16,725 research outputs found
High efficiency dark-to-bright exciton conversion in carbon nanotubes
We report that dark excitons can have a large contribution to the emission
intensity in carbon nanotubes due to an efficient exciton conversion from a
dark state to a bright state. Time-resolved photoluminescence measurements are
used to investigate decay dynamics and diffusion properties of excitons, and we
obtain intrinsic lifetimes and diffusion lengths of bright excitons as well as
diffusion coefficients for both bright and dark excitons. We find that the
dark-to-bright transition rates can be considerably high, and that more than
half of the dark excitons can be transformed into the bright excitons. The
state transition rates have a large chirality dependence with a family pattern,
and the conversion efficiency is found to be significantly enhanced by adsorbed
air molecules on the surface of the nanotubes. Our findings show the nontrivial
significance of the dark excitons on the emission kinetics in low dimensional
materials, and demonstrate the potential for engineering the dark-to-bright
conversion process by using surface interactions.Comment: 7 pages, 4 figure
Single carbon nanotubes as ultrasmall all-optical memories
Performance improvements are expected from integration of photonic devices
into information processing systems, and in particular, all-optical memories
provide a key functionality. Scaling down the size of memory elements is
desirable for high-density integration, and the use of nanomaterials would
allow for devices that are significantly smaller than the operation
wavelengths. Here we report on all-optical memory based on individual carbon
nanotubes, where adsorbed molecules give rise to optical bistability. By
exciting at the high-energy tail of the excitonic absorption resonance,
nanotubes can be switched between the desorbed state and the adsorbed state. We
demonstrate reversible and reproducible operation of the nanotube optical
memory, and determine the rewriting speed by measuring the molecular adsorption
and desorption times. Our results underscore the impact of molecular-scale
effects on optical properties of nanomaterials, offering new design strategies
for photonic devices that are a few orders of magnitude smaller than the
optical diffraction limit.Comment: 8 pages, 6 figure
Two-Nucleon Bound States in Quenched Lattice QCD
We address the issue of bound state in the two-nucleon system in lattice QCD.
Our study is made in the quenched approximation at the lattice spacing of a =
0.128 fm with a heavy quark mass corresponding to m_pi = 0.8 GeV. To
distinguish a bound state from an attractive scattering state, we investigate
the volume dependence of the energy difference between the ground state and the
free two-nucleon state by changing the spatial extent of the lattice from 3.1
fm to 12.3 fm. A finite energy difference left in the infinite spatial volume
limit leads us to the conclusion that the measured ground states for not only
spin triplet but also singlet channels are bounded. Furthermore the existence
of the bound state is confirmed by investigating the properties of the energy
for the first excited state obtained by 2x2 diagonalization method. The
scattering lengths for both channels are evaluated by applying the finite
volume formula derived by Luscher to the energy of the first excited states.Comment: 34 pages, 28 figure
Stark effect of excitons in individual air-suspended carbon nanotubes
We investigate electric-field induced redshifts of photoluminescence from
individual single-walled carbon nanotubes. The shifts scale quadratically with
field, while measurements with different excitation powers and energies show
that effects from heating and relaxation pathways are small. We attribute the
shifts to the Stark effect, and characterize nanotubes with different
chiralities. By taking into account exciton binding energies for air-suspended
tubes, we find that theoretical predictions are in quantitative agreement.Comment: 4 pages, 3 figure
Incentives vs. Control: An Analysis of U.S. Dual-Class Companies
Dual-class common stock allows for the separation of voting rights and cash flow rights across the different classes of equity. We construct a large sample of dual-class firms in the United States and analyze the relationships of insider's cash flow rights and voting rights with firm value, performance, and investment behavior. We find that relationship of firm value to cash flow rights is positive and concave and the relationship to voting rights is negative and convex. Identical quadratic relationships are found for the respective ownership variables with sales growth, capital expenditures, and the combination of R&D and advertising. Our evidence is consistent with an entrenchment effect of voting control that leads managers to underinvest and an incentive effect of cash flow ownership that induces managers to pursue more aggressive strategies.
On the Large Time Behavior of Solutions of Hamilton-Jacobi Equations Associated with Nonlinear Boundary Conditions
In this article, we study the large time behavior of solutions of first-order
Hamilton-Jacobi Equations, set in a bounded domain with nonlinear Neumann
boundary conditions, including the case of dynamical boundary conditions. We
establish general convergence results for viscosity solutions of these
Cauchy-Neumann problems by using two fairly different methods : the first one
relies only on partial differential equations methods, which provides results
even when the Hamiltonians are not convex, and the second one is an optimal
control/dynamical system approach, named the "weak KAM approach" which requires
the convexity of Hamiltonians and gives formulas for asymptotic solutions based
on Aubry-Mather sets
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