1,844 research outputs found

    Genus two partition functions of chiral conformal field theories

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    A systematic analysis of the genus two vacuum amplitudes of chiral self-dual conformal field theories is performed. It is explained that the existence of a modular invariant genus two partition function implies infinitely many relations among the structure constants of the theory. All of these relations are shown to be a consequence of the associativity of the operator product expansion, as well as the modular covariance properties of the torus one-point functions. U sing these techniques we prove that for the proposed extremal conformal field theories at c = 24k a consistent genus two vacuum amplitude exists for all k, but that this does not actually check the consistency of these theories beyond what is already testable at genus one

    Goodness beyond Reason

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    Raman investigations on single crystals and polycristalline pressed samples of organic superconducters

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    The investigation of molecular vibrations is a powerful tool which can increase our knowledge on structures and on electron molecular vibrations, which are due to charge oscillation between dimerized moIecuIes,coupled with totally symmetric intramolecular modes. Raman scattering studies account for totally symmetric vibrations. In addition, Raman spectroscopy can take advantage of resonant effects. In fact, when resonant conditions are fulfilled, selected molecular vibrations are obtained as well as information on the electronic manifold involved in the resonance process. In this paper, we report on Raman investigations of polycrystalline pressed materials of superconducting tempered -α(BEDT-TTF)2I3, called in the following, αt-{BEDT-TTF)2I3 and on untempered α(BEDT-TTF)2I3. Furthermore we report on the polarized resonant Raman scattering in the superconducting regime of (BEDT-TTF)2Cu(NCS)2 single crystals

    Temperature-dependent High-Frequency Performance of Deep Submicron Ion-Implanted AlGaN/GaN HEMTs

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    A study of the low temperature DC and RF performance of deep submicron AlGaN/GaN high electron mobility transistors (HEMTs) is reported. From 300 K to 100 K both extrinsic transconductance and drain current increase by 30%, mainly due to the lowering of the optical phonon scattering that allows higher electron mobility. Source and drain resistances improve too, which contributes to the 15-20% increase of ft and fmax. The low temperature small signal model has also been extracted accurately at every 50 K. Inductances and capacitances remain constant in the range of temperatures measured. The intrinsic transconductance can be also considered temperature independent, but the output conductance decreases from 300 K to 100 K indicating a better confinement of the 2DEG. The HEMT performance obtained at 100 K can be reached at room temperature by reducing the parasitic resistances and improving the GaN buffer isolation
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