Proper Transition Research Articles

Elastic properties of minerals and the influence of phase transitions

Elastic anomalies that accompany cation ordering and displacive phase transitions can be understood in terms of coupling between strain components and the driving order parameter in Landau free energy expansions. Non-convergent cation ordering in spinel, MgAl 2 O 4 , is accompanied by changes in individual elastic constants, shear modulus, and bulk modulus that vary linearly with the order parameter. Convergent cation ordering, such as Al/Si ordering in anorthite, is expected to give changes in elastic properties that scale with the square of the order parameter. The elastic anomalies that develop in association with displacive phase transitions show greater diversity, due to the additional influence of the order parameter susceptibility. These are illustrated for the cases of the proper ferroelastic transition at high pressure in stishovite and the improper ferroelastic transition in SrTiO 3 perovskite. Low temperature transitions in lawsonite show a more complex pattern of softening and stiffening that depends on coupling with both cation ordering and displacive processes. Variations of the spontaneous strain and elastic constants are indicative of the underlying thermodynamic mechanism for a phase transition. If any such transitions occur in minerals of the Earth's crust or mantle they should be identifiable from their distinctive influence on seismic velocities.

Doublecortin-like Kinase Controls Neurogenesis by Regulating Mitotic Spindles and M Phase Progression

The mechanisms controlling neurogenesis during brain development remain relatively unknown. Through a differential protein screen with developmental versus mature neural tissues, we identified a group of developmentally enriched microtubule-associated proteins (MAPs) including doublecortin-like kinase (DCLK), a protein that shares high homology with doublecortin (DCX). DCLK, but not DCX, is highly expressed in regions of active neurogenesis in the neocortex and cerebellum. Through a dynein-dependent mechanism, DCLK regulates the formation of bipolar mitotic spindles and the proper transition from prometaphase to metaphase during mitosis. In cultured cortical neural progenitors, DCLK RNAi Lentivirus disrupts the structure of mitotic spindles and the progression of M phase, causing an increase of cell-cycle exit index and an ectopic commitment to a neuronal fate. Furthermore, both DCLK gain and loss of function in vivo specifically promote a neuronal identity in neural progenitors. These data provide evidence that DCLK controls mitotic division by regulating spindle formation and also determines the fate of neural progenitors during cortical neurogenesis.

Misexpression of the Caenorhabditis elegans miRNA let-7 Is Sufficient to Drive Developmental Programs

The Caenorhabditis elegans microRNAs (miRNAs) lin-4 and let-7 promote transitions between stage-specific events in development by down-regulating the translation of their target genes. Expression of let-7 is required at the fourth larval stage for the proper transition from larval to differentiated, adult fates in the hypodermis; however, it was not known whether expression of let-7 is sufficient to specify these adult fates. To test this, we created fusion genes between lin-4 and let-7 that direct the expression of let-7 two stages early, at the L2 stage. We find that animals bearing the fusion genes show precocious adult development at the L4 stage, indicating that temporal misexpression of let-7 is sufficient to direct the larval-to-adult transition. Additionally, an RNA interference (RNAi)-based screen for enhancers of the precocious phenotype identified the period ortholog lin-42, among other genes, which are candidate modulators of the effects of let-7 expression. let-7 is conserved throughout bilaterian phylogeny, and orthologs of its targets have roles in vertebrate development, suggesting the importance of understanding how let-7 promotes terminal differentiation in C. elegans and other organisms.

Groucho-Associated Transcriptional Repressor Ripply1 Is Required for Proper Transition from the Presomitic Mesoderm to Somites

Concomitant with the transition from the presomitic mesoderm (PSM) to somites, the periodical gene expression characteristic of the PSM is drastically changed and translated into the segmental structure. However, the molecular mechanism underlying this transition has remained obscure. Here, we show that ripply1, encoding a nuclear protein associated with the transcriptional corepressor Groucho, is required for this transition. Zebrafish ripply1 is expressed in the anterior PSM and in several newly formed somites. Ripply1 represses mesp-b expression in the PSM through a Groucho-interacting motif. In ripply1-deficient embryos, somite boundaries do not form, the characteristic gene expression in the PSM is not properly terminated, and the initially established rostrocaudal polarity in the segmental unit is not maintained, whereas paraxial mesoderm cells become differentiated. Thus, ripply1 plays dual roles in the transition from the PSM to somites: termination of the segmentation program in the PSM and maintenance of the rostrocaudal polarity.

