M. Schlüter

Ring Currents in Icosahedral C60

A new formulation of the current within the London approximation allows the calculation of ring currents in topologically complex molecules. Application of this theory to C60 demonstrates the existence of remarkable π electron ring currents. Paramagnetic currents, in size comparable to the ones in benzene, flow within the pentagons, whereas weaker diamagnetic currents flow all around the C.60 molecule. The overall vanishing ring-current magnetic susceptibility results from a cancellation of diamagnetic and paramagnetic contributions. The presence of ring currents significantly affects chemical shifts as measured in nuclear magnetic resonance experiments. In contrast to the magnetic susceptibility, which is a property of the molecule as a whole, chemical shifts are sensitive to the local magnetic field and the effect of ring currents does not vanish.

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.

Chemical trends in metal-semiconductor barrier heights

Experimental data on metal-semiconductor interfaces are reexamined. It is found that the previously reported abrupt transition between covalent and ionic semiconductors is not that clearly defined and the outcome is diffused by data scattering. Furthermore, the data indicate nosaturation of the interface parameter Sfor S= 1. Considering the definition of S, it follows that the true Schottky limit should occur for some number S≈: 2.0-3.0 rather than for exactly S= 1 as previously claimed.

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.

Calculation of the many body interaction parameters in the highTc compound La2CuO4

Abstract The energy surface as a function of local Cu d-charge and in plane O p-charge fluctuation is calculated using the constrained local density functional approach. This is done in a supercell geometry using the LMTO-ASA method. The energy surface is then mapped onto a self consistent mean field solution of the two-band Hubbard model thus obtaining the on-site Cu and O Coulomb interaction ( U d , U p ) as well as the intersite interaction ( U pd ). Implications of the calculated values of U i on various pairing models are briefly discussed.

Gase Valence Band-Structure from Angle-Resolved Photoemission Spectroscopy

Abstract Energy-wavevector dispersion curves for the GaSe valence bands obtained from angle-resolved photoemission spectra are compared with pseudopotential band calculations. It is found that the third density-of-states feature below the top of the valence bands (peak C) is related to the Ga-Se bond rather than the Ga-Ga bond. A peak not appearing in the angle-integrated spectra appears as the most intense feature at normal emission, and helps to identify completely the nature of the valence bands down to 7–8 eV below the Fermi energy.

Density-Functional Theory of the Band Gap

Publisher Summary This chapter discusses the density-functional theory (DFT) of the band gap. Emphasizing the dependence of the density functionals on the number of particles brings out the difference between the highest occupied energy of the (N +1)-particle system and the (N + 1) th level of the N-particle system. The difference is easily shown to be the discontinuity Δ in the DFT potential when the number of particles is increased by one. The gap is then composed of two terms, the DFT gap, defined from the eigenvalues of the N-particle DFT equation and the discontinuity Δ. The discrepancy between the (local density approximation) LDA gap and the measured gap has two components: the improvement of the N-particle DFT energy values over LDA and the discontinuity. The relative importance of these two contributions is of central interest. These questions were studied by finding field-theoretic expressions for the exact exchange-correlation potential and for the discontinuity and by evaluating these expressions for a two-plane wave model for an insulator both in the exchange-only approximation and including correlation. The exchange-correlation potential and the discontinuity Δ for semiconductors and insulators from the self-energy operator within the random phase approximation were calculated.

Conduction and Valence Band Density of States of Sns2 - Theory and Experiment

Abstract The band structure of SnS2 has been investigated over a wide energy range by pseudopotential band structure calculations and synchrotron radiation photoemission spectroscopy techniques. A good correspondence has been found between energy positions of the theoretical density of states features and structure in the constant initial state (CIS) and energy distribution curves (EDCs) for the conduction and the valence bands respectively. In the energy region between — 8 eV and 15 eV from the top of the valence band we observe four valence band and six conduction band peaks.

Normal photoemission demonstration of two and three dimensionality of electron states in layer compounds

Abstract Energy spectra have been measured for photoelectrons emitted normal to the surfaces of GaSe and InSe. Some of the peaks display considerable energy dispersion ∼ 1 eV as a function of photon energy indicating that the corresponding bands are three-dimensional. Two-dimensional bands with small energy dispersion (⩽ 0.3 eV) are also observed. These features are related to the extent of Se p z -character in calculated pseudopotential wave-functions.

The electronic structure of La2CuO4: Renormalization from density functional theory to strong coupling models

Abstract The electronic structure of La2CuO4 is analyzed in terms of the Density Functional (DFT) band picture. The DFT results are mapped onto a generalized multi-band Hubbard model and numerical values for the complete set of parameters are extracted. A variety of experimebts probing the high energy (1–10 eV) scale are well described. In a subsequent mapping process based on cluster studies, several low energy (≲ 1 eV) model Hamiltonians are tested and their parameters are evaluated. The insulating phase is quantitatively described by a Heisenberg model with excitation energies in good agreement with experiment. Spectra for added electrons or holes are well described by symmetric one-band models which could be the basis for a description of the superconducting state.