7,092 research outputs found

    Large signal 2nd harmonic on wafer MESFET characterization

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    An automatic test set which performs a real time harmonic load-pull characterization is proposed. An active load technique is used in order to set the load at the test frequency and its harmonics and a complete set of device parameters useful for power amplifier design purposes can be measured versus the harmonic loads. The calibration procedure, based on substrate and coaxial standards, has been mainly developed for on wafer measurement in order to set the reference planes directly on the DU

    Save the "THRU" in the A.N.A. calibration

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    The conventional network analyzer (NWA) two-port calibration procedures require a standard thru line to be connected between the ports. Unfortunately in many applications, for example when measuring MMIC or on-wafer devices with not aligned ports, a custom thru line must be used. The procedure here applied overcomes the difficulty due to the poor knowledge of this thru element since it is based on a generic reciprocal unknown two port structure, provided that its S21 phase shift is roughly known. Some experimental comparisons with other well sound calibration techniques will be here presented where different reciprocal two-port structures were used as unknown thr

    Semi-transparent Boundary Conditions in the Worldline Formalism

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    The interaction of a quantum field with a background containing a Dirac delta function with support on a surface of codimension 1 represents a particular kind of matching conditions on that surface for the field. In this article we show that the worldline formalism can be applied to this model. We obtain the asymptotic expansion of the heat-kernel corresponding to a scalar field on Rd+1\mathbb{R}^{d+1} in the presence of an arbitrary regular potential and subject to this kind of matching conditions on a flat surface. We also consider two such surfaces and compute their Casimir attraction due to the vacuum fluctuations of a massive scalar field weakly coupled to the corresponding Dirac deltas.Comment: 12 page

    Spectral functions of non essentially selfadjoint operators

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    One of the many problems to which J.S. Dowker devoted his attention is the effect of a conical singularity in the base manifold on the behavior of the quantum fields. In particular, he studied the small-tt asymptotic expansion of the heat-kernel trace on a cone and its effects on physical quantities, as the Casimir energy. In this article we review some peculiar results found in the last decade, regarding the appearance of non-standard powers of tt, and even negative integer powers of log⁥t\log{t}, in this asymptotic expansion for the selfadjoint extensions of some symmetric operators with singular coefficients. Similarly, we show that the ζ\zeta-function associated to these selfadjoint extensions presents an unusual analytic structure.Comment: 57 pages, 1 figure. References added. Version to appear in the special volume of Journal of Physics A in honor of Stuart Dowker's 75th birthday. PACS numbers: 02.30.Tb, 02.30.Sa, 03.65.D

    Scalar Field with Robin Boundary Conditions in the Worldline Formalism

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    The worldline formalism has been widely used to compute physical quantities in quantum field theory. However, applications of this formalism to quantum fields in the presence of boundaries have been studied only recently. In this article we show how to compute in the worldline approach the heat kernel expansion for a scalar field with boundary conditions of Robin type. In order to describe how this mechanism works, we compute the contributions due to the boundary conditions to the coefficients A_1, A_{3/2} and A_2 of the heat kernel expansion of a scalar field on the positive real line.Comment: Presented at 8th Workshop on Quantum Field Theory Under the Influence of External Conditions (QFEXT 07), Leipzig, Germany, 16-21 Sep 200

    Entanglement between pairing and screening in the Gorkov-Melik-Barkhudarov correction to the critical temperature throughout the BCS-BEC crossover

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    The theoretical description of the critical temperature Tc of a Fermi superfluid dates back to the work by Gor'kov and Melik-Barkhudarov (GMB), who addressed it for a weakly-coupled (dilute) superfluid in the BCS (weak-coupling) limit of the BCS-BEC crossover. The point made by GMB was that particle-particle (pairing) excitations, which are responsible for superfluidity to occur below Tc, and particle-hole excitations, which give rise to screening also in a normal system, get effectively disentangled from each other in the BCS limit, thus yielding a reduction by a factor 2.2 of the value of Tc obtained when neglecting screening effects. Subsequent work on this topic, aimed at extending the original GMB argument away from the BCS limit with diagrammatic methods, has kept this disentangling between pairing and screening throughout the BCS-BEC crossover, without realising that the conditions for it to be valid are soon violated away from the BCS limit. Here, we reconsider this problem from a more general perspective and argue that pairing and screening are intrinsically entangled with each other along the whole BCS-BEC crossover but for the BCS limit considered by GMB. We perform a detailed numerical calculation of the GMB diagrammatic contribution extended to the whole BCS-BEC crossover, where the full wave-vector and frequency dependence occurring in the repeated in-medium two-particle scattering is duly taken into account. Our numerical calculations are tested against analytic results available in both the BCS and BEC limits, and the contribution of the GMB diagrammatic term to the scattering length of composite bosons in the BEC limit is highlighted. We calculate Tc throughout the BCS-BEC crossover and find that it agrees quite well with Quantum Monte Carlo calculations and experimental data available in the unitarity regime.Comment: 21 pages, 11 figure