22,054 research outputs found
Semiclassical theory of the tunneling anomaly in partially spin-polarized compressible quantum Hall states
Electron tunneling into a system with strong interactions is known to exhibit
an anomaly, in which the tunneling conductance vanishes continuously at low
energy due to many-body interactions. Recent measurements have probed this
anomaly in a quantum Hall bilayer of the half-filled Landau level, and shown
that the anomaly apparently gets stronger as the half-filled Landau level is
increasingly spin polarized. Motivated by this result, we construct a
semiclassical hydrodynamic theory of the tunneling anomaly in terms of the
charge-spreading action associated with tunneling between two copies of the
Halperin-Lee-Read state with partial spin polarization. This theory is
complementary to our recent work (arXiv:1709.06091) where the electron spectral
function was computed directly using an instanton-based approach. Our results
show that the experimental observation cannot be understood within conventional
theories of the tunneling anomaly, in which the spreading of the injected
charge is driven by the mean-field Coulomb energy. However, we identify a
qualitatively new regime, in which the mean-field Coulomb energy is effectively
quenched and the tunneling anomaly is dominated by the finite compressibility
of the composite Fermion liquid.Comment: 24 pages, 2 figures; Final published versio
How Will Defined Contribution Pension Plans Affect Retirement Income?
How has the emergence of defined contribution pension plans, such as 401(k)s, affected the financial security of future retirees? We consider this question using a detailed survey of pension formulas in the Survey of Consumer Finances. Our simulations show that average and median pension benefits are higher under defined contribution plans that for defined benefit plans. Defined benefit plans are slightly better at providing minimum benefits, but for plausible values of risk aversion, a defined contribution plan drawn randomly from those available in 1995 is still preferred to a defined benefit plan drawn randomly from those available in 1983. This result is robust to different assumptions regarding the spending of defined contribution balances between jobs, equity rates of return, and the date of retirement. In short, we suggest that defined contribution plans can strengthen the financial security of retirees.
Effect of magnetization on the tunneling anomaly in compressible quantum Hall states
Tunneling of electrons into a two-dimensional electron system is known to
exhibit an anomaly at low bias, in which the tunneling conductance vanishes due
to a many-body interaction effect. Recent experiments have measured this
anomaly between two copies of the half-filled Landau level as a function of
in-plane magnetic field, and they suggest that increasing spin polarization
drives a deeper suppression of tunneling. Here we present a theory of the
tunneling anomaly between two copies of the partially spin-polarized
Halperin-Lee-Read state, and we show that the conventional description of the
tunneling anomaly, based on the Coulomb self-energy of the injected charge
packet, is inconsistent with the experimental observation. We propose that the
experiment is operating in a different regime, not previously considered, in
which the charge-spreading action is determined by the compressibility of the
composite fermions.Comment: (5+1) pages, 1 figure; (v2) minor changes and added reference to our
accompanying paper arXiv:1712.02357; (v3) Final published versio
A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
Recent optical and IR studies have revealed that the heavily-reddened
starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (WR)
stars, comprising the richest WR population of any galactic cluster. We present
results of a senstive Chandra X-ray observation of Wd 1 which detected 12 of
the 22 known WR stars and the mysterious emission line star W9. The fraction of
detected WN stars is nearly identical to that of WC stars. The WN stars WR-A
and WR-B as well as W9 are exceptionally luminous in X-rays and have similar
hard heavily-absorbed spectra with strong Si XIII and S XV emission lines. The
luminous high-temperature X-ray emission of these three stars is characteristic
of colliding wind binary systems but their binary status remains to be
determined. Spectral fits of the X-ray bright sources WR-A and W9 with
isothermal plane-parallel shock models require high absorption column densities
log N = 22.56 (cm) and yield characteristic shock temperatures
kT_shock ~ 3 keV (T ~ 35 MK).Comment: ApJL, 2006, in press (3 figures, 1 table
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
DIVERSIFICATION OF REGIONAL MARKETING COOPERATIVES
Agribusiness, Marketing,
The Resolution of the Labor Scarcity Paradox
This paper reconciles the apparently contradictory evidence about American and British technology in the first half of the nineteenth century. Past studies have focused on the writings of a number of distinguished British engineers, who toured the United States during the 1850s and commented extensively on the highly mechanized state of the manufacturing sector. Other studies, however, have marshalled evidence that the interest rate was higher, and the aggregate manufacturing capital stock was lower, in the United States relative to Britain. We resolve this paradox by noting that British engineers were most impressed by only a few industries which relied on skilled workers. Using the 1849 Census of Manufactures, we estimate separate production functions for the skilled sector and for the remaining, less skilled manufacturing sector. We find strong relative complementarity between capital and natural resources in the skilled sector, and relative substitutability between skilled labor and capital. Using these parameters in a computable general equilibrium model of the U.S. and British economies indicates greater capital intensity (or labor scarcity) in the skilled manufacturing sector, but overall capital scarcity and higher interest rates, in the U.S. relative to Britain.
Chandra HETG Observations of the Colliding Stellar Wind System WR 147
We present an extended analysis of deep Chandra HETG observations of the
WR+OB binary system WR 147 that was resolved into a double X-ray source (Zhekov
& Park, 2010, ApJ, 709, L119). Our analysis of the profiles of strong emission
lines shows that their centroids are blue-shifted in the spectrum of the
northern X-ray source. We find no suppressed forbidden line in the He-like
triplets which indicates that the X-ray emitting region is not located near
enough to the stars in the binary system to be significantly affected by their
UV radiation. The most likely physical picture that emerges from the entire set
of HETG data suggests that the northern X-ray source can be associated with the
colliding stellar wind region in the wide WR+OB binary system, while the X-rays
of its southern counterpart, the WN8 star, are result from stellar wind
shocking onto a close companion (a hypothesized third star in the system).Comment: 22 pages, 6 figures, 2 Tables; accepted for publication in The
Astrophysical Journa
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