No abstract

We report a combined experimental and theoretical study of the unoccupied electronic states of the neutral molecular organic materials TTF ͑tetrathiafulvalene͒ and TCNQ ͑7,7,8,8-tetracyano-p-quinodimethane͒ and of the one-dimensional metallic charge transfer salt TTF-TCNQ. The experimental density of states ͑DOS͒ is obtained by x-ray absorption near edge spectroscopy ͑XANES͒ with synchrotron light and the predicted DOS by means of first-principles density functional theory calculations. Most of the experimentally derived elementspecific XANES features can be associated to molecular orbitals of defined symmetry. Because of the planar geometry of the TTF and TCNQ molecules and the polarization of the synchrotron light, the energy dependent or character of the orbitals can be inferred from angular dependent XANES measurements. The present work represents the state of the art analysis of the XANES spectra of this type of materials and points out the need for additional work in order to elucidate the governing selection rules in the excitation process.

In this study we use ab initio calculations and a pure silicon tip to study the tip-surface interaction with four characteristic insulating surfaces: ͑i͒ the narrow gap TiO 2 ͑110͒ surface, ͑ii͒ the classic oxide MgO ͑001͒ surface, ͑iii͒ the ionic solid CaCO 3 (101 4) surface with molecular anion, and ͑iv͒ the wide gap CaF 2 ͑111͒ surface. Generally we find that the tip-surface interaction strongly depends on the surface electronic structure due to the dominance of covalent bond formation with the silicon tip. However, we also find that in every case the strongest interaction is with the highest anion of the surface. This result suggests that, if the original silicon tip can be carefully controlled, it should be possible to immediately identify the species seen as bright in images of insulating surfaces. In order to provide a more complete picture we also compare these results to those for contaminated tips and suggest how applied voltage could also be used to probe chemical identity.

The electronic structure of the molecular superconductor ␤-(BEDT-TTF) 2 IBr 2 has been studied by means of first-principles density functional calculations. The calculated transverse cross section of the Fermi surface is in excellent agreement with that reconstructed from magnetoresistance measurements. It is shown that the cylindrical Fermi surface exhibits warping ͑the dispersion along the interlayer direction is of the order of 0.8 -1.7 % of the dispersion in the conducting plane͒ and that it does not contain any additional small pocket. These features provide support for a recent proposal concerning the much debated question of the origin of the slow magnetoresistance oscillations exhibited by this material.

The local vibrational modes ͑LVM's͒ of the oxygen chains assigned to thermal double donors ͑TDD's͒ and other related oxygen defects in silicon are studied using accurate total-energy calculations. We find that the calculated LVM frequencies as well as their isotopic shifts and charge-state dependences ͑temperature dependences͒ for the oxygen chains agree closely with the corresponding experimental quantities, which supports our assignments of the O 2i -O 2r chain to TDD1 and the O i -O nr -O i chains to TDDn (nϾ1) (O i is an interstitial oxygen and O r a threefold coordinated oxygen belonging to a ring͒.