3,962 research outputs found
Edge Effects in Finite Elongated Graphene Nanoribbons
We analyze the relevance of finite-size effects to the electronic structure
of long graphene nanoribbons using a divide and conquer density functional
approach. We find that for hydrogen terminated graphene nanoribbons most of the
physical features appearing in the density of states of an infinite graphene
nanoribbon are recovered at a length of 40 nm. Nevertheless, even for the
longest systems considered (72 nm long) pronounced edge effects appear in the
vicinity of the Fermi energy. The weight of these edge states scales inversely
with the length of the ribbon and they are expected to become negligible only
at ribbons lengths of the order of micrometers. Our results indicate that
careful consideration of finite-size and edge effects should be applied when
designing new nanoelectronic devices based on graphene nanoribbons. These
conclusions are expected to hold for other one-dimensional systems such as
carbon nanotubes, conducting polymers, and DNA molecules.Comment: 4 pages, 4 figure
Theory of Phase Transition in the Evolutionary Minority Game
We discover the mechanism for the transition from self-segregation (into
opposing groups) to clustering (towards cautious behaviors) in the evolutionary
minority game (EMG). The mechanism is illustrated with a statistical mechanics
analysis of a simplified EMG involving three groups of agents: two groups of
opposing agents and one group of cautious agents. Two key factors affect the
population distribution of the agents. One is the market impact (the
self-interaction), which has been identified previously. The other is the
market inefficiency due to the short-time imbalance in the number of agents
using opposite strategies. Large market impact favors "extreme" players who
choose fixed strategies, while large market inefficiency favors cautious
players. The phase transition depends on the number of agents (), the
reward-to-fine ratio (), as well as the wealth reduction threshold () for
switching strategy. When the rate for switching strategy is large, there is
strong clustering of cautious agents. On the other hand, when is small, the
market impact becomes large, and the extreme behavior is favored.Comment: 5 pages and 3 figure
Late-time evolution of a self-interacting scalar field in the spacetime of dilaton black hole
We investigate the late-time tails of self-interacting (massive) scalar
fields in the spacetime of dilaton black hole. Following the no hair theorem we
examine the mechanism by which self-interacting scalar hair decay. We revealed
that the intermediate asymptotic behavior of the considered field perturbations
is dominated by an oscillatory inverse power-law decaying tail. The numerical
simulations showed that at the very late-time massive self-interacting scalar
hair decayed slower than any power law.Comment: 8 pages, 4 figures, to appear in Phys. Rev.
Late-Time Evolution of Charged Gravitational Collapse and Decay of Charged Scalar Hair - II
We study analytically the initial value problem for a charged massless
scalar-field on a Reissner-Nordstr\"om spacetime. Using the technique of
spectral decomposition we extend recent results on this problem. Following the
no-hair theorem we reveal the dynamical physical mechanism by which the charged
hair is radiated away. We show that the charged perturbations decay according
to an inverse power-law behaviour at future timelike infinity and along future
null infinity. Along the future outer horizon we find an oscillatory inverse
power-law relaxation of the charged fields. We find that a charged black hole
becomes ``bald'' slower than a neutral one, due to the existence of charged
perturbations. Our results are also important to the study of mass-inflation
and the stability of Cauchy horizons during a dynamical gravitational collapse
of charged matter in which a charged black-hole is formed.Comment: Latex 15 pages, Revtex.st
Enhanced Half-Metallicity in Edge-Oxidized Zigzag Graphene Nanoribbons
We present a novel comprehensive first-principles theoretical study of the
electronic properties and relative stabilities of edge-oxidized zigzag graphene
nanoribbons. The oxidation schemes considered include hydroxyl, carboxyl,
ether, and ketone groups. Using screened exchange density functional theory, we
show that these oxidized ribbons are more stable than hydrogen-terminated
nanoribbons except for the case of the etheric groups. The stable oxidized
configurations maintain a spin-polarized ground state with antiferromagnetic
ordering localized at the edges, similar to the fully hydrogenated
counterparts. More important, edge oxidation is found to lower the onset
electric field required to induce half-metallic behavior and extend the overall
field range at which the systems remain half-metallic. Once the half-metallic
state is reached, further increase of the external electric field intensity
produces a rapid decrease in the spin magnetization up to a point where the
magnetization is quenched completely. Finally, we find that oxygen containing
edge groups have a minor effect on the energy difference between the
antiferromagnetic ground state and the above-lying ferromagnetic state.Comment: 5 pages,5 figures, 1 tabl
Asymptotic tails of massive scalar fields in a stationary axisymmetric EMDA black hole geometry
The late-time tail behavior of massive scalar fields is studied analytically
in a stationary axisymmetric EMDA black hole geometry. It is shown that the
asymptotic behavior of massive perturbations is dominated by the oscillatory
inverse power-law decaying tail at the intermediate
late times, and by the asymptotic tail at asymptotically
late times. Our result seems to suggest that the intermediate tails and the asymptotically tails
may be quite general features for evolution of massive scalar fields in any
four dimensional asymptotically flat rotating black hole backgrounds.Comment: 6 page
Mode-coupling in rotating gravitational collapse: Gravitational and electromagnetic perturbations
We consider the late-time evolution of {\it gravitational} and
electromagnetic perturbations in realistic {\it rotating} Kerr spacetimes. We
give a detailed analysis of the mode-coupling phenomena in rotating
gravitational collapse. A consequence of this phenomena is that the late-time
tail is dominated by modes which, in general, may have an angular distribution
different from the original one. In addition, we show that different types of
fields have {\it different} decaying rates. This result turns over the
traditional belief (which has been widely accepted during the last three
decades) that the late-time tail of gravitational collapse is universal.Comment: 16 page
Near-Extreme Black Holes and the Universal Relaxation Bound
A fundamental bound on the relaxation time \tau of a perturbed
thermodynamical system has recently been derived, \tau \geq \hbar/\pi T, where
is the system's temperature. We demonstrate analytically that black holes
saturate this bound in the extremal limit and for large values of the azimuthal
number m of the perturbation field.Comment: 2 Pages. Submitted to PRD on 5/12/200
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