Strain mechanism for order-parameter coupling through successive phase transitions inPrAlO3

High-resolution powder neutron diffraction data have been collected at $\ensuremath{\sim}50$ temperatures between 4.2 and 300 K for perovskites ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{AlO}}_{3}$ with $x=0$, 0.05, 0.1, and 0.25. Structural and lattice parameter data extracted by Rietveld refinement have then been used to follow details of octahedral tilting and spontaneous strains associated with the sequence of phase transitions $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c\ensuremath{\leftrightarrow}Imma\ensuremath{\leftrightarrow}C2∕m$. These are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a $Pm\overline{3}m$ reference structure with two instabilities (${\mathrm{R}}_{4}^{+}$ and ${\ensuremath{\Gamma}}_{3}^{+}$ active). Data from the literature relating to the $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c$ transition are consistent with second-order character and an extrapolated transition temperature of $1864\ifmmode\pm\else\textpm\fi{}31\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $R\overline{3}c\ensuremath{\leftrightarrow}Imma$ transition is first order in character and can be understood as occurring because coupling takes place between tilting and electronic order-parameter components via a common tetragonal strain. Strains for the $Imma\ensuremath{\leftrightarrow}C2∕m$ transition conform closely to the Landau solution for a proper ferroelastic transition with second-order character and low-temperature saturation. The acoustic anomaly reported at 118 K appears to be associated with a metrically tetragonal structure which develops, as an accidental strain degeneracy, at a temperature between $\ensuremath{\sim}110$ and $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Differences in saturation temperatures for the order parameters associated with tilting and electronic ordering are probably responsible for small additional anomalies in the evolution of strains below $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Profile analysis of expressed sequence tags derived from the ovary of tilapia, Oreochromis mossambicus

Precise regulation of gene expression is required for a proper transition through the different phases of the ovarian cycle. To study ovarian gene expression in teleosts, we constructed an adult ovary cDNA library of tilapia, Oreochromis mossambicus, and analyzed the gene expression profile using an expressed sequence tag-based strategy. Of 768 random clones, 530 clones, with insert lengths of > 100 base pairs (bp), were assembled into 474 tentative unique genes (TUGs), including 34 contigs and 440 singlets. Among these, 230 TUGs were highly homologous to known genes in public databases, while others exhibited a low sequence homology or no significant matches to known sequences; these are called novel genes here. The abundance of each identified gene was assayed to evaluate its relative importance in ovarian functions. In addition, we analyzed the expression patterns for 6 development-and reproduction-related genes, including cofilin, fibulin-4, glucosamin-6-phosphate deaminase 2 (GNPDA), MCM-6, stathmin, and zona pellucida C5 (ZPC5), as well as 9 novel genes. Results showed that GNPDA, MCM6, ZPC5, and 7 of the novel genes, including O34, O141, O148, O149, O243, O258, and O319, were predominantly expressed in the ovary and/or testis. In addition, the expression patterns of gonad-specific novel genes in tilapia larvae were differentially affected by treatment with 17 β-estradiol and 17 α-methyltestosterone during the sex-sensitive period.

