BROADBAND DIELECTRIC SPECTRA

1. Introduction

Glassformers and plastic crystals reveal a rich "zoo" of dynamic processes (see figure below and our review articles [7,9,10,18,24]). The most common ones are the α relaxation, the excess-wing or the β-relaxation, the fast process predicted by mode-coupling theory and the boson peak. By collecting dielectric spectra of different glassformers in a frequency range of more than 18 decades, we were able to investigate all those processes. The understanding of such extreme broadband spectra is a challenge for every theory of glassy dynamics.

Fig. 1: Frequency dependence of the dielectric loss at a temperature close to the glass transition (a) in a prototypical type-A glassformer with an excess-wing and (b) in a type-B glassformer with a canonical Johari-Goldstein β relaxation ('slow β'). [from: P. Lunkenheimer and A. Loidl, in The scaling of relaxation processes, edited by F. Kremer and A. Loidl (Springer, Cham, 2018), p. 23].

In addition, we have also studied a variety of other materials by broadband dielectric spectroscopy. This includes, e.g., the investigation of the many different dynamic processes in biological matter or the examination of the charge-carrier dynamics and Maxwell-Wagner relaxations in various electronic and ionic conductors.

2. Examples

The following figures show some examples of glass-forming liquids measured by us in an extremely broad frequency range:

a) Type A glass-forming liquids

Fig. 2: Broadband dielectric-loss spectra of type-A glass formers revealing an excess-wing. All spectra are shown on double-logarithmic scales for a series of temperatures, ranging from the low-viscosity regime down to the glass-transition temperature and below. (a) R=17.3% LiCl in water solution [21], (b) glycerol [5,9] and (c) salol [23]. [from: P. Lunkenheimer and A. Loidl, in The scaling of relaxation processes, edited by F. Kremer and A. Loidl (Springer, Cham, 2018), p. 23.]

b) Type B glass-forming liquids

Fig. 3: Broadband dielectric-loss spectra of type-B glass formers revealing a β relaxation. (a) tri-propylene glycol (TPG) [14], (b) xylitol [15] and (c) sorbitol [15,25]. In sorbitol, the dielectric loss is traced far down below the glass temperature, where we found some evidence for the Gardner transition, theoretically predicted to occur deep in the glassy state [25]. [from: P. Lunkenheimer and A. Loidl, in The scaling of relaxation processes, edited by F. Kremer and A. Loidl (Springer, Cham, 2018), p. 23.]

c) Plastic crystals

Fig. 4: Broadband dielectric-loss spectra of plastic-crystalline cyclo-octanol for various temperatures. [from: R. Brand, P. Lunkenheimer, and A. Loidl, Relaxations and fast dynamics of the plastic crystal cyclo-octanol investigated by broadband dielectric spectroscopy, Phys. Rev. B 56, R5713 (1997).]

Fig. 5: Broadband dielectric-loss spectra of plastic-crystalline ortho-carborane for various temperatures. [from: P. Lunkenheimer and A. Loidl, Glassy dynamics beyond the α-relaxation, in Broadband Dielectric Spectroscopy, edited by F. Kremer and A. Schönhals (Springer, Berlin, 2002), chapter 5; see also ref. [2].]

d) Conducting materials

Fig. 6: Broadband conductivity spectra at various temperatures for three different disordered materials. While the top shows the spectra for a dipolar glass former (propylene carbonate [6,7]), the data below present results for conducting materials: the ionically conducting ionic melt [Ca(NO3)2]0.4[KNO3]0.6 [3,9,10] and the electronic semiconductor Pr0.65(Ca0.8Sr0.2)0.35MnO0.35 [8]. [from: P. Lunkenheimer and A. Loidl, Response of disordered matter to electromagnetic fields, Phys. Rev. Lett. 91, 207601 (2003).]

Fig. 7: Broadband dielectric-loss spectra at various temperatures for the ionically conducting ionic melt [Ca(NO3)2]0.4[KNO3]0.6. [from: P. Lunkenheimer and A. Loidl, Glassy dynamics beyond the α-relaxation, in Broadband Dielectric Spectroscopy, edited by F. Kremer and A. Schönhals (Springer, Berlin, 2002), chapter 5.]

Fig. 8: Broadband dielectric-modulus spectra at various temperatures for [Ca(NO3)2]0.4[KNO3]0.6. This plot is based on the same data as shown in Fig. 7. Plots of the dielectric modulus lead to peaks for conducting materials, similar to the peaks in the dielectric loss found for dipolar liquids (cf. Figs. 1-5). However, the significance of this representation is controversially discussed. [from: P. Lunkenheimer and A. Loidl, Glassy dynamics beyond the α-relaxation, in Broadband Dielectric Spectroscopy, edited by F. Kremer and A. Schönhals (Springer, Berlin, 2002), chapter 5; see also refs. [3,9,10].]

3. Some relevant publications from our group: