4,157 research outputs found
Regularisation of Chiral Gauge Theories
This article gives a review of the topic of regularising chiral gauge
theories and is aimed at a general audience. It begins by clarifying the
meaning of chirality and goes on to discussing chiral projections in field
theory, parity violation and the distinction between vector and chiral field
theories. It then discusses the standard model of electroweak interactions from
the perspective of chirality. It also reviews at length the phenomenon of
anomalies in quantum field theories including the intuitive understanding of
anomalies based on the Dirac sea picture as given by Nielsen and Ninomiya. It
then raises the issue of a non-perturbative and constructive definition of the
standard model as well as the importance of such formulations. The second
Nielsen-Ninomiya theorem about the impossibility of regularising chiral gauge
theories under some general assumptions is also discussed. After a brief review
of lattice regularisation of field theories, it discusses the issue of fermions
on the lattice with special emphasis on the problem of species doubling. The
implications of these problems to introducing chiral fermions on the lattice as
well as the interpretations of anomalies within the lattice formulations and
the lattice Dirac sea picture are then discussed. Finally the difficulties of
formulating the standard model on the lattice are illustrated through detailed
discussions of the Wilson-Yukawa method, the domain wall fermions method and
the recently popular Ginsparg-Wilson method.Comment: LATEX, 26 pages, 7 fig
ScALPEL: A Scalable Adaptive Lightweight Performance Evaluation Library for application performance monitoring
As supercomputers continue to grow in scale and capabilities, it is becoming
increasingly difficult to isolate processor and system level causes of
performance degradation. Over the last several years, a significant number of
performance analysis and monitoring tools have been built/proposed. However,
these tools suffer from several important shortcomings, particularly in
distributed environments. In this paper we present ScALPEL, a Scalable Adaptive
Lightweight Performance Evaluation Library for application performance
monitoring at the functional level. Our approach provides several distinct
advantages. First, ScALPEL is portable across a wide variety of architectures,
and its ability to selectively monitor functions presents low run-time
overhead, enabling its use for large-scale production applications. Second, it
is run-time configurable, enabling both dynamic selection of functions to
profile as well as events of interest on a per function basis. Third, our
approach is transparent in that it requires no source code modifications.
Finally, ScALPEL is implemented as a pluggable unit by reusing existing
performance monitoring frameworks such as Perfmon and PAPI and extending them
to support both sequential and MPI applications.Comment: 10 pages, 4 figures, 2 table
Characterization of nano-composite M-2411/Y-123 thin films by electron backscatter diffraction and in-field critical current measurements
Thin films of nano-composite Y-Ba-Cu-O (YBCO) superconductors containing nano-sized, non-superconducting particles of Y2Ba 4CuMOx (M-2411 with M = Ag and Nb) have been prepared by the PLD technique. Electron backscatter diffraction (EBSD) has been used to analyze the crystallographic orientation of nano-particles embedded in the film microstructure. The superconducting YBa2Cu3O7 (Y-123) phase matrix is textured with a dominant (001) orientation for all samples, whereas the M-2411 phase exhibits a random orientation. Angular critical current measurements at various temperature (T) and applied magnetic field (B) have been performed on thin films containing different concentration of the M-2411 second phase. An increase in critical current density J c at T < 77 K and B < 6 T is observed for samples with low concentration of the second phase (2 mol % M-2411). Films containing 5 mol % Ag-2411 exhibit lower Jc than pure Y-123 thin films at all fields and temperatures. Samples with 5 mol % Nb-2411 show higher Jc(B) than phase pure Y-123 thin films for T < 77 K
Properties of Mg-doped Nd-Ba-Cu-O generic seed crystals for the top seeded melt growth of (RE)-Ba-Cu-O bulk superconductors