Importance of proper choice of transition rates in kinetic simulations of dynamic processes

Proper transition rates in kinetic mean-field and Monte Carlo simulations of dynamic processes are very important to obtain realistic results. We show that by proper choice of the transition rates one can unify the advantages and eliminate the disadvantages of different models used in the literature [see, e.g., Senhaji et al., Surf. Sci. 274, 297 (1992 )( kinetic tight binding Ising model), Martin, Phys. Rev. B 41, 2279 (1990) and Cserhati et al., Surf. Sci. 290, 345 (1993)]. Furthermore, we also show that this choice cannot be considered simply as an extension of the previous ones. It contains an additional parameter controlling the ratio of the transition rates in the bulk and close to the surface sGsurf / Gbulkd, which influences the kinetics of surface segregation. We show that some results obtained previously need reconsideration. We illustrate, e.g., how the composition dependence of the transition rates (diffusion coefficient) influences the ‘surfactant formation and dissolution mode’. For example, the dissolution kinetics can deviate from the parabolic law on the nanoscale in accordance with our recent results [Erdelyi et al., Phys. Rev. B 69, 113407 (2004)]. Furthermore, we also present how the sGsurf / Gbulkd ratio affects the kinetic segregation isotherm.

Optical coherence tomography can measure axonal loss in patients with ethambutol-induced optic neuropathy

To map and identify the pattern, in vivo, of axonal degeneration in ethambutol-induced optic neuropathy using optical coherence tomography (OCT). Ethambutol is an antimycobacterial agent often used to treat tuberculosis. A serious complication of ethambutol is an optic neuropathy that impairs visual acuity, contrast sensitivity, and color vision. However, early on, when the toxic optic neuropathy is mild and partly reversible, the funduscopic findings are often subtle and easy to miss. Three subjects with a history of ethambutol (EMB)-induced optic neuropathy of short-, intermediate-, and long-term visual deficits were administered a full neuro-ophthalmologic examination including visual acuity, color vision, contrast sensitivity, and fundus examination. In addition, OCT (OCT 3000, Humphrey-Zeiss, Dublin, CA) was performed on both eyes of each subject using the retinal nerve fiber layer (RNFL) analysis protocol. OCT interpolates data from 100 points around the optic nerve to effectively map out the RNFL. The results were compared to the calculated average RNFL of normal eyes accumulated from four prior studies using OCT, n=661. In all subjects with history of EMB-induced optic neuropathy, there was a mean loss of 72% nerve fiber layer thickness in the temporal quadrant (patient A, with eventual recovery of visual acuity and fields, 58% loss; patient B, with intermediate visual deficits, 68% loss; patient C, with chronic visual deficits, 90% loss), with an average mean optic nerve thickness of 26+/-16 microm. There was a combined mean loss of 46% of fibers from the superior, inferior, and nasal quadrants in the (six) eyes of all three subjects (mean average thickness of 55+/-29 microm). In both sets (four) of eyes of the subjects with persistent visual deficits (patients B and C), there was an average loss of 79% of nerve fiber thickness in the temporal quadrant. The OCT results in these patients with EMB-induced optic neuropathy show considerable loss especially of the temporal fibers. This is consistent with prior histopathological studies that show predominant loss of parvo-cellular axons (or small-caliber axons) within the papillo-macular bundle in toxic or hereditary optic neuropathies. OCT can be a valuable tool in the quantitative analysis of optic neuropathies. Additionally, in terms of management of EMB-induced optic neuropathy, it is important to properly manage ethambutol dosing in patients with renal impairment and to achieve proper transition to a maintenance dose once an appropriate loading dose has been reached.

Effect of negative acoustic refraction in a one-dimensional phononic crystal

A two-component thin-layer acoustic superlattice (one-dimensional phononic crystal), in which one of the layers of the superlattice period is near the proper ferroelastic phase transition, is considered. The necessary conditions under which a bulk elastic wave incident on the outer surface of the superlattice excites a refracted wave with negative acoustic refraction (at the fundamental or multiple frequency) have been determined.

