107 research outputs found
Cop and robber game and hyperbolicity
In this note, we prove that all cop-win graphs G in the game in which the
robber and the cop move at different speeds s and s' with s'<s, are
\delta-hyperbolic with \delta=O(s^2). We also show that the dependency between
\delta and s is linear if s-s'=\Omega(s) and G obeys a slightly stronger
condition. This solves an open question from the paper (J. Chalopin et al., Cop
and robber games when the robber can hide and ride, SIAM J. Discr. Math. 25
(2011) 333-359). Since any \delta-hyperbolic graph is cop-win for s=2r and
s'=r+2\delta for any r>0, this establishes a new - game-theoretical -
characterization of Gromov hyperbolicity. We also show that for weakly modular
graphs the dependency between \delta and s is linear for any s'<s. Using these
results, we describe a simple constant-factor approximation of the
hyperbolicity \delta of a graph on n vertices in O(n^2) time when the graph is
given by its distance-matrix
Cosmological Simulations of Normal-Branch Braneworld Gravity
We introduce a cosmological model based on the normal branch of DGP
braneworld gravity with a smooth dark energy component on the brane. The
expansion history in this model is identical to LambdaCDM, thus evading all
geometric constraints on the DGP cross-over scale r_c. This model can serve as
a first approximation to more general braneworld models whose cosmological
solutions have not been obtained yet. We study the formation of large scale
structure in this model in the linear and non-linear regime using N-body
simulations for different values of r_c. The simulations use the code presented
in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the
brane-bending mode in conjunction with the usual gravitational dynamics. The
brane-bending mode is attractive rather than repulsive in the DGP normal
branch, hence the sign of the modified gravity effects is reversed compared to
those presented in arXiv:0905.0858. We compare the simulation results with
those of ordinary LambdaCDM simulations run using the same code and initial
conditions. We find that the matter power spectrum in this model shows a
characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the
abundance of massive halos is significantly enhanced. Other results presented
here include the density profiles of dark matter halos, and signatures of the
brane-bending mode self-interactions (Vainshtein mechanism) in the simulations.
Independently of the expansion history, these results can be used to place
constraints on the DGP model and future generalizations through their effects
on the growth of cosmological structure.Comment: 17 pages, 10 figures; v2: minor changes; v3: references added; v4:
added appendix on comparison with previous results; matches published
version; v5: corrected Eqs. (2.4-2.5) and Fig. 1 following Ref. [28]; all
following results unchange
Giant Thermomechanical Bandgap Engineering in Quasi-two-dimensional Tellurium
Mechanical straining-induced bandgap modulation in two-dimensional (2D)
materials has been confined to volatile and narrow modulation due to substrate
slippage and poor strain transfer. We report the thermomechanical modulation of
the inherent bandgap in quasi-2D tellurium nanoflakes (TeNFs) via non-volatile
strain induction during hot-press synthesis. We leveraged the coefficient of
thermal expansion (CTE) mismatch between TeNFs and growth substrates by
maintaining a high-pressure enforced non-slip condition during thermal
relaxation (623 to 300K) to achieve the optimal biaxial compressive strain of
-4.6 percent in TeNFs/sapphire. This resulted in an enormous bandgap modulation
of 2.3 eV, at a rate of up to ~600 meV/%, which is two-fold larger than
reported modulation rate. Strained TeNFs display robust band-to-band radiative
excitonic blue photoemission with an intrinsic quantum efficiency (IQE) of c.a.
79.9%, making it promising for energy efficient blue LEDs and nanolasers.
Computational studies reveal that biaxial compressive strain inhibits
exciton-exciton annihilation by evading van-Hove singularities, hence promoting
radiative-recombination. Bandgap modulation by such nonvolatile straining is
scalable to other 2D semiconductors for on-demand nano(opto)-electronics
Relative Locations
The fact that physical laws often admit certain kinds of space-time symmetries is often thought to be problematic for substantivalism --- the view that space-time is as real as the objects it contains. The most prominent alternative, relationism, avoids these problems but at the cost of giving abstract objects (rather than space-time points) a pivotal role in the fundamental metaphysics. This incurs related problems concerning the relation of the physical to the mathematical. In this paper I will present a version of substantivalism that respects Leibnizian theses about space-time symmetries, and argue that it is superior to both relationism and the more orthodox form of substantivalism
Triple crossing positivity bounds, mass dependence and cosmological scalars: Horndeski theory and DHOST
Scalars are widely used in cosmology to model novel phenomena such as the
late-time cosmic acceleration. These are effective field theories with highly
nonlinear interactions, including Horndeski theory/generalized galileon and
beyond. We use the latest fully crossing symmetric positivity bounds to
constrain these cosmological EFTs. These positivity bounds, based on
fundamental principles of quantum field theory such as causality and unitarity,
are able to constrain the EFT coefficients both from above and below. We first
map the mass dependence of the fully crossing symmetric bounds, and find that a
nonzero mass generically enlarges the positivity regions. We show that
fine-tunings in the EFT construction can significantly reduce the viable
regions and sometimes can be precarious. Then, we apply the positivity bounds
to several models in the Horndeski class and beyond, explicitly listing the
ready-to-use bounds with the model parameters, and discuss the implications for
these models. The new positivity bounds are found to severely constrain some of
these models, in which positivity requires the mass to be parametrically close
to the cutoff of the EFT, effectively ruling them out. The examples include
massive galileon, the original beyond Horndeski model, and DHOST theory with
unity speed of gravity and nearly constant Newton's coupling.Comment: 45 pages, 16 figures, 1 table. To appear in JCA
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