-band offset is calculated to be 3.9 eV and found to take up most of the large band gap discontinuity. The nature of the bonding is discussed in terms of the local densities of states near the interface. The interface localized features are identified in terms of Be–C and O–C bonding and antibonding states.

Calculations with “Non-Muffin Tin” Potentials by the Green’s Function Method

A discussion is given of a procedure within the framework of the Green’s function (or KKR) method for calculating the perturbations of the energy bands due to deviations from a “muffin-tin” potential. The essential aspect of the approach is the determination of accurate wave functions in practical forms in the regions inside and outside the “muffin-tin” spheres. The forms of the perturbing potential in these regions are also discussed. This procedure was suggested by F. S. Ham and the author who applied it (in low order perturbation theory) to the case of the Mathieu potential.

Phase Shift Parametrization: Band Structure of Silver

We discuss a band parametrization scheme (within the KKR framework) specifying the phase shifts n0, n1, and n2 as functions of energy. Such an approach is particularly useful for the noble and transition metals where both d-band and free-electron-like effects are important. The nl(E) for a family of elements are expected to have characteristic energy dependences, with each nl(E) being specified over a substantial energy range by a few parameters. First, we show the existence of such characteristic behavior for the noble metals. We then use our phase shift parametrization scheme in a semi-empirical way to find the band structure of Ag. To do this, we use a first principles calculation as a guide, and adjust the parameters specifying the nl(E) to fit available Fermi surface, optical and photoemission data.

We Met at the Institute in Copenhagen

Shortly after arriving at the Institute for Theoretical Physics in Copenhagen (now known as the Niels Bohr Institute) in January 1952 at the start of a postdoctoral fellowship, I met Walter Kohn. He had already been there for about a half a year. The combination of my being abroad for the first time and being at that institution that was so renowned in the world of physics made me feel a bit overwhelmed. In his usual open, friendly and generous manner, Walter went out of his way to make me feel at home at the Institute, and ultimately to help make my stay there more profitable and enjoyable. In addition to talking to me about physics and other matters, he invited me to his home on several occasions. Considering that he and his wife were also away from home and that they had a young and growing family, the invitations were most generous. And, the warmth of the Kohn’s made the visits memorable.

Electronic Structure and Derived Linear and Nonlinear Optical Properties of Chalcopyrites

The electronic band structures were calculated for a number of chalcopyrites in both the II-IV-V 2 and I-III-VI 2 families using the linear muffin-tin orbital method. From these band structures, the second harmonic generation coefficients were calculated using a recently developed methodology in which a separation is made of inter- and intraband contributions. We found that the high value of d 36 in CdGeAs 2 is in large part due to the fact, that in this material, unlike in the other chalcopyrites, almost no compensation occurs between inter- and intraband contributions, the former one being unusually small. For the case of ZnGeP 2 , a detailed investigation of the band structure, reveals that it has an indirect band gap rather than a pseudodirect one. The implications of this for the interpretation of the optical spectra are discussed. Finally, for the I-III-VI 2 materials, we find that the Te based materials have far higher d 36 than the selenides. Combined with their potential for non-critical phase matching, this makes AgGaTe 2 an interesting compound.

Characterization of energy bands in metals by scattering phase shifts

Abstract We discuss an electron energy band parameterization scheme specifying the phase shifts η 0 , η 1 and η 2 as functions of energy. The use of this scheme has been investigated for Cu, Ag and Al.