11,085 research outputs found
Evidence on Growth, Increasing Returns and the Extent of the Market
We examine two sets of economies, (19th century U.S. states and 20th century less developed countries) where growth rates are positively correlated with initial levels of development to document how these dynamic increasing returns operate. We find that open economies do not display a positive connection between initial levels and later growth; instead, closed economies do display this positive correlation (i.e. divergence). This evidence suggests that increasing returns operate by expanding the extent of the market (as in the big push theories of Murphy, Shleifer and Vishny (1989)). For U.S. states, we also find that larger markets enhance growth by increasing the division of labor. Among LDCs, while more diversified production increases growth, diversification is negatively associated with openness for the poorest economies (as in the quality ladder theories of Boldrin and Scheinkman (1988), Young (1991) and Stokey (1991)). However, and despite the negative effect that openness has on the diversity of production and, thus, on growth, we find that openness still substantially increases growth for these poorer economies.
A ballistic pn junction in suspended graphene with split bottom gates
We have developed a process to fabricate suspended graphene devices with
local bottom gates, and tested it by realizing electrostatically controlled pn
junctions on a suspended graphene mono-layer nearly 2 micrometers long.
Measurements as a function of gate voltage, magnetic field, bias, and
temperature exhibit characteristic Fabry-Perot oscillations in the cavities
formed by the pn junction and each of the contacts, with transport occurring in
the ballistic regime. Our results demonstrate the possibility to achieve a high
degree of control on the local electronic properties of ultra-clean suspended
graphene layers, a key aspect for the realization of new graphene
nanostructures.Comment: 10 pages, 4 figure
Heralding two- and four-photon path entanglement on chip
Generating quantum entanglement is not only an important scientific endeavor,
but will be essential to realizing quantum-enhanced technologies, in
particular, quantum-enhanced measurements with precision beyond classical
limits. We investigate the heralded generation of multiphoton entanglement for
quantum metrology using a reconfigurable integrated waveguide device in which
projective measurement of auxiliary photons heralds the generation of
path-entangled states. We use four and six-photon inputs, to analyze the
heralding process of two- and four-photon NOON states-a superposition of N
photons in two paths, capable of enabling phase supersensitive measurements at
the Heisenberg limit. Realistic devices will include imperfections; as part of
the heralded state preparation, we demonstrate phase superresolution within our
chip with a state that is more robust to photon loss
Lattice effects on the spin dynamics in antiferromagnetic molecular rings
We investigate spin dynamics in antiferromagnetic (AF) molecular rings at
finite temperature in the presence of spin-phonon (s-p) interaction. We derive
a general expression for the spin susceptibility in the weak s-p coupling limit
and then we focus on the low-frequency behavior, in order to discuss a possible
microscopic mechanism for nuclear relaxation in this class of magnetic
materials. To lowest order in a perturbative expansion, we find that the
susceptibility takes a Lorentzian profile and all spin operators (, ) contribute to spin dynamics at wave vectors . Spin anisotropies
and local s-p coupling play a key role in the proposed mechanism. Our results
prove that small changes in the spatial symmetry of the ring induce qualitative
changes in the spin dynamics at the nuclear frequency, providing a novel
mechanism for nuclear relaxation. Possible experiments are proposed.Comment: 4 pages, 2 figures. to appear in PR
Capture-zone scaling in island nucleation: phenomenological theory of an example of universal fluctuation behavior
In studies of island nucleation and growth, the distribution of capture
zones, essentially proximity cells, can give more insight than island-size
distributions. In contrast to the complicated expressions, ad hoc or derived
from rate equations, usually used, we find the capture-zone distribution can be
described by a simple expression generalizing the Wigner surmise from random
matrix theory that accounts for the distribution of spacings in a host of
fluctuation phenomena. Furthermore, its single adjustable parameter can be
simply related to the critical nucleus of growth models and the substrate
dimensionality. We compare with extensive published kinetic Monte Carlo data
and limited experimental data. A phenomenological theory sheds light on the
result.Comment: 5 pages, 4 figures, originally submitted to Phys. Rev. Lett. on Dec.
15, 2006; revised version v2 tightens and focuses the presentation,
emphasizes the importance of universal features of fluctuations, corrects an
error for d=1, replaces 2 of the figure
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