Cohesive‐zone models, higher‐order continuum theories and reliability methods for computational failure analysis

AbstractA concise overview is given of various numerical methods that can be used to analyse localization and failure in engineering materials. The importance of the cohesive‐zone approach is emphasized and various ways to incorporate the cohesive‐zone methodology in discretization methods are discussed. Numerical representations of cohesive‐zone models suffer from a certain mesh bias. For discrete representations this is caused by the initial mesh design, while for smeared representations it is rooted in the ill‐posedness of the rate boundary value problem that arises upon the introduction of decohesion. A proper representation of the discrete character of cohesive‐zone formulations which avoids any mesh bias can be obtained elegantly when exploiting the partition‐of‐unity property of finite element shape functions. The effectiveness of the approach is demonstrated for some examples at different scales. Moreover, examples are shown how this concept can be used to obtain a proper transition from a plastifying or damaging continuum to a shear band with gross sliding or to a fully open crack (true discontinuum). When adhering to a continuum description of failure, higher‐order continuum models must be used. Meshless methods are ideally suited to assess the importance of the higher‐order gradient terms, as will be shown. Finally, regularized strain‐softening models are used in finite element reliability analyses to quantify the probability of the emergence of various possible failure modes. Copyright © 2004 John Wiley & Sons, Ltd.

Main transition strategies and its logic: Actors, alternatives and prospects

In the article author deals with ideological background of two transition strategies under the influence of three grand ideologies: liberalism conservativism, Marxism. Two prevailing strategies of shock therapy and gradualism are revisited with reference to the geopolitical context shaped after the fall of communism and vividly marked with conflict of liberalism and conservativism during the last 15 years in East Europe. Neoliberal strategy of shock therapy provides renewal of the conditions that this very strategy tries to avoid, namely condition of rise of socialism and conservativism. On the other hand, occurrences in East Europe in the same period, marked with failures in efforts to establish free market privatization and free flow of capital in economy, are unfolding in well known logic of confrontation of grand ideologies. In other words, after the fall of Berlin wall we could notice the rise of liberalism on the global scene akin to the form known from the XIX and beginning of the XX century. Ex socialist countries under the pressure of IMF and World Bank policies are disposed to the reiteration of neoliberal experiment that with its economic political and social effects paves the way to the renewal of left and right ideologies. Author maintains that enduring of ideological conflicts presents great obstacle to the rational politics. In addition to that, authors in the social sciences, due to their sharp divisions along ideological lines mostly failed in its historical opportunity to analyze and predict events. They were mostly pleading for democracy, regardless of concrete political and economic conditions, and were neglecting the care for institution building. But due to the lack in proper institutional frame transition amounts to one misfortune more in societies already ruined with outcomes of socialism. Without institutional background free market amounts to the black market, privatization turn into great robbery as a form of primary accumulation of capital, and instead of capital we see readiness of political elites to sold out the countries and their resources, including leading the people into voluntary serfdom toward the West.

Abrupt changes and hysteretic behavior of90°domains in epitaxial ferroelectric thin films with misfit dislocations

The influence of misfit dislocations on the statics and dynamics of 90° domains (twins) in epitaxial thin films is studied theoretically. The misfit dislocations create additional potential barriers, which hinder the movements of domain walls along the film/substrate interface. For ferroelastic 90° walls, these barriers are considerable because an elastic interaction exists between 90° domains and misfit dislocations owing to the coupling of the dislocation stresses to the spontaneous strains generated at the proper or improper ferroelastic phase transition. The energy of elastic interaction is evaluated for a single 90° domain embedded into a tetragonal film containing a periodic array of edge misfit dislocations. On this basis. the equilibrium width of a single embedded 90° domain is calculated as a function of the misfit strain in the film/substrate systems. It is found that, at some threshold values of the misfit strain, the gradual variation of the domain width is interrupted by steplike changes. The evolution of the 90° domain pattern during the polarization switching in ferroelectric films with misfit dislocations is analyzed in detail. The calculations show that the electric-field-induced growth of an embedded 90° domain involves three different stages. Remarkably, at some critical value of the applied electric field, one of the 90° domain walls jumps over the neighboring misfit dislocation, which is accompanied by abrupt changes of the domain width and position in the film. The steplike movements of 90° domain walls over the misfit dislocations are proposed as a possible microscopic mechanism of the Barkhausen effect occurring in ferroelectric thin films during the polarization reversal. The hysteretic behavior of 90° domains at the cyclic variation of an applied electric field is also demonstrated.

