Browsing by Author "Scheffler, Marc"

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    Broadband microwave spectroscopy on correlated electrons
    (2004) Scheffler, Marc; Dressel, Martin (Prof. Dr. rer. nat.)
    Heavy-fermion compounds are prime examples of strongly correlated electron systems. To study their electrodynamic properties, optical spectroscopy is an established tool and has shown that the characteristic energy scales of heavy fermions are much lower than those of normal metals. However, conventional spectrometers do not allow for investigations of the particularly interesting excitations of the mobile charge carriers, i.e. the heavy fermions, because the corresponding frequencies are in the microwave range: experiments using geometrical optics are not possible any more for these low frequencies, but the electromagnetic waves have to be guided, for example using coaxial cables. In the present study these problems were solved, and a broadband microwave spectrometer was developed that is particularly suited to study metals at low temperatures. It employs the Corbino geometry, i.e. the flat sample terminates the open end of a coaxial cable, thus reflecting the microwave signal propagating in the cable. The reflection coefficient then yields the electrodynamic properties of the sample. Unfortunately the microwave signal is also influenced by the transmission properties of the coaxial cable, and therefore one has to calibrate the spectrometer carefully. Here we measure up to three different calibration samples at the same low temperatures as the sample, i.e. we perform a full low-temperature calibration. Following this procedure, the precision of the obtained reflection coefficient is typically 0.001. The frequency range of the spectrometer is 45 MHz to 20 GHz and its temperature range 1.65 K to 300 K. Using this spectrometer, the microwave properties of the heavy-fermion system UPd2Al3 were studied, in particular the complex conductivity at low temperatures. Here a frequency dependence was observed that follows closely the prediction of the Drude model, but exhibits an extremely low scattering rate. This was expected for heavy fermions in general and for this particular material UPd2Al3, but here for the first time this could be unambiguously verified. The temperature dependence of the directly observable scattering rate scales with the temperature dependence of the resistivity, allowing a determination of the otherwise hardly accessible charge carrier density.
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    Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators
    (2023) Beydeda, Cenk; Nikolaou, Konstantin; Tochtermann, Marius; Ebensperger, Nikolaj G.; Untereiner, Gabriele; Farag, Ahmed; Karl, Philipp; Ubl, Monika; Giessen, Harald; Dressel, Martin; Scheffler, Marc
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    Formen und Kräfte : ein mathematisch-physikalischer Gang zur Kunst auf dem Campus Vaihingen
    (Stuttgart : Fakultät 8 - Mathematik und Physik, Universität Stuttgart, 2022) Stroppel, Markus; Scheffler, Marc; Engstler, Katja Stefanie; Engstler, Katja Stefanie (Konzept und Gestaltung)
    Der Rundgang erläutert und interpretiert einzelne Objekte und künstlerische Elemente der Lernstraße auf dem Campus Vaihingen aus mathematischer und physikalischer Sicht für die interessierte Allgemeinheit, aber auch für Schüler­innen und Schüler und für Studierende.