10,496 research outputs found
Global Structure of Exact Scalar Hairy Dynamical Black Holes
We study the global structure of some exact scalar hairy dynamical black
holes which were constructed in Einstein gravity either minimally or
non-minimally coupled to a scalar field. We find that both the apparent horizon
and the local event horizon (measured in luminosity coordinate) monotonically
increase with the advanced time as well as the Vaidya mass. At late advanced
times, the apparent horizon approaches the event horizon and gradually becomes
future outer. Correspondingly, the space-time arrives at stationary black hole
states with the relaxation time inversely proportional to the power
of the final black hole mass, where is the space-time dimension. These
results strongly support the solutions describing the formation of black holes
with scalar hair. We also obtain new charged dynamical solutions in the
non-minimal theory by introducing an Maxwell field which is non-minimally
coupled to the scalar.
The presence of the electric charge strongly modifies the dynamical evolution
of the space-time.Comment: 18 pages and 6 figures; minor corrections, publised versio
Criticality in Einstein-Gauss-Bonnet Gravity: Gravity without Graviton
General Einstein-Gauss-Bonnet gravity with a cosmological constant allows two
(A)dS spacetimes as its vacuum solutions. We find a critical point in the
parameter space where the two (A)dS spacetimes coalesce into one and the
linearized perturbations lack any bilinear kinetic terms. The vacuum
perturbations hence loose their interpretation as linear graviton modes at the
critical point. Nevertheless, the critical theory admits black hole solutions
due to the nonlinear effect. We also consider Einstein gravity extended with
general quadratic curvature invariants and obtain critical points where the
theory has no bilinear kinetic terms for either the scalar trace mode or the
transverse modes. Such critical phenomena are expected to occur frequently in
general higher derivative gravities.Comment: 21 pages, no figures;refereces adde
Quantum Game with Restricted Matrix Strategies
We study a quantum game played by two players with restricted multiple
strategies. It is found that in this restricted quantum game Nash equilibrium
does not always exist when the initial state is entangled. At the same time, we
find that when Nash equilibrium exists the pay off function is usually
different from that in the classical counterpart except in some special cases.
This presents an explicit example where quantum game and classical game may
differ. When designing a quantum game with limited strategies, the allowed
strategy should be carefully chosen according to the type of initial state.Comment: 5 pages and 3 figure
Boron Nitride Nanosheets for Metal Protection
Although the high impermeability of graphene makes it an excellent barrier to
inhibit metal oxidation and corrosion, graphene can form a galvanic cell with
the underlying metal that promotes corrosion of the metal in the long term.
Boron nitride (BN) nanosheets which have a similar impermeability could be a
better choice as protective barrier, because they are more thermally and
chemically stable than graphene and, more importantly, do not cause galvanic
corrosion due to their electrical insulation. In this study, the performance of
commercially available BN nanosheets grown by chemical vapor deposition as a
protective coating on metal has been investigated. The heating of the copper
foil covered with the BN nanosheet at 250 {\deg}C in air over 100 h results in
dramatically less oxidation than the bare copper foil heated for 2 h under the
same conditions. The electrochemical analyses reveal that the BN nanosheet
coating can increase open circuit potential and possibly reduce oxidation of
the underlying copper foil in sodium chloride solution. These results indicate
that BN nanosheets are a good candidate for oxidation and corrosion protection,
although conductive atomic force microscopy analyses show that the
effectiveness of the protection relies on the quality of BN nanosheets.Comment: With Supporting Informatio
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