Electronic spectra, solvatochromic behavior and acid–base properties of some azo cinnoline compounds

The electronic spectra of three azo cinnoline derivatives have been studied in pure and mixed organic solvents of different characteristics as well as the effect of concentration of the compounds in the different solvents. The different bands observed have been assigned to the proper electronic transition. The longer wavelength band displayed by the para nitro cinnoline derivative in dimethylformamide (DMF) solution is assigned to an intermolecular CT transition. The solvated H-bonding complexes formed between DMF and the para nitro derivative were investigated. Δ G and K f values of these complexes have been determined. The acidity constants of the para nitro compound were determined from the spectra in aquous-methanolic solution of varying pH values. The effect of temperature on the longer wavelength visible band of p-NO 2 has been investigated.

Subacute ruminal acidosis (SARA): a review.

Subacute ruminal acidosis (SARA) is likely to arise when an easily palatable, high-energy diet meets a ruminal environment not adapted to this type of substrate. Increase of short-chained fatty acids (SCFA) will occur. Eventually, this may result in a transient nadir of ruminal pH below 5.5. Two situations are likely to represent the risk of SARA. First, fresh lactating cows are confronted with a diet considerably differing from that in the dry-period. A diet change carried out too rapidly or without proper transition management will put the animals at risk. Secondly, further in lactation, inaccurate calculation of dry-matter-intake (DMI) leading to wrong roughage/concentrate ratio, an inadequate content of structure within the diet or mistakes in preparing of total mixed rations may produce SARA. The consequences of SARA are diverse and complex. Laminitis is regularly connected to SARA and the negative impact of organic acids on the ruminal wall may lead to parakeratosis enabling translocation of pathogens into the bloodstream provoking inflammation and abscessation throughout the ruminant body. Moreover, milk-fat depression (MFD) can be related to SARA. In order to achieve a proper diagnosis, SARA has to be understood as a herd-management problem. A screening of the herd for SARA by means of a rumenocentesis, performed on a sample-group, preferably 12 individuals, may reveal the presence of SARA. The herd screening should include the risk group suspected, preferably. The prevention of SARA applies to the principles of ruminant feeding. Careful transition management from the dry to the lactation period and control of fibre-content and ration quality should be more yielding than the use of buffers or antibiotic drugs.

On the generation of catalytic antibodies by transition state analogues.

The effective design of catalytic antibodies represents a major conceptual and practical challenge. It is implicitly assumed that a proper transition state analogue (TSA) can elicit a catalytic antibody (CA) that will catalyze the given reaction in a similar way to an enzyme that would evolve (or was evolved) to catalyze this reaction. However, in most cases it was found that the TSA used produced CAs with relatively low rate enhancement as compared to the corresponding enzymes, when these exist. The present work explores the origin of this problem, by developing two approaches that examine the similarity of the TSA and the corresponding transition state (TS). These analyses are used to assess the proficiency of the CA generated by the given TSA. Both approaches focus on electrostatic effects that have been found to play a major role in enzymatic reactions. The first method uses molecular interaction potentials to look for the similarity between the TSA and the TS and, in principle, to help in designing new haptens by using 3D quantitative structure-activity relationships. The second and more quantitative approach generates a grid of Langevin dipoles, which are polarized by the TSA, and then uses the grid to bind the TS. Comparison of the resulting binding energy with the binding energy of the TS to the grid that was polarized by the TS provides an estimate of the proficiency of the given CA. Our methods are used in examining the origin of the difference between the catalytic power of the 1F7 CA and chorismate mutase. It is demonstrated that the relatively small changes in charge and structure between the TS and TSA are sufficient to account for the difference in proficiency between the CA and the enzyme. Apparently the environment that was preorganized to stabilize the TSA charge distribution does not provide a sufficient stabilization to the TS. The general implications of our findings and the difficulties in designing a perfect TSA are discussed. Finally, the possible use of our approach in screening for an optimal TSA is pointed out.

