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Spektroskopie dynamisch ungeordneter Festkörper
Prof. Dr. Roland Böhmer


Experimentelle Physik III - Arbeitsgruppe Prof. R. Böhmer

Nonresonant dielectric hole burning

Most of the experiments in our laboratory are performed in the linear regime in which the polarization response is proportional to the electrical excitation. In addition, we developed and applied non-linear techniques such as nonresonant dielectric hole burning. This pump and probe technique is able to distinguish homogeneous from heterogeneous scenarios of relaxation.
Developed together with Ralph V. Chamberlin from Arizona State University, this pump, wait, and probe experiment is able to reveal whether the broadening of a dielectric loss spectrum is due to a distribution of relaxation times or not. In the latter case the spectrum can be modified by a frequency selective pump employing very strong electrical fields with amplitudes in the kilovolt range. The altered correlation function is probed as the response to a small voltage step. The detection of the modified differential response is based on the shown phase cycle. This experiment can be considered as the low-frequency variant of related optical or nuclear-magnetic hole burning techniques. In combination with appropriate theoretical modeling this experiment has been successful in clarifying the nature of various relaxation processes in systems as diverse as supercooled liquids, relaxor ferroelectrics, orientationally disordered crystals, and ionic conductors, to name a few.

Selected references

B. Schiener, R. Böhmer, A. Loidl, R. V. Chamberlin, Nonresonant spectral hole burning in the slow dielectric response of supercooled liquids, Science 274, 752-754 (1996)
R. Richert, R. Böhmer, Heterogeneous and homogeneous diffusivity in an ion conducting glass, Phys. Rev. Lett. 83, 4337-4340 (1999)
R. Böhmer, G. Diezemann, Principles and applications of pulsed dielectric spectroscopy and nonresonant dielectric hole burning, in: Broadband dielectric spectroscopy, edited by F. Kremer and A. Schönhals (Springer, Berlin, 2002), p. 523-569

Multiple-time NMR correlation functions

Using the stimulated echo which can be pulse sequence of a radio-frequency pulse we study the slow dynamics, mostly via quadrupolar nucleus deuterium and lithium as probes. Hence we can generate signals of the type

(a) Recorded as a function of the mixing time tm the two-time correlation function F2 provides direct access to motional times scales.
(b) At a fixed mixing time this 3-pulse sequence can often be regarded as a low-pass filter which selects molecules or ions which did not move during tm. Using a fourth pulse the magnetization can be stored for latter use.
(c) The two-time correlation of the selected subensemble is tested via the G4(tm3) function.
(d) Another four-time correlation function is obtained by using the same low-pass filter twice. It enables us to detect whether the correlation time of a molecule or ion changes during the time interval tm2 and has given valuable information on dynamical heterogeneities in glass-forming materials and ion conductors.

Selected references

G. Hinze, R. Böhmer, G. Diezemann, H. Sillescu, Experimental determination of four-time stimulated echoes in liquids, colloidal suspensions, and crystals, J. Magn. Reson. 131, 218-223 (1998)
R. Böhmer, Multiple-time correlation functions from spin-3/2 solid-state NMR spectroscopy, J. Magn. Reson. 147, 78-88 (2000).