466 research outputs found
Constructing the Cubic Interaction Vertex of Higher Spin Gauge Fields
We propose a method of construction of a cubic interaction in massless Higher
Spin gauge theory both in flat and in AdS space-times of arbitrary dimensions.
We consider a triplet formulation of the Higher Spin gauge theory and
generalize the Higher Spin symmetry algebra of the free model to the
corresponding algebra for the case of cubic interaction. The generators of this
new algebra carry indexes which label the three Higher Spin fields involved
into the cubic interaction. The method is based on the use of oscillator
formalism and on the Becchi-Rouet-Stora-Tyutin (BRST) technique. We derive
general conditions on the form of cubic interaction vertex and discuss the
ambiguities of the vertex which result from field redefinitions. This method
can in principle be applied for constructing the Higher Spin interaction vertex
at any order. Our results are a first step towards the construction of a
Lagrangian for interacting Higher Spin gauge fields that can be holographically
studied.Comment: Published Version; comments added in introduction; minor typos and
references correcte
First principles study on the segregation of metallic solutes and non-metallic impurities in Cu grain boundary
Metallic dopants have the potential to increase the mechanical strength of
polycrystalline metals. These elements are expected to aggregate in regions of
lower coordination, such as grain boundaries. At the grain boundaries, they can
have a beneficial (toughening) or detrimental effect (e.g. grain boundary
embrittlement). In this study, we employ Density Functional Theory (DFT) to
compute the segregation energies of various metallic and other non-metallic
elements to determine their effect when introduced in a symmetric Cu grain
boundary. The study results may be used to qualitatively rank the beneficial
effect of certain metallic elements, such as V, Zr, and Ag, as well as the
strong weakening effect of non-metallic impurities like O, S, F and P.
Furthermore, the induced local distortion is found to be proportional to the
weakening effect of the elements
BRST approach to Lagrangian formulation of bosonic totally antisymmeric tensor fields in curved space
We apply the BRST approach, previously developed for higher spin field
theories, to gauge invariant Lagrangian construction for antisymmetric massive
and massless bosonic fields in arbitrary d-dimensional curved space. The
obtained theories are reducible gauge models both in massless and massive cases
and the order of reducibility grows with the value of the rank of the
antisymmetric field. In both the cases the Lagrangians contain the sets of
auxiliary fields and possess more rich gauge symmetry in comparison with
standard Lagrangian formulation for the antisymmetric fields. This serves
additional demonstration of universality of the BRST approach for Lagrangian
constructions in various field models.Comment: 12 page
On manifolds admitting the consistent Lagrangian formulation for higher spin fields
We study a possibility of Lagrangian formulation for free higher spin bosonic
totally symmetric tensor field on the background manifold characterizing by the
arbitrary metric, vector and third rank tensor fields in framework of BRST
approach. Assuming existence of massless and flat limits in the Lagrangian and
using the most general form of the operators of constraints we show that the
algebra generated by these operators will be closed only for constant curvature
space with no nontrivial coupling to the third rank tensor and the strength of
the vector fields. This result finally proves that the consistent Lagrangian
formulation at the conditions under consideration is possible only in constant
curvature Riemann space.Comment: 11 pages; v2: minor typos corrected, a reference adde
Energies and structures of Cu/Nb and Cu/W interfaces from density functional theory and semi-empirical calculations
Cu/Me multilayer systems, with Me referring to a body-centered cubic () metal, such as Nb and W, are widely used for nuclear, electrical, and electronic applications. Despite making up only a small percentage of the volume, interfaces in such systems play a major role in determining their electrical, mechanical, thermal and diffusion properties. Face-centered cubic () Cu often forms Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) type interfaces with metals or variations thereof. For the Cu/Nb system, these interface relationships have been extensively studied with semi-empirical methods. Surprisingly, the energetics and interface properties of Cu/W have not yet been studied in detail, in spite of extensive applications. In this study, we employ both periodic Embedded Atom Method (EAM) and Density Functional Theory (DFT) simulations to explore the geometric and energetic properties of the KS and NW interfaces of Cu/Nb and Cu/W. To assess the reliability of our approach, the dependence of the results on the size of periodic cells is examined for coherent and incoherent interfaces. We provide the interface energies and the work of separation for the Cu/W and Cu/Nb interfaces at DFT accuracy. The results of calculations with two EAM potentials are in qualitative agreement with those obtained using DFT and allow investigating the convergence of interfacial properties. These key energetic quantities can be used for future thermodynamic and mechanical modeling of Cu/Me interfaces