Frequency-dependent elastic response at the ferroelectric phase transition ofAgNa(NO2)2

The elastic dynamics of $\mathrm{AgNa}({\mathrm{NO}}_{2}{)}_{2}$ crystals around the proper ferroelectric phase transition at ${T}_{C}=38\ifmmode^\circ\else\textdegree\fi{}C$ was investigated by ultrasonic $(f=10\mathrm{MHz})$ and dynamic mechanical analysis (f=0.6-50 Hz) techniques. The system represents a unique example of a ferroelectric crystal with extremely slow dielectric relaxation. Due to this reason it reveals a substantially different elastic behavior in megahertz and hertz frequency regions. We describe our data using a phenomenological model that includes both the effect of order parameter (polarization) relaxation with a characteristic time ${\ensuremath{\tau}}_{p}$ and thermal (entropy) relaxation with a characteristic time ${\ensuremath{\tau}}_{\mathrm{th}}.$

Mechanism of occurrence of soft mode at proper ferroelastic phase transitions

It is shown that softening the acoustic mode, which brings about the occurrence of the proper ferroelastic phase transition in some crystals, is substantially due to the ``kinematic'' anharmonicity. The latter arises at the transition from the natural curvilinear coordinates of interatomic separation to the Cartesian atomic displacements. The equation for such crystals has been obtained, with whose help it appears possible to predict the pressure of the phase transtion to a triclinic phase if the initial elastic moduli are known.

Proper ferroelastic phase transitions in thin epitaxial films with symmetry-conserving and symmetry-breaking misfit strains

We study how the ferroelastic domain structure sets in in an epitaxial film of a material with second order proper ferroelastic transition. The domain structures considered are similar to either $a_{1}/a_{2}/a_{1}/a_{2}$ or $c/a/c/a$ structures in perovskite ferroelectrics. If the "extrinsic" misfit strain, not associated with the transition, does not break the symmetry of the high-temperature phase, the phase transition in the film occurs at somewhat lower temperature compared to the bulk. The loss of stability then occurs with respect to a sinusoidal strain wave, which evolves into the domain structure with practically the same geometry and approximately the same period. In the presence of the symmetry-breaking component of the misfit strain ("extrinsic" misfit) the character of the phase transition is qualitatively different. In this case it is a {\em topological} transition between single-domain and multi-domain states, which starts from a low density of the domain walls.

Phase-field model of domain structures in ferroelectric thin films

A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic→tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution.

Dielectric properties in the vicinity of the ferroelectric phasetransition in a mixed crystal of deuteratedBP0.01BPI0.99

The dielectric behaviour of ferroelectric hydrogen bonded deuterated betainephosphate0.01 betaine phosphite0.99 is investigated in the region ofthe ferroelectric phase transition. Dielectric dispersion is investigated inthe frequency range up to 12 GHz. The dielectric dynamics of a small admixturesuch as 1% BP is already able to change noticeably the character of the phasetransition as well as the quasi-one-dimensional feature of deuterated betainephosphite. The Debye type dispersion shows a critical slowing down. Thefrequency of the relaxational soft mode in the paraelectric phase variesaccording to the quasi-one-dimensional Ising model and decreases down to0.55 GHz at the phase transition temperature (TC = 299 K). The activationenergy for the deuteron flipping motion amounts to ΔU = 3.6kTC = 0.095 eV. The obtained results show the order-disorder characterof the proper ferroelectric phase transition. However, the small admixture of1% betaine phosphate changes noticeably the character of the phase transitionas well as the quasi-one-dimensional feature in comparison to pure deuteratedbetaine phosphite.