Oscillations, Waves, and Interactions - GWDG

14 M. R. Schroeder, D. Guicking and U. Kaatze
12 Organic conductors and deformed semiconductors
The transition from more applied research in radio-frequency and microwave techniques
to applications of such techniques in basic research also happened by the
orientation towards organic conductors. Interest in these materials springs from
their molecular structure which promotes electrical conductivity in one direction.
This confinement to quasi-one-dimensional behaviour opens insights in interesting
phenomena which do not exist or can hardly be studied in three dimensions [159].
Additionally, one-dimensional conductors are considered potential materials for applications.
Hans-Wilhelm Helberg and his group investigated a multitude of organic conductors,
semiconductors, and superconductors, comprising polymers, charge transfer
complexes, and doped materials [160–164]. DC and microwave conductivity as well
as electric permittivity measurements were performed in wide ranges of frequency
(0.5 to 60 GHz) and temperature (1.7 to 700 K) in order to elucidate the mechanisms
of electronic excitation and conductivity. Intra- and intermolecular excitations have
been considered using polarisation microscope techniques in the visible and near infrared
spectral ranges to determine the indicatrix orientation and the directions of
optical absorption. One-dimensional solids are further on studied by Martin Dressel,
a former student of the Helberg group, at the University of Stuttgart [159].
13 Biophysics
Biologically inspired or medically motivated topics were considered by several groups
of the institute. Molecular aspects apply to head group reorientations of zwitterions,
hydrocarbon chain isomerisations, domain structure fluctuations and defect formations
in lipid membranes [165–167] as well as to the molecular dynamics and conformational
kinetics of carbohydrates in solution [168,169]. Some of these investigations
benefited much from close cooperation with colleagues at the Max-Planck-Institut
für Biophysikalische Chemie [170–174].
A quite different approach to biophysics was the treatment of nonlinear differential
equations to gain a better understanding of the biological clock [175]. The numerical
solution of the homogeneous and inhomogeneous van der Pol equation was aimed
at entrainment-synchronization phenomena of a self-excited oscillator by an external
one.
Increasing demands in the monitoring of organ properties during ischemia sparked
interest in the non-invasive dielectric spectroscopy of organ tissue [176] and in the
modelling of the electrical impedance of cellular media [177].
In the 1980s use of focused pulses from a neodynium:YAG laser enabled noninvasive
intraocular surgery by photodisruption. This new instrument called for a
careful investigation of acoustical transients and cavitation bubbles by laser-induced
breakdown in liquids on conditions relevant to ophthalmologic applications [178,179].
Lithotripter shock waves cause cavitation inside the human body. The action of
cavitation bubbles leads to tissue damage as side effect of narrow-focus extracorporal
shock wave lithotripsy [103]. Shock waves and ultrasound may also enhance the
transient permeability of cell membranes and may facilitate drug delivery without