We have recently developed a new generic seed crystal that has been used successfully to fabricate any oriented, single grain (RE)-Ba-Cu-O bulk superconductor by a cold seeding technique. In this paper we report the chemical, structural and microstructural properties of these seed crystals, including the variation of melting point, crystallographic parameters and volume fraction of Mg-rich inclusions in the Nd1 + xBa 2-x(Cu1-yMgy)3Oz matrix as a function of externally added MgO content. The influence of Mg-doping on the superconducting transition temperatures of YBCO grains fabricated using these seeds is investigated. Finally, an optimum MgO content of the generic seed that effectively controls the orientation of the seeded grain without compromising its superconducting properties is suggested from the many seed crystals fabricated with a wide range of Mg-rich addition
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Processing of advanced Al/SiC particulate metal matrix composites under intensive shearing – A novel rheo process
Particulate Metal Matrix Composites (PMMCs) have attracted interest for application in numerous fields. The current processing methods often produce agglomerated particles in the ductile matrix and as a result these composites exhibit extremely low ductility. The key idea to solve the current problem is to adopt a novel Rheo-process allowing the application of sufficient shear stress () on particulate clusters embedded in liquid metal to overcome the average cohesive force or the tensile strength of the cluster. In this study, cast A356/SiCp composites were produced using a conventional stir casting technique and a novel Rheo-process. The microstructure and properties were evaluated. The adopted Rheo-process significantly improved the distribution of the reinforcement in the matrix. A good combination of improved Ultimate Tensile Strength (UTS) and tensile elongation (ε) is obtained
Entanglement-Assisted Quantum Error Correction with Linear Optics
We construct a theory of continuous-variable entanglement-assisted quantum
error correction. We present an example of a continuous-variable
entanglement-assisted code that corrects for an arbitrary single-mode error. We
also show how to implement encoding circuits using passive optical devices,
homodyne measurements, feedforward classical communication, conditional
displacements, and off-line squeezers.Comment: 8 pages, 1 figure, major expansion of paper with detailed exampl
Pairing symmetry signatures of T1 in superconducting ferromagnets
We study the nuclear relaxation rate 1/T1 as a function of temperature for a
superconducting-ferromagnetic coexistent system using a p-wave triplet model
for the superconducting pairing symmetry. This calculation is contrasted with a
singlet s-wave one done previously, and we see for the s-wave case that there
is a Hebel-Slichter peak, albeit reduced due to the magnetization, and no peak
for the p-wave case. We then compare these results to a nuclear relaxation rate
experiment on UGe2 to determine the possible pairing symmetry signatures in
that material. It is seen that the experimental data is inconclusive to rule
out the possibility of s-wave pairing in .Comment: 4 pages, 4 figure
Different haemodynamic (24-h ambulatory blood-pressure monitoring) and rennin-inhibiting effect of a 1 week treatment with enalapril and lisinopril
Ambulatory blood pressure and heart rate monitoring were used for comparing the antihypertensive effect of a 1-week treatment with enalapril and lisinopril 10 mg once daily (double-blind crossover placebo-controlled d y ). Twelve outpatients with mild to moderate hypertension were treated. Both drugs had a significant and identical hypotensive effect. Neither drug affected the diurnal rhythm of blood pressure or heart rate. Therefore the two drugs are equipotent antihypertensive agents. Both drugs inhibited ACE activity to a highly significant extent, but in this regard lisinopril was more effective than enalapril. However, lisinopril's greater ACE inhibition was not accompanied by a greater hypotensive effect. The clinical value of this difference is not yet established
Incommensurate spin resonance in URu2Si2
We focus on inelastic neutron scattering in and argue that
observed gap in the fermion spectrum naturally leads to the spin feature
observed at energies at momenta at \bQ^* = (1\pm 0.4,
0,0). We discuss how spin features seen in can indeed be thought
of in terms of {\em spin resonance} that develops in HO state and is {\em not
related} to superconducting transition at 1.5K. In our analysis we assume that
the HO gap is due to a particle-hole condensate that connects nested parts of
the Fermi surface with nesting vector . Within this approach we can
predicted the behavior of the spin susceptibility at \bQ^* and find it to be
is strikingly similar to the phenomenology of resonance peaks in high-T and
heavy fermion superconductors. The energy of the resonance peak scales with
. We discuss observable consequences
spin resonance will have on neutron scattering and local density of states.Comment: 8 pgaes latex, 4 fig
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