20,392 research outputs found
Ultrafast Resonant Polarization Interferometry: Towards the First Direct Detection of Vacuum Polarization
Vacuum polarization, an effect predicted nearly 70 years ago, is still yet to
be directly detected despite significant experimental effort. Previous attempts
have made use of large liquid-helium cooled electromagnets which inadvertently
generate spurious signals that mask the desired signal. We present a novel
approach for the ultra-sensitive detection of optical birefringence that can be
usefully applied to a laboratory detection of vacuum polarization. The new
technique has a predicted birefringence measurement sensitivity of in a 1 second measurement. When combined with the extreme
polarizing fields achievable in this design we predict that a vacuum
polarization signal will be seen in a measurement of just a few days in
duration.Comment: 9 pages, 2 figures. submitted to PR
Methods for measuring the citations and productivity of scientists across time and discipline
Publication statistics are ubiquitous in the ratings of scientific
achievement, with citation counts and paper tallies factoring into an
individual's consideration for postdoctoral positions, junior faculty, tenure,
and even visa status for international scientists. Citation statistics are
designed to quantify individual career achievement, both at the level of a
single publication, and over an individual's entire career. While some academic
careers are defined by a few significant papers (possibly out of many), other
academic careers are defined by the cumulative contribution made by the
author's publications to the body of science. Several metrics have been
formulated to quantify an individual's publication career, yet none of these
metrics account for the dependence of citation counts and journal size on time.
In this paper, we normalize publication metrics across both time and discipline
in order to achieve a universal framework for analyzing and comparing
scientific achievement. We study the publication careers of individual authors
over the 50-year period 1958-2008 within six high-impact journals: CELL, the
New England Journal of Medicine (NEJM), Nature, the Proceedings of the National
Academy of Science (PNAS), Physical Review Letters (PRL), and Science. In
comparing the achievement of authors within each journal, we uncover
quantifiable statistical regularity in the probability density function (pdf)
of scientific achievement across both time and discipline. The universal
distribution of career success within these arenas for publication raises the
possibility that a fundamental driving force underlying scientific achievement
is the competitive nature of scientific advancement.Comment: 25 pages in 1 Column Preprint format, 7 Figures, 4 Tables. Version
II: changes made in response to referee comments. Note: change in definition
of "Paper shares.
Basel III Liquidity Risk Measures and Bank Failure
Basel III banking regulation emphasizes the use of liquidity coverage and nett stable funding ratios as measures of liquidity risk. In this paper, we approximate these measures by using global liquidity data for 391 hand-selected, LIBOR-based, Basel II compliant banks in 36 countries for the period 2002 to 2012. In particular, we compare the risk sensitivity of the aforementioned Basel III liquidity risk measures to those of traditional measures such as the nonperforming assets ratio, return-on-assets, LIBOR-OISS, Basel II Tier 1 capital ratio, government securities ratio, and brokered deposits ratio. Furthermore, we use a discrete-time hazard model to study bank failure. In this regard, we find that Basel III risk measures have limited ability to predict bank failure when compared with their traditional counterparts. An important result is that a higher liquidity coverage ratio is associated with a higher bank failure rate. We also find that market-wide liquidity risk (proxied by LIBOR-OISS) was the major predictor of bank failures in 2009 and 2010 while idiosyncratic liquidity risk (proxied by other liquidity risk measures) was less. In particular, our contribution is the first to achieve these results on a global scale over a relatively long period for a variety of banks
Does Function Follow Organizational Form? Evidence From the Lending Practices of Large and Small Banks
Theories based on incomplete contracting suggest that small organizations may do better than large organizations in activities that require the processing of soft information. We explore this idea in the context of bank lending to small firms, an activity that is typically thought of as relying heavily on soft information. We find that large banks are less willing than small banks to lend to informationally 'difficult' credits, such as firms that do not keep formal financial records. Moreover, controlling for the endogeneity of bank-firm matching, large banks lend at a greater distance, interact more impersonally with their borrowers, have shorter and less exclusive relationships, and do not alleviate credit constraints as effectively. All of this is consistent with small banks being better able to collect and act on soft information than large banks.
Does Function Follow Organzizational Form? Evidence From the Lending Practices of Large and Small Banks
Theories based on incomplete contracting suggest that small organizations may do better than large organizations in activities that require the processing of soft information. We explore this idea in the context of bank lending to small firms, an activity that is typically thought of as relying heavily on soft information. We find that large banks are less willing than small banks to lend to informationally “difficult†credits, such as firms that do not keep formal financial records. Moreover, controlling for the endogeneity of bank-firm matching, large banks lend at a greater distance, interact more impersonally with their borrowers, have shorter and less exclusive relationships, and do not alleviate credit constraints as effectively. All of this is consistent with small banks being better able to collect and act on soft information than large banks. The opinions in this paper do not necessarily reflect those of the Federal Reserve Board or its staff. This work has been supported by the National Science Foundation (Rajan, Stein), and the George J. Stigler Center for Study of the State and Economy (Rajan). Thanks also to seminar participants at Yale, the Federal Reserve Bank of New York, Tulane, Babson, the University of Illinois, the Federal Reserve Bank of Chicago Bank Structure Conference, the NBER and the Western Finance Association meetings, as well as to Abhijit Banerjee, Michael Kremer, David Scharfstein, Andrei Shleifer, Greg Udell, Christopher Udry and James Weston for helpful comments and suggestions.
