15 Fakultätsübergreifend / Sonstige Einrichtung

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/16

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Author: search.filters.author.Donavan, Steve

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    Microwave cavity perturbation technique. Part 3, Applications
    (1993) Dressel, Martin; Klein, Olivier; Donavan, Steve; Grüner, George
    The resonant cavity perturbation method as described in the preceding two parts of this series is applied to study the electrodynamical properties of different materials in the microwave and millimeter wave spectral range. We briefly discuss the relevant uncertainties which are asociated with the different measurement techniques and we find that employing the amplitude technique it is possible to measure both the width and frequency to nearly the same precision. We then demonstrate the broad range of applicability of this technique by showing results obtained on several different materials, ranging from an insulator to a superconductor. The performance limitations of this technique are discussed in detail.
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    Microwave cavity perturbation technique. Part 1, Principles
    (1993) Klein, Olivier; Donavan, Steve; Dressel, Martin; Grüner, George
    This report reviews the analysis used to extract the complex conductivity of a compound from a microwave cavity perturbation measurement. We intend to present a generalized treatment valid for any spheroidally shaped sample of arbitrary conductivity which is placed at either the electric or magnetic field antinode of the cavity. To begin with, we establish the relationship between the measured parameters and the conductivity for a spherical sample. Next, we extend these results to the case of spheroids; and for the first time, we cover all different configurations that one can possibly use to study an arbitrary conducting sample inside a cavity: in particular, all possible orientations of the sample with respect to the applied field are solved.
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    The electrodynamics of the spin density wave ground state : optical experiments on (TMTSF)2PF6
    (1994) Donavan, Steve; Kim, Yong; Degiorgi, Leonardo; Dressel, Martin; Grüner, George; Wonneberger, Wolfgang
    Conductivity measurements are reported in the organic linear-chain compound (TMTSF)2PF6 , in both the metallic and spin-density-wave states. The components of the complex conductivity were established by measurements in the radio-frequency, micro- and millimeter wave, and infrared spectral ranges. At temperatures above the spin-density-wave transition, a Drude-like metallic behavior was found together with a temperature-independent feature at higher frequencies. An observed Drude scattering rate of 3 cm-1 was found, placing the material well into the clean limit. In the spin-density-wave state, the low field dc resistivity shows an activated behavior similar to a standard semiconductor with a gap value 2Δ/kB≈45 K. The ac response shows a strong frequency dependence, and most importantly, we observe two subgap modes: a very broad one in the radio frequency range, due to internal deformations of the spin density wave, and a narrow mode near 0.1 cm-1, which we interpret as the response of the q=O phason. Furthermore, as expected for a material in the clean limit, we do not see evidence for a single particle gap in the infrared spectral ange. In this paper, we will compare our experimental results with the various models of spindensity-wave dynamics and comment on the current status of the understanding of the dynamical response of spin density waves.
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    Microwave cavity perturbation technique. Part 2, Experimental scheme
    (1993) Donavan, Steve; Klein, Olivier; Dressel, Martin; Holczer, Karoly; Grüner, George
    In this paper, the second in a three part series, we describe an experimental scheme used to measure the electrodynamical response of a material in the millimeter wave range of frequency. In particular, with this technique we can directly evaluate the complex conductivity from a measurement ofboth the bandwidth and characteristic frequency of a resonator containing the specimen. We will describe in detail all the technical improvements achieved which provide the required accuracy.