Spatial patterns mediated by subcritical and supercritical thermal fluctuations in the splay Fréedericksz transition

The magnetically induced splay Fréedericksz transition is reexamined to look for pattern forming phenomena slightly above or below criticality. By using our traditional scheme of stochastic nematodynamic equations, situations are, respectively, found of transient and permanent predominance of transversal periodicities (wave numbers) along the direction perpendicular to the initial orientation within the sample. The relevance of these predictions in relation with recent observations in the electrically driven splay Fréedericksz transition, and in general with other pattern forming phenomena, is stressed.

State-to-state reaction probabilities within the quantum transition state framework

Rigorous quantum dynamics calculations of reaction rates and initial state-selected reaction probabilities of polyatomic reactions can be efficiently performed within the quantum transition state concept employing flux correlation functions and wave packet propagation utilizing the multi-configurational time-dependent Hartree approach. Here, analytical formulas and a numerical scheme extending this approach to the calculation of state-to-state reaction probabilities are presented. The formulas derived facilitate the use of three different dividing surfaces: two dividing surfaces located in the product and reactant asymptotic region facilitate full state resolution while a third dividing surface placed in the transition state region can be used to define an additional flux operator. The eigenstates of the corresponding thermal flux operator then correspond to vibrational states of the activated complex. Transforming these states to reactant and product coordinates and propagating them into the respective asymptotic region, the full scattering matrix can be obtained. To illustrate the new approach, test calculations study the D + H2(ν, j) → HD(ν′, j′) + H reaction for J = 0.

Lateglacial evolution of the Brenno glacier (Southern Swiss Alps) : preliminary results

The Lateglacial evolution of the Ticino glacier and tributaries is poorly known because of the lack of research by Quaternary geomorphologists during the last decades. In spite of the interest for the cryosphere reactions during the Lateglacial climate warming, only few scientific studies were carried out about the history of the northern valleys of the Ticino Alps during the deglaciation (e.g. Seiffert 1953, Renner 1982, Hantke 1983). Within the framework of geomorphological investigations on the Lateglacial and Holocene glacier/permafrost evolution in the Ticino Alps, the history of the Brenno glacier (Blenio Valley, Eastern Ticino Alps) during the end of the Pleistocene has been studied. The deglaciation sequence of the Blenio Valley is still not complete (Scapozza et al. 2009). Only the first glacial stadial of the Brenno glacier and the last Lateglacial stadials of the Greina region (northern Blenio valley, see Fontana et al. 2008) and of the upper Malvaglia Valley (eastern Blenio Valley, see Scapozza et al. 2008) have been unequivocally defined. For every stadial, the surface of the palaeoglacier and the depression of the Equilibrium Line Altitude (ELA) have been reconstructed on the base of geomorphological mapping. The first individual glacial stadial of the Brenno glacier corresponds to the Biasca stadial of the Ticino glacier defined by Hantke (1983). The ELA depression of 1100-1200 meters and its morphological and glaciological characteristics allow us to correlate this stadial with the Weissbad stadial defined by Keller (1988). In the Greina region, three stadials corresponding to the end of the Lateglacial have been identified, with an ELA depression of 110, 210 and 310-350 meters (Fontana et al. 2008). In the upper Malvaglia Valley, three stadials corresponding to the end of the Oldest Dryas and the Younger Dryas have been identified for the Orino glacier, with an ELA depression of 290, 400-420 and 470-560 meters (Scapozza et al. 2008). If we consider the other (fragmentary) glacial deposits of the Blenio Valley, it is possible to define a regression sequence of the Brenno glacier with 8 stadials, from the Biasca stadial to the end of the Younger Dryas. An attempt of correlation with the model "Gothard" developed by Renner (1982) and Hantke (1983) and with the model "Eastern Swiss Alps" developed by Maisch (1982) is proposed in Table 1. The following chronological conclusions are, therefore, proposed: (1) the Biasca stadial is probably the first stadial after the transition Pleniglacial - Lateglacial; (2) the stadials BRE 7 to BRE 3 are positioned between the beginning of the Lateglacial and the Bølling-Allerød interstadial; (3) the stadials BRE 2 and BRE 1 are assumed to be related to the Younger Dryas event.

Fréedericksz transition in a periodic magnetic field

We study the Fréedericksz transition in a twist geometry under the effect of a periodic modulation of the magnitude of the applied magnetic field. We find a shift of the effective instability point and a time-periodic state with anomalously large orientational fluctuations. This time-periodic state occurs below threshold and it is accompanied by a dynamically stabilized spatial pattern. Beyond the instability the emergence of a transient pattern can be significantly delayed by a fast modulation, allowing the observation of pattern selection by slowing down the reorientational dynamics.

Dynamics of orientational fluctuations in a homeotropic Fréedericksz transition under a rotating magnetic field

We study the problem of the Fréedericksz transition under a rotating magnetic field by using a dynamical model which incorporates thermal fluctuations into the whole set of nematodynamic equations. In contrast to other geometries, nonuniform textures in the plane of the sample do not appear favored. The proper consideration of thermal noise enables us to describe the dynamics of orientational fluctuations both below and above the shifted instability.

Dynamics of transient domain structures in nematic liquid crystals: The splay Fréedericksz transition

We discuss the dynamics of the transient pattern formation process corresponding to the splay Fréedericksz transition. The emergence and subsequent evolution of the spatial periodicity is here described in terms of the temporal dependence of the wave numbers corresponding to the maxima of the structure factor. Situations of perpendicular as well as oblique field-induced stripes relative to the initial orientation of the director are both examined with explicit indications of the time scales needed for their appearance and posterior development.

Ionic-covalent transition in titanium oxides

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, Ti2O3, and TiO2 series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, TiO2 is better viewed as a rather covalent oxide.

Transient dynamics of orientational fluctuations in the Fréedericksz transition

Transient dynamics of spatial fluctuations of the director field in the pure twist Fréedericksz transition is studied. A nonlinear calculation is presented. Anomalous transient fluctuations are shown. Different stages of evolution and the domain of validity of linear theories are discussed.

X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model.

A cryptic heterogametic transition revealed by sex-linked DNA markers in Palearctic green toads.

In sharp contrast to birds and mammals, most cold-blooded vertebrates have homomorphic (morphologically undifferentiated) sex chromosomes. This might result either from recurrent X-Y recombination (occurring e.g. during occasional events of sex reversal) or from frequent turnovers (during which sex-determining genes are overthrown by new autosomal mutations). Evidence for turnovers is indeed mounting in fish, but very few have so far been documented in amphibians, possibly because of practical difficulties in identifying sex chromosomes. Female heterogamety (ZW) has long been established in Bufo bufo, based on sex reversal and crossing experiments. Here, we investigate a sex-linked marker identified from a laboratory cross between Palearctic green toads (Bufo viridis subgroup). The F(1) offspring produced by a female Bufo balearicus and a male Bufo siculus were phenotypically sexed, displaying an even sex ratio. A sex-specific marker detected in highly reproducible AFLP genotypes was cloned. Sequencing revealed a noncoding, microsatellite-containing fragment. Reamplification and genotyping of families of this and a reciprocal cross showed B. siculus to be male heterogametic (XY) and suggested the same system for B. balearicus. Our results thus reveal a cryptic heterogametic transition within bufonid frogs and help explain patterns of hybrid fitness within the B. viridis subgroup. Turnovers of genetic sex-determination systems may be more frequent in amphibians than previously thought and thus contribute to the prevalence of homomorphic sex chromosomes in this group.