Browsing by Author "Dressel, Martin (Prof. Dr. rer. nat.)"

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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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    Microwave investigation of low-dimensional organic conductors
    (2003) Petukhov, Konstantin; Dressel, Martin (Prof. Dr. rer. nat.)
    We present measurements of electrodynamical response of (TMTSF)2PF6, a representative member of the organic quasi-one dimensional Bechgaard salts, in both the normal (T > 12 K) and spin density wave state (T < 12 K). We report on the investigations performed along the chain axis "a", and transverse crystallographic axes "b'" and "c*". For the measurements along the "b'" axis the single crystals samples were utilized for the first time. These measurements have been made at 24 and 33.5 GHz frequency and were anticipated by the DC transport measurements. For the measurements at microwave frequencies the contactless cavity perturbation technique, employing the cylindrical copper cavity operated in the TE011 transmission mode, was developed. The design of the cavities allowed us to rotate the sample inside it, resulting in a precise sample alignment. The measurements were collected in the temperature range from 300 K down to 2 K. The normal state transport properties of (TMTSF)2PF6 were analyzed in the framework of Luttinger liquid and Fermi liquid. The strong discrepancies forward the description within the Luttinger liquid picture were evidenced. The formation of the SDW ground state in all three directions was observed below 12 K. The thermal activation energy value at zero frequency (21 K) is in a good agreement with literature single particle gap value, while at the microwave frequencies the valuable reduction of the activation behavior (6 K) was observed along the "a" and "b'" crystallographic axes. Such behavior is indicative of a strongly frequency-dependent response and was attributed to the vicinity of the SDW pinned mode in the microwave frequency range. The low-temperature anomalies, below T < 3.5 K, were evidenced deeply in the SDW state at 33.5 GHz, and we believe that these anomalies are due to the exceeding of the threshold electric field when performing the microwave measurements.
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    Optische Eigenschaften des Spinketten-Spinleiter-Systems (Sr, Ca, La) 14 Cu 24 O 41
    (2004) Haas, Philipp; Dressel, Martin (Prof. Dr. rer. nat.)
    In dieser Arbeit wurde systematisch der Einfluß von Ca- und La-Substitution im Spinketten-Spinleiter-System (Sr, Ca, La)14Cu24O41 auf die temperaturabhängigen (5 - 300 K) optischen Spektren im Submillimeter bis infraroten Bereich (5 - 10000 cm-1) entlang der 3 Kristallrichtungen untersucht. Dieses System ist bei hoher Ca-Konzentration unter Druck supraleitend und eng mit den Hochtemperatur-Supraleitern verwandt. Ca-Substitution transferiert Löcher (Ladungsträger) von den Ketten in die Leitern. Im Rahmen dieser Arbeit konnte die Ausbildung einer Ladungsdichte-Welle entlang der Leiterbeine und der Leitersprossen bei niedrigen Ca-Konzentration nachgewiesen werden, die mit zunehmender Ca-Konzentration unterdrückt wird und somit in Konkurrenz zur Supraleitung steht. La-Substitution entfernt Löcher aus dem System, es konnte ein Hopping-Transport im Ketten-Untersystem gezeigt werden.
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    Optische Leitfähigkeit dünner Goldfilme im infraroten Spektralbereich
    (2008) Brandt, Tobby; Dressel, Martin (Prof. Dr. rer. nat.)
    Die optische Leitfähigkeit von Goldfilmen mit Schichtdicken von 9nm bis hin zu einer Monolagenbedeckung wurde im Frequenzbereich zwischen 600 Wellenzahlen bis 6000 Wellenzahlen bei Temperaturen vom 300K bis 4,5K aus Reflexionsmessungen mittels Fouriertransformationsspektroskopie ermittelt. Die Goldfilme wurden unter UHV Bedingungen auf Si(111)Au-7x7-Oberflächen präpariert, mit LEED charakterisiert und optisch untersucht. Ein Metall-Isolator Übergang sowohl in der Frequenz- als auch in der Temperaturabhängigkeit der Leitfähigkeit wurde bei einer nominellen Schichtdicke von 2nm beobachtet. Für Filmdicken größer 3nm lassen sich die Goldfilme mit der Drudetheorie beschreiben. Bei diesen metallischen Goldfilmen wurden frequenzabhängige Sizeeffekte in der Leitfähigkeit identifiziert. Bei Auswertung der Streurate wurde weiterhin ein Anstieg der Debeye-Temperatur in Abhängigkeit der Filmdicke beobachtet. Für Filmdicken kleiner 2nm wird ein ausgeprägtes Maximum in der frequenzabhängigen Leitfähigkeit beobachtet. Generell steigt die Leitfähigkeit in Abhängigkeit zur inversen Filmdicke stark an. Die Frequenz- und Temperaturabhängigkeit der Leitfähigkeit dieser granularen, nicht geschlossenen Filme lässt sich nicht wiederspruchsfrei mit einer der bekannten Theorien zum Elektronentransport beschreiben.