12,609 research outputs found

    Thermoelectric Amplification of Phonons in Graphene

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    Amplification of acoustic phonons due to an external temperature gredient (T\nabla T) in Graphene was studied theoretically. The threshold temperature gradient (T)0g(\nabla T)_0^{g} at which absorption switches over to amplification in Graphene was evaluated at various frequencies ωq\omega_q and temperatures TT. For T=77KT = 77K and frequency ωq=12THz\omega_q = 12THz, (T)0g=0.37Km1(\nabla T)_0^{g} = 0.37Km^{-1}. The calculation was done in the regime at ql>>1ql >> 1. The dependence of the normalized (Γ/Γ0\Gamma/\Gamma_0) on the frequency ωq\omega_q and the temperature gradient (T/T)(\nabla T/T) are evaluated numerically and presented graphically. The calculated (T)0g(\nabla T)_0^{g} for Graphene is lower than that obtained for homogeneous semiconductors (nInSbn-InSb) (T)0hom103Kcm1(\nabla T)_0^{hom} \approx 10^3Kcm^{-1}, Superlattices (T)0SL=384Kcm1(\nabla T)_0^{SL} = 384Kcm^{-1}, Cylindrical Quantum Wire (T)0cqw102Kcm1(\nabla T)_0^{cqw} \approx 10^2Kcm^{-1}. This makes Graphene a much better material for thermoelectric phonon amplifier.Comment: 12 Pages, 6 figure

    Relating on-shell and off-shell formalism in perturbative quantum field theory

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    In the on-shell formalism (mostly used in perturbative quantum field theory) the entries of the time ordered product T are on-shell fields (i.e. the basic fields satisfy the free field equations). With that, (multi)linearity of T is incompatible with the Action Ward identity. This can be circumvented by using the off-shell formalism in which the entries of T are off-shell fields. To relate on- and off-shell formalism correctly, a map sigma from on-shell fields to off-shell fields was introduced axiomatically by Duetsch and Fredenhagen. In that paper it was shown that, in the case of one real scalar field in N=4 dimensional Minkowski space, these axioms have a unique solution. However, this solution was given there only recursively. We solve this recurrence relation and give a fully explicit expression for sigma in the cases of the scalar, Dirac and gauge fields for arbitrary values of the dimension N.Comment: The case of gauge fields was added. 16 page

    Technology Development in Water Resource Management

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    In developed nations, technological advancements rapidlyare changing every aspect of our lives: how we work, howwe communicate with each other, and even how we areentertained. The influence of technology is readilyapparent. The subtleties of technology and its lessapparent influences also are tremendous. Advances incomputer technology, communications, and manufacturingare affecting all sectors, including engineering. In thearena of water resources, technological advances havehelped to not only develop a better understanding of ourphysical systems but have allowed improved operationaland institutional tools to be developed to support watermanagement. These advances have the potential toprovide large and meaningful benefits to less developedinfrastructures.The objective of this paper will be to describe severaltechnological advances in water resources, specifically inareas such as Flood Warning, Water Administration, andMulti-Objective Water Management. Examples oftechnologies implemented which have direct applicabilityto developing infrastructures are described. Important tothe discussion of these advances are the ways in whichimplementation and use of these technologies can protectand save lives, extend and optimally use limited resources,and provide useful inform ation to assist in the sustainab legrowth and development of our natural resources

    Causal perturbation theory in terms of retarded products, and a proof of the Action Ward Identity

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    In the framework of perturbative algebraic quantum field theory a local construction of interacting fields in terms of retarded products is performed, based on earlier work of Steinmann. In our formalism the entries of the retarded products are local functionals of the off shell classical fields, and we prove that the interacting fields depend only on the action and not on terms in the Lagrangian which are total derivatives, thus providing a proof of Stora's 'Action Ward Identity'. The theory depends on free parameters which flow under the renormalization group. This flow can be derived in our local framework independently of the infrared behavior, as was first established by Hollands and Wald. We explicitly compute non-trivial examples for the renormalization of the interaction and the field.Comment: 76 pages, to appear in Rev. Math. Phy

    Explicit Zeta Functions for Bosonic and Fermionic Fields on a Noncommutative Toroidal Spacetime

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    Explicit formulas for the zeta functions ζα(s)\zeta_\alpha (s) corresponding to bosonic (α=2\alpha =2) and to fermionic (α=3\alpha =3) quantum fields living on a noncommutative, partially toroidal spacetime are derived. Formulas for the most general case of the zeta function associated to a quadratic+linear+constant form (in {\bf Z}) are obtained. They provide the analytical continuation of the zeta functions in question to the whole complex ss-plane, in terms of series of Bessel functions (of fast, exponential convergence), thus being extended Chowla-Selberg formulas. As well known, this is the most convenient expression that can be found for the analytical continuation of a zeta function, in particular, the residua of the poles and their finite parts are explicitly given there. An important novelty is the fact that simple poles show up at s=0s=0, as well as in other places (simple or double, depending on the number of compactified, noncompactified, and noncommutative dimensions of the spacetime), where they had never appeared before. This poses a challenge to the zeta-function regularization procedure.Comment: 15 pages, no figures, LaTeX fil

    Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars

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    We investigate the stability of differentially rotating proto-neutron stars (PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric MHD instabilities are derived using a local linear analysis. PNSs are expected to have much stronger radial shear in the rotation velocity compared to normal stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a large azimuthal wavenumber mm is dominant over the kink mode (m=1m=1) in differentially rotating PNSs. The growth rate of the NMRI is of the order of the angular velocity Ω\Omega which is faster than that of the kink-type instability by several orders of magnitude. The stability criteria are analogous to those of the axisymmetric magnetorotational instability with a poloidal field, although the effects of leptonic gradients are considered in our analysis. The NMRI can grow even in convectively stable layers if the wavevectors of unstable modes are parallel to the restoring force by the Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify the magnetic fields and drive MHD turbulence in PNSs, which may lead to enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical Journal (December 12, 2005

    Continuity of the four-point function of massive ϕ44\phi_4^4-theory above threshold

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    In this paper we prove that the four-point function of massive \vp_4^4-theory is continuous as a function of its independent external momenta when posing the renormalization condition for the (physical) mass on-shell. The proof is based on integral representations derived inductively from the perturbative flow equations of the renormalization group. It closes a longstanding loophole in rigorous renormalization theory in so far as it shows the feasibility of a physical definition of the renormalized coupling.Comment: 23 pages; to appear in Rev. Math. Physics few corrections, two explanatory paragraphs adde

    A note on supersymmetric Yang-Mills thermodynamics

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    The thermodynamics of supersymmetric Yang-Mills theories is studied by computing the two-loop correction to the canonical free energy and to the equation of state for theories with 16, 8 and 4 supercharges in any dimension 4d104\leq d\leq 10, and in two dimensions at finite volume. In the four-dimensional case we also evaluate the first non-analytic contribution in the 't Hooft coupling to the free energy, arising from the resummation of ring diagrams. To conclude, we discuss some applications to the study of the Hagedorn transition in string theory in the context of Matrix strings and speculate on the possible physical meaning of the transition.Comment: 19 pages, harvmac, epsf. 1 figure included. Minor changes: typos corrected; references, a footonote and a note adde
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