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

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

Dynamics of a Semiflexible Polymer or Polymer Ring in Shear Flow

Polymers exposed to shear flow exhibit a rich tumbling dynamics. While rigid rods rotate on Jeffery orbits, flexible polymers stretch and coil up during tumbling. Theoretical results show that in both of these asymptotic regimes the tumbling frequency f_c in a linear shear flow of strength \gamma scales as a power law Wi^(2/3) in the Weissenberg number Wi=\gamma \tau, where \tau is a characteristic time of the polymer's relaxational dynamics. For flexible polymers these theoretical results are well confirmed by experimental single molecule studies. However, for the intermediate semiflexible regime the situation is less clear. Here we perform extensive Brownian dynamics simulations to explore the tumbling dynamics of semiflexible polymers over a broad range of shear strength and the polymer's persistence length l_p. We find that the Weissenberg number alone does not suffice to fully characterize the tumbling dynamics, and the classical scaling law breaks down. Instead, both the polymer's stiffness and the shear rate are relevant control parameters. Based on our Brownian dynamics simulations we postulate that in the parameter range most relevant for cytoskeletal filaments there is a distinct scaling behavior with f_c \tau*=Wi^(3/4) f_c (x) with Wi=\gamma \tau* and the scaling variable x=(l_p/L)(Wi)^(-1/3); here \tau* is the time the polymer's center of mass requires to diffuse its own contour length L. Comparing these results with experimental data on F-actin we find that the Wi^(3/4) scaling law agrees quantitatively significantly better with the data than the classical Wi^(2/3) law. Finally, we extend our results to single ring polymers in shear flow, and find similar results as for linear polymers with slightly different power laws.Comment: 17 pages, 14 figure

Three-dimensional electronic instabilities in polymerized solid A1C60

The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of polymerized C60 chains, with magnetic properties that suggest a non-metallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual three-dimensional antiferromagnetically ordered structure with a semi-metallic electronic spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from [email protected]