5,261 research outputs found
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
Instability of spatial patterns and its ambiguous impact on species diversity
Self-arrangement of individuals into spatial patterns often accompanies and
promotes species diversity in ecological systems. Here, we investigate pattern
formation arising from cyclic dominance of three species, operating near a
bifurcation point. In its vicinity, an Eckhaus instability occurs, leading to
convectively unstable "blurred" patterns. At the bifurcation point, stochastic
effects dominate and induce counterintuitive effects on diversity: Large
patterns, emerging for medium values of individuals' mobility, lead to rapid
species extinction, while small patterns (low mobility) promote diversity, and
high mobilities render spatial structures irrelevant. We provide a quantitative
analysis of these phenomena, employing a complex Ginzburg-Landau equation.Comment: 4 pages, 3 figures and supplementary information. To appear in Phys.
Rev. Lett
A rigorous solution concept for geodesic and geodesic deviation equations in impulsive gravitational waves
The geodesic as well as the geodesic deviation equation for impulsive
gravitational waves involve highly singular products of distributions
(\theta\de, \theta^2\de, \de^2). A solution concept for these equations
based on embedding the distributional metric into the Colombeau algebra of
generalized functions is presented. Using a universal regularization procedure
we prove existence and uniqueness results and calculate the distributional
limits of these solutions explicitly. The obtained limits are regularization
independent and display the physically expected behavior.Comment: RevTeX, 9 pages, final version (minor corrections, references added
The Dynamical Fingerprint of Core Scouring in Massive Elliptical Galaxies
The most massive elliptical galaxies have low-density centers or cores that
differ dramatically from the high-density centers of less massive ellipticals
and bulges of disk galaxies. These cores have been interpreted as the result of
mergers of supermassive black hole binaries, which depopulate galaxy centers by
gravitationally slingshotting central stars toward large radii. Such binaries
naturally form in mergers of luminous galaxies. Here, we analyze the population
of central stellar orbits in 11 massive elliptical galaxies that we observed
with the integral field spectrograph SINFONI at the European Southern
Observatory Very Large Telescope. Our dynamical analysis is orbit-based and
includes the effects of a central black hole, the mass distribution of the
stars, and a dark matter halo. We show that the use of integral field
kinematics and the inclusion of dark matter is important to conclude upon the
distribution of stellar orbits in galaxy centers. Six of our galaxies are core
galaxies. In these six galaxies, but not in the galaxies without cores, we
detect a coherent lack of stars on radial orbits in the core region and a
uniform excess of radial orbits outside of it: when scaled by the core radius,
the radial profiles of the classical anisotropy parameter beta are nearly
identical in core galaxies. Moreover, they match quantitatively the predictions
of black hole binary simulations, providing the first convincing dynamical
evidence for core scouring in the most massive elliptical galaxies.Comment: 8 pages, 3 figures, accepted by Ap
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]
Stochastic Yield Catastrophes and Robustness in Self-Assembly
A guiding principle in self-assembly is that, for high production yield,
nucleation of structures must be significantly slower than their growth.
However, details of the mechanism that impedes nucleation are broadly
considered irrelevant. Here, we analyze self-assembly into finite-sized target
structures employing mathematical modeling. We investigate two key scenarios to
delay nucleation: (i) by introducing a slow activation step for the assembling
constituents and, (ii) by decreasing the dimerization rate. These scenarios
have widely different characteristics. While the dimerization scenario exhibits
robust behavior, the activation scenario is highly sensitive to demographic
fluctuations. These demographic fluctuations ultimately disfavor growth
compared to nucleation and can suppress yield completely. The occurrence of
this stochastic yield catastrophe does not depend on model details but is
generic as soon as number fluctuations between constituents are taken into
account. On a broader perspective, our results reveal that stochasticity is an
important limiting factor for self-assembly and that the specific
implementation of the nucleation process plays a significant role in
determining the yield
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
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