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

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

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

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

Delocalized Chern character for stringy orbifold K-theory

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 $K^*_G(G)$ of a finite group $G$ 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

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 $\Delta^{\rm MC}_{\rm XX}$ and radiative width $\Gamma^{\rm MC}_{\rm XX}$, are large, about $10-30$ of the molecule binding energy $\epsilon_{\rm XX}$, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1$\lambda$-mode and 0$\lambda$-mode cavity polaritons. The later decay channel, ``excitonic molecule $\to$ 0$\lambda$-mode polariton + 0$\lambda$-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 $\epsilon_{\rm XX} \simeq 0.9-1.1$ meV, is $\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3$ meV in the microcavities and $\Gamma^{\rm QW}_{\rm XX} \simeq 0.1$ meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the $T_2 = 2 T_1$ 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.