1. Broad-Band Dielectric Spectroscopy
We are able to perform dielectric studies utilizing a large variety of techniques which nearly continuously cover an exceptionally broad frequency range from 3 µHz up to 360 THz. Different cooling and heating devices allow for measurements from 1.2 to 1000 K. Moreover, measurements in external magnetic field up to 14 T are possible.
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Overview of the devices and techniques used in the dielectric laboratory at the
University of Augsburg for broadband dielectric spectroscopy. [adapted from: P. Lunkenheimer et al., in "Structural Glasses and Supercooled Liquids: Theory, Experiment, and Applications", edited by P.G. Wolynes and V. Lubchenko (Wiley, Hoboken, 2012)]. |
In the coaxial reflection technique (up to 3 GHz) the sample is mounted at the end of a coaxial line, thereby bridging inner and outer conductor (e.g., (b) in the figure below). A special variant is the open-end coaxial reflection method where the end of a coaxial line is completely immersed into the liquid under investigation. This method works up to about 40 GHz.
In the coaxial transmission technique the sample completely fills the space between inner and outer conductor of a coaxial line ((c) in the figure below), whose transmission properties are determined. This method was especially adapted in our group to also enable the investigation of low-loss samples. For frequencies up to 40 GHz, network analyzers are employed.
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Schematic plots of typical sample geometries used for the different measurement techniques
(the sample material is indicated by the filled black regions): (a) parallel-plate capacitor
for low-frequency measurements; (b) coaxial reflection setup where the sample forms the end
of the inner conductor (also other configurations can be used); (c) coaxial transmission line,
filled with sample material; (d) scheme of Mach-Zehnder spectrometer used in the submillimeter
wavelength range; (e) transmission measurements in the Fourier transform infrared spectrometer. [from: P. Lunkenheimer et al., in "Structural Glasses and Supercooled Liquids: Theory, Experiment, and Applications", edited by P.G. Wolynes and V. Lubchenko (Wiley, Hoboken, 2012)]. |
For more details on linear dielectric techniques employed in our group see, e.g.:
R. Böhmer, M. Maglione, P. Lunkenheimer, and A. Loidl
J. Appl. Phys. 65, 901 (1989).
U. Schneider, P. Lunkenheimer, A. Pimenov, R. Brand, and A. Loidl
Ferroelectrics 249, 89 (2001).
P. Lunkenheimer, M. Köhler, S. Kastner, and A. Loidl
in "Structural Glasses and Supercooled Liquids:
Theory, Experiment, and Applications", edited by P.G. Wolynes and V. Lubchenko (Wiley, Hoboken, 2012).
2. Nonlinear Dielectric Measurements
In broadband dielectric spectroscopy, usually relatively low voltages of up to several volts are applied to the samples. Thus, these measurements are performed in the linear regime of the P(E) (polarization vs. field) curve. In addition, a variety of field-dependent and pyroelectric measurements can be performed in our group. These methods can be used to investigate, e.g., P(E) hysteresis loops or spontaneous polarization in ferroelectrics or multiferroics. Also the non-linear properties of supercooled liquids are investigated, which helps understanding the dynamic processes at the glass transition.
Dielectric spectra can be collected applying maximum ac voltages of 4000 V (peak to peak) corresponding to electrical fields of up to several 100 kV/cm, depending on sample thickness and electric strength of the material. In this way, in addition to the conventional dielectric susceptibility at high fields, its higher-harmonic components can also be detected.
In addition, polarization vs. field hysteresis-curves can be measured, also allowing for a determination of the high-field dielectric permittivity, including its higher-harmonic components, at voltages up to 10 kV. Pyrocurrent measurements allow for the determination of field-cooled/zero-field-cooled polarizations.
Devices: