42,695 research outputs found
Implications of Color Gauge Symmetry For Nucleon Spin Structure
We study the chromodynamical gauge symmetry in relation to the internal spin
structure of the nucleon. We show that 1) even in the helicity eigenstates the
gauge-dependent spin and orbital angular momentum operators do not have
gauge-independent matrix element; 2) the evolution equations for the gluon spin
take very different forms in the Feynman and axial gauges, but yield the same
leading behavior in the asymptotic limit; 3) the complete evolution of the
gauge-dependent orbital angular momenta appears intractable in the light-cone
gauge. We define a new gluon orbital angular momentum distribution
which {\it is} an experimental observable and has a simple scale evolution.
However, its physical interpretation makes sense only in the light-cone gauge
just like the gluon helicity distribution y.Comment: Minor corrections are made in the tex
Period halving of Persistent Currents in Mesoscopic Mobius ladders
We investigate the period halving of persistent currents(PCs) of
non-interacting electrons in isolated mesoscopic M\"{o}bius ladders without
disorder, pierced by Aharonov-Bhom flux. The mechanisms of the period halving
effect depend on the parity of the number of electrons as well as on the
interchain hopping. Although the data of PCs in mesoscopic systems are
sample-specific, some simple rules are found in the canonical ensemble average,
such as all the odd harmonics of the PCs disappear, and the signals of even
harmonics are non-negative. {PACS number(s): 73.23.Ra, 73.23.-b, 68.65.-k}Comment: 6 Pages with 3 EPS figure
A Temperature Analysis of High-power AlGaN/GaN HEMTs
Galliumnitride has become a strategic superior material for space, defense
and civil applications, primarily for power amplification at RF and mm-wave
frequencies. For AlGaN/GaN high electron mobility transistors (HEMT), an
outstanding performance combined together with low cost and high flexibility
can be obtained using a System-in-a-Package (SIP) approach. Since thermal
management is extremely important for these high power applications, a hybrid
integration of the HEMT onto an AlN carrier substrate is proposed. In this
study we investigate the temperature performance for AlGaN/GaN HEMTs integrated
onto AlN using flip-chip mounting. Therefore, we use thermal simulations in
combination with experimental results using micro-Raman spectroscopy and
electrical dc-analysis.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Electromagnetic fields in a 3D cavity and in a waveguide with oscillating walls
We consider classical and quantum electromagnetic fields in a
three-dimensional (3D) cavity and in a waveguide with oscillating boundaries of
the frequency . The photons created by the parametric resonance are
distributed in the wave number space around along the axis of the
oscillation. When classical waves propagate along the waveguide in the one
direction, we observe the amplification of the original waves and another wave
generation in the opposite direction by the oscillation of side walls. This can
be understood as the classical counterpart of the photon production. In the
case of two opposite walls oscillating with the same frequency but with a phase
difference, the interferences are shown to occur due to the phase difference in
the photon numbers and in the intensity of the generated waves.Comment: 8 pages revTeX including 1 eps fi
Quark Orbital-Angular-Momentum Distribution in the Nucleon
We introduce gauge-invariant quark and gluon angular momentum distributions
after making a generalization of the angular momentum density operators. From
the quark angular momentum distribution, we define the gauge-invariant and
leading-twist quark {\it orbital} angular momentum distribution . The
latter can be extracted from data on the polarized and unpolarized quark
distributions and the off-forward distribution in the forward limit. We
comment upon the evolution equations obeyed by this as well as other orbital
distributions considered in the literature.Comment: 8 pages, latex, no figures, minor corrections mad
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