Light-Front-Quantized QCD in Covariant Gauge

The light-front (LF) canonical quantization of quantum chromodynamics in covariant gauge is discussed. The Dirac procedure is used to eliminate the constraints in the gauge-fixed front form theory quantum action and to construct the LF Hamiltonian formulation. The physical degrees of freedom emerge naturally. The propagator of the dynamical $\psi_+$ part of the free fermionic propagator in the LF quantized field theory is shown to be causal and not to contain instantaneous terms. Since the relevant propagators in the covariant gauge formulation are causal, rotational invariance---including the Coulomb potential in the static limit---can be recovered, avoiding the difficulties encountered in light-cone gauge. The Wick rotation may also be performed allowing the conversion of momentum space integrals into Euclidean space forms. Some explicit computations are done in quantum electrodynamics to illustrate the equivalence of front form theory with the conventional covariant formulation. LF quantization thus provides a consistent formulation of gauge theory, despite the fact that the hyperplanes $x^{\pm}=0$ used to impose boundary conditions constitute characteristic surfaces of a hyperbolic partial differential equation.Comment: LaTex, 16 page

Lattice thermal conduction in ultra-thin nanocomposites

This is the final version of the article. Available from the publisher via the DOI in this record.We have studied the lattice thermal conductivity of Si/Ge periodic nanocomposites (superlattice, nanowire, and nanodot structures) of sample sizes in the range of 30 nm-30 μm, periodicities 1.1 nm and 2.2 nm, with reasonably dirty interfaces, and n-type doping concentration in the range of 1023-1026m-3. Our calculations employ a judicious combination of ab initio and physically sound semi-empirical methods for detailed calculations of estimates of phonon scattering rates due to anharmonicity and interface formation. Based upon our results we conclude that the formation of ultra-thin nanocomposites in any of the three structures is capable of reducing the conductivity below the alloy limit. This can be explained as a result of combination of the sample length dependence, the on-set of mini-Umklapp three-phonon processes, mass mixing at the interfaces between Si and Ge regions, and the sample doping level.We are grateful to the EPSRC (UK) for supporting this project via the Grant Award No.r EP/H046690/1. Quantum Espresso calculations were performed on the Intel Nehalem (i7) cluster (ceres) at the University of Exeter. I.O.T. also acknowledges support from the John Templeton Foundation as part of the Durham Emergence Project during the final stages of this work

Shell model description of Ge isotopes

A shell model study of the low energy region of the spectra in Ge isotopes for $38\leq N\leq 50$ is presented, analyzing the excitation energies, quadrupole moments, $B(E2)$ values and occupation numbers. The theoretical results have been compared with the available experimental data. The shell model calculations have been performed employing three different effective interactions and valence spaces.We have used two effective shell model interactions, JUN45 and jj44b, for the valence space $f_{5/2} \, p \,g_{9/2}$ without truncation. To include the proton subshell $f_{7/2}$ in valence space we have employed the $fpg$ effective interaction due to Sorlin {\it et al.}, with $^{48}$Ca as a core and a truncation in the number of excited particles.Comment: 10 pages, 10 figures, Proc. of the XXXV Nuclear Physics Symposium, January 3-6 2012, Cocoyoc, Morelos, Mexico. IOP Journal of Physics: Conference Series (in press

Quantum atomic delocalization vs. structural disorder in amorphous silicon

Quantum effects on the atom delocalization in amorphous silicon have been studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum delocalization is appreciable vs. topological disorder, as seen from structural observables such as the radial distribution function (RDF). At low temperatures, the width of the first peak in the RDF increases by a factor of 1.5 due to quantum effects. The overall anharmonicity of the solid vibrations at finite temperatures in amorphous silicon is clearly larger than in the crystalline material. Low-energy vibrational modes are mainly located on coordination defects in the amorphous material.Comment: 5 pages, 5 PS figures, REVTE

Transcriptomics profiling of Indian mustard (Brassica juncea) under arsenate stress identifies key candidate genes and regulatory pathways

© 2015 Srivastava, Srivastava, Sablok, Deshpande and Suprasanna. Arsenic (As) is a non-essential element, a groundwater pollutant, whose uptake by plants produces toxic effects. The use of As-contaminated groundwater for irrigation can affect the crop productivity. Realizing the importance of the Brassica juncea as a crop plant in terms of oil-yield, there is a need to unravel mechanistic details of response to As stress and identify key functional genes and pathways. In this research, we studied time-dependent (4–96 h) transcriptome changes in roots and shoots of B. juncea under arsenate [As(V)] stress using Agilent platform. Among the whole transcriptome profiled genes, a total of 1,285 genes showed significant change in expression pattern upon As(V) exposure. The differentially expressed genes were categorized to various signaling pathways including hormones (jasmonate, abscisic acid, auxin, and ethylene) and kinases. Significant effects were also noticed on genes related to sulfur, nitrogen, CHO, and lipid metabolisms along with photosynthesis. Biochemical assays were conducted using specific inhibitors of glutathione and jasmonate biosynthesis, and kinases. The inhibitor studies revealed interconnection among sulfur metabolism, jasmonate, and kinase signaling pathways. In addition, various transposons also constituted a part of the altered transcriptome. Lastly, we profiled a set of key functional up- and down-regulated genes using real-time RT-PCR, which could act as an early indicators of the As stress

Human papillomavirus 16/18 and nasopharyngeal carcinoma

Sixteen cases of nasopharyngeal carcinoma (eight anaplastic and eight well differentiated squamous types) were examined for the presence of human papillomavirus types 16 and 18 genomes using the polymerase chain reaction on paraffin wax embedded biopsy specimens. Although nasopharyngeal carcinoma, particularly the anaplastic type, is strongly associated with Epstein-Barr virus, other factors may be involved in its pathogenesis. No DNA of either human papillomavirus subtype was detected. It is concluded, therefore, that these two 'high risk' types of human papillomavirus are not implicated in the pathogenesis of nasopharyngeal carcinoma. The number of cases in this series was small, however, and further studies are warranted using fresh biopsy material and including other viral subtypes.published_or_final_versio