Reformulation of the LDA+U method for a local orbital basis

We present a new approach to the evaluation of the on-site repulsion energy U for use in the LDA+U method of Anisimov and collaborators. Our objectives are to make the method more firmly based, to concentrate primarily on ground state properties rather than spectra, and to test the method in cases where only modest changes in orbital occupations are expected, as well as for highly correlated materials. Because of these objectives, we employ a differential definition of U. We also define a matrix U, which we find is very dependent on the environment of the atom in question. The formulation is applied to evaluate U for transition metal monoxides from VO to NiO using a local orbital basis set. The resulting values of U are typically only 40-65% as large as values currently in use. We evaluate the U matrix for the e_g and t_{2g} subshells in paramagnetic FeO, and illustrate the very different charge response of the e_g and t_{2g} states. The sensitivity of the method to the choice of the d orbitals, and to the basis set in general, is discussed.Comment: 6 figure

Electronic Structure of Superconducting Ba6c60

We report the results of first-principles electronic-structure calculations for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound shows strong Ba-C hybridization in the valence and conduction regions, mixed covalent/ionic bonding character, partial charge transfer, and insulating zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with REVTE

Transverse waves in a post-flare supra-arcade

Observations of propagating transverse waves in an open magnetic field structure with the Transition Region And Coronal Explorer (TRACE) are presented. Waves associated with dark tadpole-like sunward moving structures in the post-flare supra-arcade of NOAA active region 9906 on the 21st of April 2002 are analysed. They are seen as quasi-periodic transverse displacements of the dark tadpole tails, with periods in the range of 90–220 s. Their phase speeds and displacement amplitudes decrease as they propagate sunwards. At heights of 90 and 60 Mm above the post-flare loop footpoints the phase speeds are in the ranges 200–700 km s −1 and 90–200 km s −1 respectively. Furthermore, for consecutive tadpoles the phase speeds decrease and periods increase as a function of time. The waves are interpreted as propagating fast magnetoacoustic kink waves guided by a vertical, evolving, open structure

Important role of alkali atoms in A4C60

We show that hopping via the alkali atoms plays an important role for the t1u band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is broadened by more than 40 % by the presence of the alkali atoms. The difference between A4C60 and A3C60 is in particular due to the less symmetric location of the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene

Fast magnetoacoustic waves in curved coronal loops. II, Tunneling modes

Aims. Fast magnetoacoustic waves in curved coronal loops are investigated and the role of lateral leakage in wave damping, which includes the mechanism of wave tunneling, is explored. Methods. A coronal loop is modeled as a curved, magnetic slab in the zero plasma-β limit. In this model and for an arbitrary piece-wise continuous power law equilibrium density profile, the wave equation governing linear vertically polarised fast magnetoacoustic waves is solved analytically. An associated dispersion relation is derived and the frequencies and eigenfunctions of the wave modes are characterised. Results. For some equilibria, the waves are shown to be all damped due to lateral leakage. It is demonstrated that waves either leak straight out into the external medium or have to overcome an evanescent barrier, which is linked to wave tunneling. The wave solutions consist of alternating vertically polarised kink and sausage branches. Fast kink oscillations may have a non-zero density perturbation when averaged across the loop. The calculated damping rate of fast magnetoacoustic kink oscillations is shown to be consistent with related numerical simulations and show that lateral leakage may explain the observed damping of (vertically polarised) fast magnetoacoustic kink oscillations