8,597 research outputs found
Space charge and charge trapping characteristics of cross-linked polyethylene subjected to ac electric stresses
This paper reports on the result of space charge evolution in cross-linked polyethylene (XLPE) planar samples of approximately 220 ?m thick. The space charge measurement technique used in this study is the PEA method. There are two phases to this experiment. In the first phase, the samples were subjected to dc 30 kVdc/mm and ac (sinusoidal) electric stress level of 30 kVpk/mm at frequencies of 1 Hz, 10 Hz and 50 Hz ac. In addition, ac space charge under 30 kVrms/mm and 60 kVpk/mm electric stress at 50 Hz was also investigated. The volts off results showed that the amount of charge trapped in XLPE sample under dc electric stress is significantly bigger than samples under ac stress even when the applied ac stresses are substantially higher. The second phase of the experiment involves studying the dc space charge evolution in samples that were tested under ac stress during the first phase of the experiment. Ac ageing causes positive charge to become more dominant over negative charge. It was also discovered that ac ageing creates deeper traps, particularly for negative charge. This paper also gave a brief overview of the data processing methods used to analyse space charge under ac electric stress
Multiscale Technicolor and the Zbb-bar Vertex
We estimate the correction to the Zbb-bar vertex arising from the exchanges
of the sideways extended technicolor (ETC) boson and the flavor-diagonal ETC
boson in the multiscale walking technicolor model. The obtained result is too
large to explain the present data. However, if we introduce a new self-
interaction for the top quark to induce the top quark condensate serving as the
origin of the large top quark mass, the corrected R_b=Gamma_b/Gamma_h can be
consistent with the recent LEP data. The corresponding correction to
R_c=Gamma_c/Gamma_h is shown to be negligibly small.Comment: 9-page LaTex fil
Emergent particle-hole symmetry in spinful bosonic quantum Hall systems
When a fermionic quantum Hall system is projected into the lowest Landau
level, there is an exact particle-hole symmetry between filling fractions
and . We investigate whether a similar symmetry can emerge in bosonic
quantum Hall states, where it would connect states at filling fractions
and . We begin by showing that the particle-hole conjugate to a
composite fermion `Jain state' is another Jain state, obtained by reverse flux
attachment. We show how information such as the shift and the edge theory can
be obtained for states which are particle-hole conjugates. Using the techniques
of exact diagonalization and infinite density matrix renormalization group, we
study a system of two-component (i.e., spinful) bosons, interacting via a
-function potential. We first obtain real-space entanglement spectra
for the bosonic integer quantum Hall effect at , which plays the role of
a filled Landau level for the bosonic system. We then show that at
the system is described by a Jain state which is the particle-hole conjugate of
the Halperin (221) state at . We show a similar relationship between
non-singlet states at and . We also study the case of
, providing unambiguous evidence that the ground state is a composite
Fermi liquid. Taken together our results demonstrate that there is indeed an
emergent particle-hole symmetry in bosonic quantum Hall systems.Comment: 10 pages, 8 figures, 4 appendice
Structural relaxation in a system of dumbbell molecules
The interaction-site-density-fluctuation correlators, the dipole-relaxation
functions, and the mean-squared displacements of a system of symmetric
dumbbells of fused hard spheres are calculated for two representative
elongations of the molecules within the mode-coupling theory for the evolution
of glassy dynamics. For large elongations, universal relaxation laws for states
near the glass transition are valid for parameters and time intervals similar
to the ones found for the hard-sphere system. Rotation-translation coupling
leads to an enlarged crossover interval for the mean-squared displacement of
the constituent atoms between the end of the von Schweidler regime and the
beginning of the diffusion process. For small elongations, the superposition
principle for the reorientational -process is violated for parameters
and time intervals of interest for data analysis, and there is a strong
breaking of the coupling of the -relaxation scale for the diffusion
process with that for representative density fluctuations and for dipole
reorientations.Comment: 15 pages, 14 figures, Phys. Rev. E in pres
Origins of ferromagnetism in transition-metal doped Si
We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis
A mode-coupling theory for the glassy dynamics of a diatomic probe molecule immersed in a simple liquid
Generalizing the mode-coupling theory for ideal liquid-glass transitions,
equations of motion are derived for the correlation functions describing the
glassy dynamics of a diatomic probe molecule immersed in a simple glass-forming
system. The molecule is described in the interaction-site representation and
the equations are solved for a dumbbell molecule consisting of two fused hard
spheres in a hard-sphere system. The results for the molecule's arrested
position in the glass state and the reorientational correlators for
angular-momentum index and near the glass transition are
compared with those obtained previously within a theory based on a
tensor-density description of the molecule in order to demonstrate that the two
approaches yield equivalent results. For strongly hindered reorientational
motion, the dipole-relaxation spectra for the -process can be mapped on
the dielectric-loss spectra of glycerol if a rescaling is performed according
to a suggestion by Dixon et al. [Phys. Rev. Lett. {\bf 65}, 1108 (1990)]. It is
demonstrated that the glassy dynamics is independent of the molecule's inertia
parameters.Comment: 19 pages, 10 figures, Phys. Rev. E, in prin
Improvement of dielectric loss of doped Ba0.5Sr0.5TiO3 thin films for tunable microwave devices
Al2O3-Ba0.5Sr0.5TiO3 (Al2O3-BST) thin films, with different Al2O3 contents,
were deposited on (100) LaAlO3 substrate by pulsed laser deposition (PLD)
technique. The Al2O3-BST films was demosnstrated to be a suitable systems to
fabricate ferroelectric thin films with low dielectric loss and higher figure
of merit for tunable microwave devices. Pure BST thin films were also
fabricated for comparison purpose. The films' structure and morphology were
analyzed by X-ray diffractiopn and scanning electron microscopy, respectively;
nad showed that the surface roughness for the Al2O3-BST films increased with
the Al2O3 content. Apart from that, the broadening in the intensity peak in XRD
result indicating the grain size of the Al2O3-BST films reduced with the
increasing of Al2O3 dopant. We measured the dielctric properties of Al2O3-BST
films with a home-made non-destructive dual resonator method at frequency ~ 7.7
GHZ. The effect of doped Al2O3 into BST thin films significantly reduced the
dielectric constant, dielectric loss and tunability compare to pure BST thin
film. Our result shows the figure of merit (K), used to compare the films with
varied dielectric properties, increased with the Al2O3 content. Therefore
Al2O3-BST films show the potential to be exploited in tunable microwave
devices.Comment: 8 pages, 4 figures, 1 table. Accepted & tentatively for Feb 15 2004
issue, Journal of Applied Physic
Superconductivity and Magnetism in REFeAsO1-xFx (RE=Rare Earth Elements)
Fluoride-doped iron-based oxypnictides containing rare-earth gadolinium
(GdFeAsO0.8F0.2) and co-doping with yttrium (Gd0.8Y0.2FeAsO0.8F0.2) have been
prepared via conventional solid state reaction at ambient pressure. The
non-yttrium substituted oxypnictide show superconducting transition as high as
43.9 K from temperature dependent resistance measurements with the Meissner
effect observed at a lower temperature of 40.8 K from temperature dependent
magnetization measurements. By replacing a small amount of gadolinium with
yttrium Tc was observed to be lowered by 10 K which might be caused by a change
in the electronic or magnetic structures since the crystal structure was not
altered.Comment: 4 pages, 4 figures, Journal of Physics: Conference Series
(Proceedings in the LT25 Low Temperature Physics Conference) Submitte
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