8,492 research outputs found
Novel Quark Fragmentation Functions and the Nucleon's Transversity Distribution
We define twist-two and twist-three quark fragmentation functions in Quantum
Chromodynamics (QCD) and study their physical implications. Using this
formalism we show how the nucleon's transversity distribution can be measured
in single pion inclusive electroproduction.Comment: 10 pages, uses PHYZZX macro package, 2 PostScript figures (added
using FIGURES). MIT-CTP-215
A superconductor to superfluid phase transition in liquid metallic hydrogen
Although hydrogen is the simplest of atoms, it does not form the simplest of
solids or liquids. Quantum effects in these phases are considerable (a
consequence of the light proton mass) and they have a demonstrable and often
puzzling influence on many physical properties, including spatial order. To
date, the structure of dense hydrogen remains experimentally elusive. Recent
studies of the melting curve of hydrogen indicate that at high (but
experimentally accessible) pressures, compressed hydrogen will adopt a liquid
state, even at low temperatures. In reaching this phase, hydrogen is also
projected to pass through an insulator-to-metal transition. This raises the
possibility of new state of matter: a near ground-state liquid metal, and its
ordered states in the quantum domain. Ordered quantum fluids are traditionally
categorized as superconductors or superfluids; these respective systems feature
dissipationless electrical currents or mass flow. Here we report an analysis
based on topological arguments of the projected phase of liquid metallic
hydrogen, finding that it may represent a new type of ordered quantum fluid.
Specifically, we show that liquid metallic hydrogen cannot be categorized
exclusively as a superconductor or superfluid. We predict that, in the presence
of a magnetic field, liquid metallic hydrogen will exhibit several phase
transitions to ordered states, ranging from superconductors to superfluids.Comment: for a related paper see cond-mat/0410425. A correction to the front
page caption appeared in Oct 14 issue of Nature:
http://www.nature.com/nature/links/041014/041014-11.htm
Implications of the Babinet Principle for Casimir Interactions
We formulate the Babinet Principle (BP) as a relation between the scattering
amplitudes for electromagnetic waves, and combine it with multiple scattering
techniques to derive new properties of Casimir forces. We show that the Casimir
force exerted by a planar conductor or dielectric on a self- complementary
perforated planar mirror is approximately half that on a uniform mirror
independent of the distance between them. The BP suggests that Casimir edge
effects are anomalously small, supporting results obtained earlier in special
cases. Finally, we illustrate how the BP can be used to estimate Casimir forces
between perforated planar mirrors
The Nucleon ``Tensor Charges'' and the Skyrme Model
The lowest moment of the twist-two, chiral-odd parton distribution
of the nucleon can be related to the so-called ``tensor charges'' of the
nucleon. We consider the tensor charges in the Skyrme model, and find that in
the large-, SU(3)-symmetric limit, the model predicts that the octet
isosinglet tensor charge, , is of order with respect to the
octet isovector tensor charge, . The predicted ratio is then 1/3,
in the large- limit. These predictions coincide with the Skyrme model
predictions for the octet charges, and . (The
prediction for the axial charges differs from the commonly quoted
prediction of 5/9, which is based on an inconsistent treatment of the
large- limit.) The model also predicts that the singlet tensor charge,
, is of order with respect to .Comment: 9 single-spaced pages, no figures, MIT-CTP-212
Hindcasting of decadal-timescale estuarine bathymetric change with a tidal-timescale model
This paper is not subject to U.S. copyright. The definitive version was published in Journal of Geophysical Research 114 (2009): F04019, doi:10.1029/2008JF001191.Hindcasting decadal-timescale bathymetric change in estuaries is prone to error due to limited data for initial conditions, boundary forcing, and calibration; computational limitations further hinder efforts. We developed and calibrated a tidal-timescale model to bathymetric change in Suisun Bay, California, over the 1867–1887 period. A general, multiple-timescale calibration ensured robustness over all timescales; two input reduction methods, the morphological hydrograph and the morphological acceleration factor, were applied at the decadal timescale. The model was calibrated to net bathymetric change in the