Likelihood-ratio ranking of gravitational-wave candidates in a non-Gaussian background
We describe a general approach to detection of transient gravitational-wave
signals in the presence of non-Gaussian background noise. We prove that under
quite general conditions, the ratio of the likelihood of observed data to
contain a signal to the likelihood of it being a noise fluctuation provides
optimal ranking for the candidate events found in an experiment. The
likelihood-ratio ranking allows us to combine different kinds of data into a
single analysis. We apply the general framework to the problem of unifying the
results of independent experiments and the problem of accounting for
non-Gaussian artifacts in the searches for gravitational waves from compact
binary coalescence in LIGO data. We show analytically and confirm through
simulations that in both cases the likelihood ratio statistic results in an
improved analysis.Comment: 10 pages, 6 figure
Remarks on Two-Loop Free Energy in N=4 Supersymmetric Yang-Mills Theory at Finite Temperature
The strong coupling behavior of finite temperature free energy in N=4
supersymmetric SU(N) Yang-Mills theory has been recently discussed by Gubser,
Klebanov and Tseytlin in the context of AdS-SYM correspondence. In this note,
we focus on the weak coupling behavior. As a result of a two-loop computation
we obtain, in the large N 't Hooft limit, . Comparison with the strong
coupling expansion provides further indication that free energy is a smooth
monotonic function of the coupling constant.Comment: 5 pages, 1 figure; final form, Physical Review
Exploring improved holographic theories for QCD: Part I
Various holographic approaches to QCD in five dimensions are explored using
input both from the putative non-critical string theory as well as QCD. It is
argued that a gravity theory in five dimensions coupled to a dilaton and an
axion may capture the important qualitative features of pure QCD. A part of the
higher alpha' corrections are resummed into a dilaton potential. The potential
is shown to be in one-to-one correspondence with the exact beta-function of
QCD, and its knowledge determines the full structure of the vacuum solution.
The geometry near the UV boundary is that of AdS_5 with logarithmic corrections
reflecting the asymptotic freedom of QCD. We find that all relevant confining
backgrounds have an IR singularity of the "good" kind that allows unambiguous
spectrum computations. Near the singularity the 't Hooft coupling is driven to
infinity. Asymptotically linear glueball masses can also be achieved. The
classification of all confining asymptotics, the associated glueball spectra
and meson dynamics are addressed in a companion paper, ArXiv:0707.1349Comment: 37+23 pages, 11 figures. (v3) Some clarifications and typo
corrections. Journal versio
Structure and Migration Mechanisms of Small Vacancy Clusters in Cu: A Combined EAM and DFT Study
Voids in face-centered cubic (fcc) metals are commonly assumed to form via the aggregation of vacancies; however, the mechanisms of vacancy clustering and diffusion are not fully understood. In this study, we use computational modeling to provide a detailed insight into the structures and formation energies of primary vacancy clusters, mechanisms and barriers for their migration in bulk copper, and how these properties are affected at simple grain boundaries. The calculations were carried out using embedded atom method (EAM) potentials and density functional theory (DFT) and employed the site-occupation disorder code (SOD), the activation relaxation technique nouveau (ARTn) and the knowledge led master code (KLMC). We investigate stable structures and migration paths and barriers for clusters of up to six vacancies. The migration of vacancy clusters occurs via hops of individual constituent vacancies with di-vacancies having a significantly smaller migration barrier than mono-vacancies and other clusters. This barrier is further reduced when di-vacancies interact with grain boundaries. This interaction leads to the formation of self-interstitial atoms and introduces significant changes into the boundary structure. Tetra-, penta-, and hexa-vacancy clusters exhibit increasingly complex migration paths and higher barriers than smaller clusters. Finally, a direct comparison with the DFT results shows that EAM can accurately describe the vacancy-induced relaxation effects in the Cu bulk and in grain boundaries. Significant discrepancies between the two methods were found in structures with a higher number of low-coordinated atoms, such as penta-vacancies and di-vacancy absortion by grain boundary. These results will be useful for modeling the mechanisms of diffusion of complex defect structures and provide further insights into the structural evolution of metal films under thermal and mechanical stress
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