Dielectric Spectroscopy at High Frequencies on Glass Forming Liquids
- PDF / 703,982 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 104 Downloads / 210 Views
DIELECTRIC SPECTROSCOPY AT HIGH FREQUENCIES ON GLASS FORMING LIQUIDS P. LUNKENHEIMER', A. PIMENOV', M. DRESSEL', B. GORSI-UNOV"*, U. SCHNEIDER', B. SCHIENER2 , R. BOHMER 2'**,A. LOIDL' 2'Experimentalphysik V, Universitat Augsburg, Universitatsstr. 2, D-86 135 Augsburg, Germany Inst. fur Festkorperphysik, TH Darmstadt, Hochschulstr. 6, D-64289 Darmstadt, Germany ABSTRACT Dielectric spectroscopy up to 950 GHz has been performed on various glass formers as glycerol, propylene-carbonate, and Salol. Special attention is given to the dielectric loss, e", in the crossover regime from the nt-relaxation to the far-infrared (FIR) response where it can be directly compared to the dynamic susceptibilities obtained by neutron and light scattering techniques. We observe a minimum in s"(v) at high frequencies which cannot be explained by a simple transition from on-relaxation peak to the FIR bands but has to be attributed to additional fast processes. In all materials investigated, c"(v) increases significantly sublinear above the minimum. The ratio of the intensity of the nx-process and the fast process as determined from our dielectric experiments is significantly higher compared to the results from the scattering experiments. INTRODUCTION Stimulated by recent theoretical approaches, the fast dynamics in glass-forming liquids came into the focus of interest. Various scenarios have been proposed to describe or predict the dynamic susceptibility at high frequencies in the GHz-TI-z region [1-5]. Maybe the most controversially discussed approach is the mode coupling theory (MCT) of the glass transition [1] which makes distinct predictions for the high-frequency region. The relevant frequency range up to now has been investigated mainly by neutron and light scattering experiments (see, e.g., [611]). Only recently, we were able to extend the frequency range of dielectric experiments on glass-forming liquids up to 370 GHz [12] using backward wave oscillators as coherent sources of electromagnetic radiation [13]. These experiments, which have been performed on glycerol, for the first time revealed the existence of a relatively broad minimum in the frequency dependence of the dielectric loss, &"(v), at frequencies above GHz. Its functional form and temperature dependence cannot be explained by a simple crossover from the structural (ct-) relaxation to the FIR response but is indicative of additional fast processes prevailing in this glass-forming liquid. In the present paper we will provide results on glycerol in an extended frequency range up to 950 GHz. In addition we will show first results on two other molecular glass-formers: propylene carbonate (PC) and Salol. All results are compared to the susceptibilities as obtained from the scattering experiments, as well as to theoretical predictions. EXPERIMENT Broadband dielectric measurements involve the use of various techniques. At frequencies 10 1Hz:!_ v •_ I kI-z measurements were performed in the time domain using a spectrometer which is based on a design described by Mopsik [14]. The
Data Loading...
