19,283 research outputs found
Optical and transport gaps in gated bilayer graphene
We discuss the effect of disorder on the band gap measured in bilayer
graphene in optical and transport experiments. By calculating the optical
conductivity and density of states using a microscopic model in the presence of
disorder, we demonstrate that the gap associated with transport experiments is
smaller than that associated with optical experiments. Intrinsic bilayer
graphene has an optical conductivity in which the energy of the peaks
associated with the interband transition are very robust against disorder and
thus provide an estimate of the band gap. In contrast, extraction of the band
gap from the optical conductivity of extrinsic bilayer graphene is almost
impossible for significant levels of disorder due to the ambiguity of the
transition peaks. The density of states contains an upper bound on the gap
measured in transport experiments, and disorder has the effect of reducing this
gap which explains why these experiments have so far been unable to replicate
the large band gaps seen in optical measurements.Comment: 5 pages, 5 figures, RevTeX. Published versio
Quantization of Gauge Field Theories on the Front-Form without Gauge Constraints I : The Abelian Case
Recently, we have proposed a new front-form quantization which treated both
the and the coordinates as front-form 'times.' This
quantization was found to preserve parity explicitly. In this paper we extend
this construction to local Abelian gauge fields . We quantize this theory using
a method proposed originally by Faddeev and Jackiw . We emphasize here the
feature that quantizing along both and , gauge theories does not
require extra constraints (also known as 'gauge conditions') to determine the
solution uniquely.Comment: 18 pages, phyzz
Deformation and crystallization of Zr-based amorphous alloys in homogeneous flow regime
The purpose of this study is to experimentally investigate the interaction of inelastic deformation and microstructural changes of two Zr-based bulk metallic glasses (BMGs): Zr_(41.25)Ti_(13.75)Cu_(12.5)Ni_(10)Be_(22.5) (commercially designated as Vitreloy 1 or Vit1) and Zr_(46.75)Ti_(8.25)Cu_(7.5)Ni_(10)Be_(27.5) (Vitreloy 4, Vit4). High-temperature uniaxial compression tests were performed on the two Zr alloys at various strain rates, followed by structural characterization using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Two distinct modes of mechanically induced atomic disordering in the two alloys were observed, with Vit1 featuring clear phase separation and crystallization after deformation as observed with TEM, while Vit4 showing only structural relaxation with no crystallization. The influence of the structural changes on the mechanical behaviors of the two materials was further investigated by jump-in-strain-rate tests, and flow softening was observed in Vit4. A free volume theory was applied to explain the deformation behaviors, and the activation volumes were calculated for both alloys
The Gluon Spin in the Chiral Bag Model
We study the gluon polarization contribution at the quark model
renormalization scale to the proton spin, , in the chiral bag model. It
is evaluated by taking the expectation value of the forward matrix element of a
local gluon operator in the axial gauge . It is shown that the confining
boundary condition for the color electric field plays an important role. When a
solution satisfying the boundary condition for the color electric field, which
is not the conventionally used but which we favor, is used, the has a
positive value for {\it all} bag radii and its magnitude is comparable to the
quark spin polarization. This results in a significant reduction in the
relative fraction of the proton spin carried by the quark spin, which is
consistent with the small flavor singlet axial current measured in the EMC
experiments.Comment: Corrections to figure
Scattered light mapping of protoplanetary disks
High-contrast scattered light observations have revealed the surface
morphology of several dozens of protoplanetary disks at optical and
near-infrared wavelengths. Inclined disks offer the opportunity to measure part
of the phase function of the dust grains that reside in the disk surface which
is essential for our understanding of protoplanetary dust properties and the
early stages of planet formation. We aim to construct a method which takes into
account how the flaring shape of the scattering surface of an (optically thick)
protoplanetary disk projects onto the image plane of the observer. This allows
us to map physical quantities (scattering radius and scattering angle) onto
scattered light images and retrieve stellar irradiation corrected (r^2-scaled)
images and dust phase functions. We apply the method on archival polarized
intensity images of the protoplanetary disk around HD 100546 that were obtained
with VLT/SPHERE in R'-band and VLT/NACO in H- and Ks-band. The brightest side
of the r^2-scaled R'-band polarized intensity image of HD 100546 changes from
the far to the near side of the disk when a flaring instead of a geometrically
flat disk surface is used for the r^2-scaling. The decrease in polarized
surface brightness in the scattering angle range of ~40-70 deg is likely a
result of the dust phase function and degree of polarization which peak in
different scattering angle regimes. The derived phase functions show part of a
forward scattering peak which indicates that large, aggregate dust grains
dominate the scattering opacity in the disk surface. Projection effects of a
protoplanetary disk surface need to be taken into account to correctly
interpret scattered light images. Applying the correct scaling for the
correction of stellar irradiation is crucial for the interpretation of the
images and the derivation of the dust properties in the disk surface layer.Comment: Accepted for publication in A&A, 6 pages, 3 figure
A Forage Area of Expertise Team: The Michigan Approach to Applied Research and Extension
Agricultural Experiment Stations and the Cooperative Extension Service have traditionally contributed to the economic, social, human, and environmental capital of the United States. Despite this, both institutions have experienced declining federal budget support and increasing competition for resources (Hamm 1997; Hood & Schutjer 1990; Knutson & Outlaw 1994; Paarlberg 1992). Michigan State University Extension, in partnership with the Michigan Agricultural Experiment Station, implemented self-directed area of expertise (AOE) teams as its major educational development and delivery model. AOE teams grew out of experiences with previous temporary research/Extension teams and quick response professional groups operating within traditional line responsibilities for research and Extension units. An extension forage area of expertise team was organized in 1999 to deliver educational programmes in forage management and conduct applied forage research. Funds were made available directly to the forage team from central extension administration
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