DOI 10.1140/epje/i2001-10097-3 Eur. Phys. J. E 7, 387–392 (2002) T HE EUROPEAN P HYSICAL JOURNAL E c  EDP Sciences Societ`a Italiana di Fisica Springer-Verlag 2002 Glassy and fluidlike behavior of the isotropic phase of n-cyanobiphenyls in broad-band dielectric relaxation studies S.J. Rzoska 1, a , M. Paluch 1 , A. Drozd-Rzoska 1 , J. Ziolo 1 , P. Janik 1 , and K. Czupry´ nski 2 1 Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland 2 Military Technical University, ul. S. Kaliskiego, Warsaw, Poland Received 22 November 2001 Abstract. It is shown that the temperature behavior of peaks ( fp,ε ′′ p ) of dielectric loss curves in the isotropic phase of n-cyanobiphenyls (n =8, 9, 10) with isotropic-nematic and isotropic-smectic A transitions exhibits features characterisic for both supercooled, glass-forming liquids and critical, binary mixtures. The behavior of f p (T ) can be portrayed by the Vogel-Fulcher-Tamman relation and the “critical-like”, mode- coupling theory (MCT) equation. The latter is supported by the novel analysis of electric conductivity σ (T ). The obtained f p (T ) and σ (T ) dependencies can be related by using the fractional Debye-Einstein- Stokes law. For all tested mesogens the static dielectric permittivities ε ′ (T ) and ε ′′ p (T ) are described by dependencies resembling those applied in the homogeneous phase of critical mixtures but with specific-heat critical exponent α ≈ 0.5. This behavior agrees with the novel fluidlike description for the isotropic-nematic transition (P.K. Mukherjee, Phys. Rev. E 51, 5745 (1995); A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)). The obtained glassy features of dielectric relaxation support the recent simulation analysis carried out by M. Letz et al. (Phys. Rev. E 62, 5173 (2000)). PACS. 64.70.Md Transitions in liquid crystals – 64.70.Pf Glass transitions – 64.60.Ht Dynamic critical phenomena Supercooling and glass-forming liquids are still one of the great mysteries of the soft-condensed-matter physics. One of their characteristic features is the strong, non- Arrhenius, slowing-down of the primary “alpha” relax- ation time on cooling. For portraying this dependence the Vogel-Fulcher-Tamman (VFT) relation is usually ap- plied [1–4]: τ VFT = τ ∞ exp  DT 0 T − T 0  , (1) where the singular temperature T 0 is the ideal glass tem- perature and D is the fragility parameter classifying glass- forming materials. The glass temperature T g can be taken from dielec- tric tests: T g = T (τ VFT = 100 s) [1]. It is noteworthy that recent studies showed that T g might also be a singular temperature in the VFT relation for nonlinear dielectric relaxation times [5]. The mode-coupling theory (MCT) in- troduced another singularity [1–4, 6]: τ = τ 0 (T − T X ) −γ for T>T X + 20 K , (2) where T X denotes the MCT “singular” temperature ( T 0 < T g <T X ) describing the hypothetical crossover from the a e-mail: rzoska@us.edu.pl ergodic to non-ergodic regimes [1]. The power exponent 2 <γ< 4 is a non-universal parameter [1–4,6]. The non-Debye shape of the absorption curve and its broadening on cooling constitute the other typical features of glass-forming liquids [1–4]. At first sight the above fea- tures seem to be absent in dielectric tests in the isotropic phase of liquid crystalline materials with the isotropic- nematic (I-N) transition, for instance in such “classical” rod-like materials as 4-cyano-4-n -alkylbiphenyls (n CB). The vast majority of references suggest the almost-Debye shape of absorption curves and the clear Arrhenius evolu- tion of relaxation times ([7–13] and Refs. therein). These results coincide with the simple mean-field description of the isotropic phase, which dominated until recently [9– 14]. However, in the opinion of the authors the experi- mental situation is ambiguous. Firstly, the majority of re- sults covers a small range of temperatures above the I-N clearing point [7–13]. Secondly, there are few results in- dicating the possible validity of the VFT dependence of dielectric relaxation times, mainly in mesophases [9, 15]. Finally, the possibilities of the mean-field approximation for describing the surrounding of the I-N transition seems to be limited [16–18,20–23]. In the nineties novel experi- mental [17, 18, 20–24] and theoretical [16, 18, 19, 23] studies showed the possible complex liquid [18, 22, 23, 25], fluidlike