2,188,849 research outputs found

    Frequency Scaling of Microwave Conductivity in the Integer Quantum Hall Effect Minima

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    We measure the longitudinal conductivity σxx\sigma_{xx} at frequencies 1.246GHzf10.051.246 {\rm GHz} \le f \le 10.05 GHz over a range of temperatures 235mKT4.2235 {\rm mK} \le T \le 4.2 K with particular emphasis on the Quantum Hall plateaus. We find that Re(σxx)Re(\sigma_{xx}) scales linearly with frequency for a range of magnetic field around the center of the plateaus, i.e. where σxx(ω)σxxDC\sigma_{xx}(\omega) \gg \sigma_{xx}^{DC}. The width of this scaling region decreases with higher temperature and vanishes by 1.2 K altogether. Comparison between localization length determined from σxx(ω)\sigma_{xx}(\omega) and DC measurements on the same wafer show good agreement.Comment: latex 4 pages, 4 figure

    Delocalized Chern character for stringy orbifold K-theory

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    In this paper, we define a stringy product on K^*_{orb}(\XX) \otimes \C , the orbifold K-theory of any almost complex presentable orbifold \XX. We establish that under this stringy product, the de-locaized Chern character ch_{deloc} : K^*_{orb}(\XX) \otimes \C \longrightarrow H^*_{CR}(\XX), after a canonical modification, is a ring isomorphism. Here H^*_{CR}(\XX) is the Chen-Ruan cohomology of \XX. The proof relies on an intrinsic description of the obstruction bundles in the construction of Chen-Ruan product. As an application, we investigate this stringy product on the equivariant K-theory KG(G)K^*_G(G) of a finite group GG with the conjugation action. It turns out that the stringy product is different from the Pontryajin product (the latter is also called the fusion product in string theory).Comment: 34 pages. Final version to appear in Trans. of AMS. Improve the expositions and Change of the title thanks the referee

    Radiative corrections to the excitonic molecule state in GaAs microcavities

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    The optical properties of excitonic molecules (XXs) in GaAs-based quantum well microcavities (MCs) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift ΔXXMC\Delta^{\rm MC}_{\rm XX} and radiative width ΓXXMC\Gamma^{\rm MC}_{\rm XX}, are large, about 103010-30 % of the molecule binding energy ϵXX\epsilon_{\rm XX}, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1λ\lambda-mode and 0λ\lambda-mode cavity polaritons. The later decay channel, ``excitonic molecule \to 0λ\lambda-mode polariton + 0λ\lambda-mode polariton'', deals with the short-wavelength MC polaritons invisible in standard optical experiments, i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy ϵXX0.91.1\epsilon_{\rm XX} \simeq 0.9-1.1 meV, is ΓXXMC0.20.3\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3 meV in the microcavities and ΓXXQW0.1\Gamma^{\rm QW}_{\rm XX} \simeq 0.1 meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the T2=2T1T_2 = 2 T_1 limit, relevant to the XX states, holds at temperatures below 10 K, and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning, and propose to use the critical points for high-precision measurements of the molecule bindingenergy and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.

    Profile statt Rankings: Eine Methode zur Darstellung von Aktivitäten institutioneller Einheiten der Kommunikationswissenschaft

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    Evaluationen und Rechenschaftspflicht für Forschende nehmen im Hochschulumfeld zu; gerade in Feldern der Geistes- und Sozialwissenschaften stoßen die klassischen Instrumente – oft auf der Basis bibliometrischer Analysen – auf Grenzen, da sie wichtige Eigenschaften der Felder, wie z. B. die Ausrichtung auf das lokale und sprachregionale Umfeld, die hohe Lehrbelastung oder das Gewicht der Publikationen in Buchform nicht berücksichtigen. In diesem Text stellen wir ein Instrument vor, das entwickelt wurde, um den Forschungsoutput kommunikationswissenschaftlicher Einheiten an Schweizer Hochschulen darzustellen. Dieses Instrument erstellt Forschungsprofile, welche die Aktivitäten in verschiedenen Dimensionen – Wissenschaft, Forschungsausbildung, Lehre und Transfer – abbilden. Solche Profile können als Grundlage für interne Standortbestimmungen, aber auch als Input für (Selbst-)Evaluationen verwendet werden. Der Text präsentiert das Instrument und diskutiert Herausforderungen bei seiner Erstellung und Implementation, illustriert anhand von Resultaten der Studie