Crystal Structures and Electronic Properties of Haloform-Intercalated C60

Using density functional methods we calculated structural and electronic properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3 (X=Cl,Br). Both compounds are narrow band insulator materials with a gap between valence and conduction bands larger than 1 eV. The calculated widths of the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively. The orbitals of the haloform molecules overlap with the $\pi$ orbitals of the fullerene molecules and the p-type orbitals of halogen atoms significantly contribute to the valence and conduction bands of C60 2CHX3. Charging with electrons and holes turns the systems to metals. Contrary to expectation, 10 to 20 % of the charge is on the haloform molecules and is thus not completely localized on the fullerene molecules. Calculations on different crystal structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at the Fermi energy are sensitive to the orientation of the haloform and C60 molecules. At a charging of three holes, which corresponds to the superconducting phase of pure C60 and C60 2CHX3, the calculated density of states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60 2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table

Use of the Generalized Gradient Approximation in Pseudopotential Calculations of Solids

We present a study of the equilibrium properties of $sp$-bonded solids within the pseudopotential approach, employing recently proposed generalized gradient approximation (GGA) exchange correlation functionals. We analyze the effects of the gradient corrections on the behavior of the pseudopotentials and discuss possible approaches for constructing pseudopotentials self-consistently in the context of gradient corrected functionals. The calculated equilibrium properties of solids using the GGA functionals are compared to the ones obtained through the local density approximation (LDA) and to experimental data. A significant improvement over the LDA results is achieved with the use of the GGA functionals for cohesive energies. For the lattice constant, the same accuracy as in LDA can be obtained when the nonlinear coupling between core and valence electrons introduced by the exchange correlation functionals is properly taken into account. However, GGA functionals give bulk moduli that are too small compared to experiment.Comment: 15 pages, latex, no figure

Reconstructions of surface ocean conditions from the North East Atlantic and Nordic Seas during the last millennium

We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the �Medieval Climate Anomaly� warming was most pronounced before AD 1200, with a long-term cooling trend apparent after AD 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the �Medieval Climate Anomaly�. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions