Balázs Fekete

Suppression of spatially periodic patterns by dc voltage

The effect of superposed dc and ac applied voltages on two types of spatially periodic instabilities in nematic liquid crystals, flexoelectric domains (FD), and electroconvection (EC) was studied. The onset characteristics, threshold voltages, and critical wave vectors were determined. We found that in general the superposition of driving with different time symmetries inhibits the pattern forming mechanisms for FD and EC as well. As a consequence, the onset extends to much higher voltages than the individual dc or ac thresholds. A dc-bias-induced reduction of the crossover frequency from the conductive to the dielectric EC regimes and a peculiar transition between two types of flexodomains with different wavelengths were detected. Direct measurements of the change of the electrical conductivity and its anisotropy, induced by the applied dc voltage component, showed that the dc bias substantially affects both parameters. Taking into account the experimentally detected variations of the conductivity in the linear stability analysis of the underlying nematohydrodynamic equations, a qualitative agreement with the experimental findings on the onset behavior of spatially periodic instabilities was obtained.

Patterns driven by combined ac and dc electric fields in nematic liquid crystals

The effect of superimposed ac and dc electric fields on the formation of electroconvection and flexoelectric patterns in nematic liquid crystals was studied. For selected ac frequencies, an extended standard model of the electrohydrodynamic instabilities was used to characterize the onset of pattern formation in the two-dimensional parameter space of the magnitudes of the ac and dc electric field components. Numerical as well as approximate analytical calculations demonstrate that depending on the type of patterns and on the ac frequency, the combined action of ac and dc fields may either enhance or suppress the formation of patterns. The theoretical predictions are qualitatively confirmed by experiments in most cases. Some discrepancies, however, seem to indicate the need to extend the theoretical description.

Inhibited pattern formation by asymmetrical high voltage excitation in nematic fluids

In contrast to the predictions of the standard theory of electroconvection (EC), our experiments showed that the action of superposed ac and dc voltages rather inhibits pattern formation than favors the emergence of instabilities; the patternless region may extend to much higher voltages than the individual ac or dc thresholds. The pattern formation induced by such asymmetrical voltage was explored in a nematic liquid crystal in a wide frequency range. The findings could be qualitatively explained for the conductive EC, but represent a challenging problem for the dielectric EC.

Influence of dc voltage on the dielectric properties of nematics.

We report on precise impedance measurements, with the aim of exploring the influence of a dc bias voltage on the dielectric permittivity and electrical conductivity of liquid crystals and on their anisotropies. We prove that the dielectric permittivity is not affected by a dc bias; however, the electrical conductivity suffers a substantial reduction upon increasing the superposed dc voltage. Moreover, we show that the relative conductivity anisotropy also diminishes at increasing dc bias. Introduction Nematic liquid crystals are anisotropic dielectric materials with a low electrical conductivity, which originates from a small concentration of ionic contaminants. Experimental studies of the electrooptical properties of nematics, as well as most display applications of them utilize ac driving voltage; investigations under dc voltage are rather scarce. Recently, electroconvection and flexoelectric patterns occurring under superposed action of ac and dc voltages have been studied and, unexpectedly, an inhibition of the pattern formation at high voltages (an increase of the thresholds) was found [1-2]. The fact that conductivity is an important factor determining the threshold voltage, motivated the present testing of the effect of dc bias on the electrical conductivity. Experimental Method Measurements were performed on the compound 4-n-octyl-oxyphenyl 4-n-methyloxybenzoate (1OO8), having a nematic phase in the temperature range between 53 C and 77 C and a chemical structure shown in Fig. 1. It is a material used for studying pattern formation in electric field [2-3]. Precise impedance measurements were carried out with the dielectric analyzer Novocontrol Alpha equipped with a ZG4 test interface. The liquid crystal was filled into a plane condenser (glass plates with transparent electrodes of A = 1 cm size, separated with a gap of d = 1 mm). The electrodes were coated with polyimide layer, in order to mimic the electrical boundary conditions of the thinner commercial cells employed in pattern formation studies. The cell was placed in a shielded chamber, with its temperature kept at T = 58 ± 0.05 C. Fig. 1. The chemical structure of the nematic 4-n-octyloxyphenyl 4-n-methyloxybenzoate (1OO8). As liquid crystals are anisotropic materials, the dielectric permittivity (electrical conductivity) can be characterized by the quantities ε|| (σ||) and ε⊥ (σ⊥), measured with an electric field E parallel with and perpendicular to the director n, respectively. These two orthogonal experimental geometries can be easily realized, if the chamber is put into a magnetic field H. For the large cell thickness used, a magnetic induction value of B = 1 T is large enough to align the director parallel to H. Thus applying Dielectric Materials and Applications: ISyDMA’2016 Materials Research Forum LLC Materials Research Proceedings 1 (2016) 42-44 doi: http://dx.doi.org/10.21741/2474-395X/1/11 43 a magnetic field parallel to the electrode surface (H, n ⊥ E), the perpendicular component is obtainable; when the chamber is rotated so that the magnetic field becomes normal to the electrodes (H, n || E), the parallel component can be measured. The complex impedances of the empty and the liquidcrystal-containing cell were measured at the frequency of 1 kHz, with a probing ac rms voltage of Uac = 0.2 V. The liquid crystal sample was interpreted as a parallel RC circuit, with

How to Become a Judge in Hungary? From the Professionalism of the Judiciary to the Political Ties of the Constitutional Court

This chapter discusses the rules for the selection of Hungarian judges and constitutional court justices. Besides providing an in-depth analysis of the relevant legal provisions, it also aims to shed light on their broader socio-political context. In conclusion, this paper argues that the selection of constitutional court justices qualitatively differs from that of ordinary judges. In the case of judges, mostly professional requirements prevail, while the appointment of constitutional court justices is deeply pervaded by political motives. Therefore, it can be argued that the political preferences of the government could certainly distort the selection of the members of the constitutional court, whilst professionalism is certainly a value of the ordinary judiciary. Lastly, it should also be mentioned that the representation of social and political diversity is definitely not a main criterion of judicial selection in general.

The Revival of Comparative Law in a Socialist Country: The Impact of Imre Szabó and Gyula Eörsi on the Development of Hungarian Comparative Law

This article discusses the revival of comparative law in Hungarian Socialist jurisprudence. Prior to World War II, the development of comparative law generally had followed international trends; however, it was disrupted at both a personal and an institutional level at the end of the 1940s due to the Marxist-Leninist turn of legal thinking that accompanied the introduction of a Communist regime in the country. Nonetheless, this rejection of comparative law was gradually replaced by a more open attitude that strongly supported participation in the international comparative-law movement from the 1960s. Imre Szabo and Gyula Eorsi played a prominent role in this transformation. They legitimized the use of comparative methods in Socialist jurisprudence and, also, created a plausible conceptual framework for Socialist comparative law.