Chalcogenide-glass polarization-maintaining photonic crystal fiber for mid-infrared supercontinuum generation
In this paper, we report the design and fabrication of a highly birefringent
polarization-maintaining photonic crystal fiber (PM-PCF) made from chalcogenide
glass, and its application to linearly-polarized supercontinuum (SC) generation
in the mid-infrared region. The PM fiber was drawn using the casting method
from As38Se62 glass which features a transmission window from 2 to 10
and a high nonlinear index of 1.13.10mW. It has a
zero-dispersion wavelength around 4.5 and, at this wavelength, a large
birefringence of 6.10 and consequently strong polarization maintaining
properties are expected. Using this fiber, we experimentally demonstrate
supercontinuum generation spanning from 3.1-6.02 and 3.33-5.78
using femtosecond pumping at 4 and 4.53 , respectively. We
further investigate the supercontinuum bandwidth versus the input pump
polarization angle and we show very good agreement with numerical simulations
of the two-polarization model based on two coupled generalized nonlinear
Schr\"odinger equations.Comment: 13 pages, 8 figure
Drop Axis Ratio Distributions in Stratiform and Convective Rain
A fully calibrated low profile 2D video disdrometer (2DVD) has been recording many different rainfall events in Northern Alabama (USA) since June 2007. An earlier publication reported drop shapes and axis ratio distributions determined for some of the events. For one of the cases examined, a noticeable shift in the 3.5 - 3.75 mm drop axis ratio distribution was noted. In this paper, we extend the earlier work by separating the 2DVD measurements into stratiform and convective rain. The separation is made possible by using the minute-by-minute drop size distribution (DSD) measured by the 2DVD. The 1-minute DSDs are fitted to a gamma distribution, and using a simple indexing technique which involves two of the fitted parameters, periods of convective and stratiform rain are separated for a given event. The output of the DSD indexing technique is qualitatively confirmed by comparing with simultaneous time series observations from a co-located UHF profiler which continuously records height profiles of reflectivity, Doppler mean and spectral width, all of which enable the identification of bright-band periods and, furthermore, periods of moderate and deep convection. Excellent consistency is found between the output of the DSD-based separation method and the profiler observations. Next, we utilize the output of DSD index-based separation method to flag the periods of severe convection for a given event. Drop axis ratios during the flagged periods are derived and compared with those during stratiform rain periods. Five cases have been considered. Axis ratio distributions do not show appreciable differences between stratiform and convective periods for four of the cases. The fifth case (the same case as reported earlier) shows a shift in the 3.5 - 3.75 mm drop axis ratios during a prolonged period of convection. The contoured shapes for these drops determined from the 2DVD camera data indicate the possibility of non-axisymmetric oscillations, compared with the contoured images for other events which fit well to our reference drop shapes. For all of above cases, observations from a C-band polarimetric radar - situated 15 km away are examined. The variations between the co-polar radar reflectivity and the differential reflectivity as well as the specific differential phase are compared with the 2DVD data based scattering calculations for the 5 events. The implications will be discussed
Detection of Vacuum Birefringence with Intense Laser Pulses
We propose a novel technique that promises hope of being the first to
directly detect a polarization in the quantum electrodynamic (QED) vacuum. The
technique is based upon the use of ultra-short pulses of light circulating in
low dispersion optical resonators. We show that the technique circumvents the
need for large scale liquid helium cooled magnets, and more importantly avoids
the experimental pitfalls that plague existing experiments that make use of
these magnets. Likely improvements in the performance of optics and lasers
would result in the ability to observe vacuum polarization in an experiment of
only a few hours duration.Comment: 4 pages, 1 figur
Analogue of cosmological particle creation in an ion trap
We study phonons in a dynamical chain of ions confined by a trap with a
time-dependent (axial) potential strength and demonstrate that they behave in
the same way as quantum fields in an expanding/contracting universe. Based on
this analogy, we present a scheme for the detection of the analogue of
cosmological particle creation which should be feasible with present-day
technology. In order to test the quantum nature of the particle creation
mechanism and to distinguish it from classical effects such as heating, we
propose to measure the two-phonon amplitude via the red side-band
and to compare it with the one-phonon amplitude ( red side-band).
PACS: 04.62.+v, 98.80.-k, 42.50.Vk, 32.80.Pj.Comment: 4 pages, 2 figure
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