No Mass Gap Phase Transition in Novel Massless Dirac Fermion Material

Using an organic massless Dirac fermion system, we found that massless Dirac fermions undergo a quantum phase transition without creating any mass gap even in the strong coupling regime.

Unifying the mechanism of mitotic exit control in a spatiotemporal logical model

The transition from mitosis into the first gap phase of the cell cycle in budding yeast is controlled by the Mitotic Exit Network (MEN). The network interprets spatiotemporal cues about the progression of mitosis and ensures that release of Cdc14 phosphatase occurs only after completion of key mitotic events. The MEN has been studied intensively; however, a unified understanding of how localisation and protein activity function together as a system is lacking. In this paper, we present a compartmental, logical model of the MEN that is capable of representing spatial aspects of regulation in parallel to control of enzymatic activity. We show that our model is capable of correctly predicting the phenotype of the majority of mutants we tested, including mutants that cause proteins to mislocalise. We use a continuous time implementation of the model to demonstrate that Cdc14 Early Anaphase Release (FEAR) ensures robust timing of anaphase, and we verify our findings in living cells. Furthermore, we show that our model can represent measured cell–cell variation in Spindle Position Checkpoint (SPoC) mutants. This work suggests a general approach to incorporate spatial effects into logical models. We anticipate that the model itself will be an important resource to experimental researchers, providing a rigorous platform to test hypotheses about regulation of mitotic exit.

Canopy Gap phase regeneration: A study in moist temperate forests of Central Himalayas, India

Broad leaved forests continuously have some asynchronous and localized disturbance events.Tree fall gaps are the dominant form of such disturbances in these forests. Moist temperate forests dominated by Quercus leucotricophora in Garhwal were investigated for study. Ten random transect belts were established in a mixed broadleaved forest patch. The forest comprised of gaps, created by various natural processes. Gaps varied in size ranging from 48 to 589 m2. Microclimatic and edaphic variables of canopy gap and understorey areas were recorded to assess the parameters that influence gap phase regeneration. Forests of the studyarea comprised of gaps that varied in size and shape. Light intensity, soil moisture and soil temperature were prime micro-climatic variables influencing species recruitment in gaps.Species assemblage was recorded higher from gaps than forest understorey (20 and 17 tree species, respectively). Overall recruitment of tree species in understorey and canopy gaps was 3054 seedling ha-1 and 2277 saplings ha-1, respectively. The results obtained during our study show that gap formation process in moist temperate forests of Garhwal promotes a strong but temporary variations in microclimatic environment. This heterogeneity factor determines the biological diversity but needs more long term systematic research in Garhwal for better understanding.

Evidence that the cell cycle is a series of uncoupled, memoryless phases

The cell cycle is canonically described as a series of 4 phases: G1 (gap phase 1), S (DNA synthesis), G2 (gap phase 2), and M (mitosis). Various models have been proposed to describe the durations of each phase, including a two-state model with fixed S-G2-M duration and random G1 duration1,2; a “stretched” model in which phase durations are proportional3; and an inheritance model in which sister cells show correlated phase durations2,4. A fundamental challenge is to understand the quantitative laws that govern cell-cycle progression and to reconcile the evidence supporting these different models. Here, we used time-lapse fluorescence microscopy to quantify the durations of G1, S, G2, and M phases for thousands of individual cells from three human cell lines. We found no evidence of correlation between any pair of phase durations. Instead, each phase followed an Erlang distribution with a characteristic rate and number of steps. These observations suggest that each cell cycle phase is memoryless with respect to previous phase durations. We challenged this model by perturbing the durations of specific phases through oncogene activation, inhibition of DNA synthesis, reduced temperature, and DNA damage. Phase durations remained uncoupled in individual cells despite large changes in durations in cell populations. To explain this behavior, we propose a mathematical model in which the independence of cell-cycle phase durations arises from a large number of molecular factors that each exerts a minor influence on the rate of cell-cycle progression. The model predicts that it is possible to force correlations between phases by making large perturbations to a single factor that contributes to more than one phase duration, which we confirmed experimentally by inhibiting cyclin-dependent kinase 2 (CDK2). We further report that phases can show coupling under certain dysfunctional states such as in a transformed cell line with defective cell cycle checkpoints. This quantitative model of cell cycle progression explains the paradoxical observation that phase durations are both inherited and independent and suggests how cell cycle progression may be altered in disease states.

О рекомбинационных процессах в пленках CdS-PbS

The transverse and longitudinal photoconductivity, photoluminescence, and cathodoluminescence of sublimated (CdS)_0.9–(PbS)_0.1 films at room temperature and upon cooling are studied. The role of inclusions of the narrow-gap phase in the processes is shown. The films are excited over the entire active surface and pointwise (within one crystallite). The surface recombination rate and the lifetime of majority charge carriers at different generation rates and characters of excitation are estimated. A comparative table of recombination parameters of CdS and CdS–PbS films is presented.

Экситонные линии поглощения PbS в композите CdS-PbS

The spectral dependences of transmission, absorption, and reflection in CdS−PbS semiconductor layers, which are promising for the development of photodetectors and emitters in the region of an atmospheric- transparency window, were studied. It has been established that a series of thin lines are observed in the infrared spectrum on an absorption edge of the narrow-gap phase (PbS). A method for identification of exciton series was used, and exciton-binding energy E _ ex , exciton concentration N , and value of valence-band splitting Δ E _ v were determined.