entire basin; average error for bathymetric change over individual depth ranges was 37%. On a model cell-by-cell basis, performance for spatial amplitude correlation was poor over the majority of the domain, though spatial phase correlation was better, with 61% of the domain correctly indicated as erosional or depositional. Poor agreement was likely caused by the specification of initial bed composition, which was unknown during the 1867–1887 period. Cross-sectional bathymetric change between channels and flats, driven primarily by wind wave resuspension, was modeled with higher skill than longitudinal change, which is driven in part by gravitational circulation. The accelerated response of depth may have prevented gravitational circulation from being represented properly. As performance criteria became more stringent in a spatial sense, the error of the model increased. While these methods are useful for estimating basin-scale sedimentation changes, they may not be suitable for predicting specific locations of erosion or deposition. They do, however, provide a foundation for realistic estuarine geomorphic modeling applications.This study was supported by the U.S. Geological
Survey’s Priority Ecosystems Science program, CALFED Bay/Delta
Program, and the University of California Center for Water Resources. Use
of ROMS and the CSTMS was supported by the U.S. Geological Survey,
with assistance from John Warner
Discontinuous hindcast simulations of estuarine bathymetric change : a case study from Suisun Bay, California
This paper is not subject to U.S. copyright. The definitive version was published in Estuarine, Coastal and Shelf Science 93 (2011): 142-150, doi:10.1016/j.ecss.2011.04.004.Simulations of estuarine bathymetric change over decadal timescales require methods for idealization and reduction of forcing data and boundary conditions. Continuous simulations are hampered by computational and data limitations and results are rarely evaluated with observed bathymetric change data. Bathymetric change data for Suisun Bay, California span the 1867–1990 period with five bathymetric surveys during that period. The four periods of bathymetric change were modeled using a coupled hydrodynamic-sediment transport model operated at the tidal-timescale. The efficacy of idealization techniques was investigated by discontinuously simulating the four periods. The 1867–1887 period, used for calibration of wave energy and sediment parameters, was modeled with an average error of 37% while the remaining periods were modeled with error ranging from 23% to 121%. Variation in post-calibration performance is attributed to temporally variable sediment parameters and lack of bathymetric and configuration data for portions of Suisun Bay and the Delta. Modifying seaward sediment delivery and bed composition resulted in large performance increases for post-calibration periods suggesting that continuous simulation with constant parameters is unrealistic. Idealization techniques which accelerate morphological change should therefore be used with caution in estuaries where parameters may change on sub-decadal timescales. This study highlights the utility and shortcomings of estuarine geomorphic models for estimating past changes in forcing mechanisms such as sediment supply and bed composition. The results further stress the inherent difficulty of simulating estuarine changes over decadal timescales due to changes in configuration, benthic composition, and anthropogenic forcing such as dredging and channelization.This study was supported by the U.S Geological Survey’s
Priority Ecosystems Science program, CALFED Bay/Delta Program,
and the University of California Center forWater Resources
Low-temperature phase transformations of PZT in the morphotropic phase-boundary region
We present anelastic and dielectric spectroscopy measurements of
PbZr(1-x)Ti(x)O(3) with 0.455 < x < 0.53, which provide new information on the
low temperature phase transitions. The tetragonal-to-monoclinic transformation
is first-order for x < 0.48 and causes a softening of the polycrystal Young's
modulus whose amplitude may exceed the one at the cubic-to-tetragonal
transformation; this is explainable in terms of linear coupling between shear
strain components and tilting angle of polarization in the monoclinic phase.
The transition involving rotations of the octahedra below 200 K is visible both
in the dielectric and anelastic losses, and it extends within the tetragonal
phase, as predicted by recent first-principle calculations.Comment: 4 pages, 4 figure
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