Small Bipolarons in the 2-dimensional Holstein-Hubbard Model. II Quantum Bipolarons

We study the effective mass of the bipolarons and essentially the possibility to get both light and strongly bound bipolarons in the Holstein-Hubbard model and some variations in the vicinity of the adiabatic limit. Several approaches to investigate the quantum mobility of polarons and bipolarons are proposed for this model. It is found that the bipolaron mass generally remains very large except in the vicinity of the triple point of the phase diagram, where the bipolarons have several degenerate configurations at the adiabatic limit (single site (S0), two sites (S1) and quadrisinglet (QS)), while the polarons are much lighter. This degeneracy reduces the bipolaron mass significantly. The triple point of the phase diagram is washed out by the lattice quantum fluctuations which thus suppress the light bipolarons. We show that some model variations, for example a phonon dispersion may increase the stability of the (QS) bipolaron against the quantum lattice fluctuations. The triple point of the phase diagram may be stable to quantum lattice fluctuations and a very sharp mass reduction may occur, leading to bipolaron masses of the order of 100 bare electronic mass for realistic parameters. Thus such very light bipolarons could condense as a superconducting state at relatively high temperature when their interactions are not too large, that is, their density is small enough. This effect might be relevant for understanding the origin of the high Tc superconductivity of doped cuprates far enough from half filling.Comment: accepted Eur. Phys. J. B (january 2000) Ref. B960

Small Bipolarons in the 2-dimensional Holstein-Hubbard Model. I The Adiabatic Limit

The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard interaction $\upsilon \geq 0$, which tends to break them, generates many different ground-states. There are four domains in the $g,\upsilon$ phase diagram delimited by first order transition lines. Except for the domain at weak electron-phonon coupling (small g) where the electrons remain free, the electrons form bipolarons which can 1) be mostly located on a single site (small $\upsilon$, large g); 2) be an anisotropic pair of polarons lying on two neighboring sites in the magnetic singlet state (large $\upsilon$, large g); or 3) be a "quadrisinglet state" which is the superposition of 4 electronic singlets with a common central site. This quadrisinglet bipolaron is the most stable in a small central domain in between the three other phases. The pinning modes and the Peierls-Nabarro barrier of each of these bipolarons are calculated and the barrier is found to be strongly depressed in the region of stability of the quadrisinglet bipolaron

Kavalactones and flavokavins profiles contribute to quality assessment of kava (Piper methysticum G. Forst.), the Traditional beverage of the Pacific

Kava (Piper methysticum) is increasingly traded internationally and there is need for a rapid method to analyze kava raw material before export. The objectives of the present study were: (i) to develop a simple and robust protocol for high throughput simultaneous quantification of kavalactones (KLs) and flavokavins (FKs) in kava and (ii) to assess its potential for quality control. Methysticin; dihydromethysticin; kavain; desmethoxyyangonin; dihydrokavain; yangonin; and flavokavin A, B and C were quantified using HPTLC in acetonic extracts of 174 kava varieties. UHPLC analysis was conducted on a subset of six varieties representing the genetic variation of the species. The genetically distinct groups of nobles, two-day and wichmannii varieties were clearly differentiated and multivariate analyses of UHPLC and HPTLC data were congruent. Noble varieties have significantly low FKs/KLs (0.13) and high kavain/flavokavin B (K/FKB = 7.31). Two-day and wichmannii varieties are characterized by high FKs/KLs (0.36, 0.21) and low K/FKB (1.5, 1.7). A high-throughput HPTLC protocol was developed with a total analytical time of 50 min for 20 samples and only 10 mL of mobile phase. The use of acetone, sonication and two different detection wavelengths improves the accuracy compared to previous HPLC studies and confirms that kava varieties exhibit distinct chemotypes clearly differentiated by their FKs/KLs profiles. These results will strengthen the use of Codex Alimentarius regional standards

A Stretching Algorithm for Parallel Real-time DAG Tasks on Multiprocessor Systems

International audienceParallelism is becoming more important nowadays due to the increasing use of multiprocessor systems. In this paper, we study the problem of scheduling periodic parallel real-time Directed Acyclic graph (DAG) tasks on m homogeneous multiprocessor systems. A DAG task is an example of inter-subtask parallelism. It consists of a collection of dependent subtasks under precedence constraints. The dependencies between subtasks make scheduling process more challenging. We propose a stretching algorithm applied on each DAG tasks to transform them into a set of independent sequential threads with intermediate offsets and deadlines. The threads obtained with the transformation are two types, (i) fully-stretched master threads with utilization equal to 1 and (ii) constrained-deadline independent threads. The fully-stretched master threads are assigned to dedicated processors and the remaining processors m' ≤ m, are scheduled using global EDF scheduling algorithm. Then, we prove that preemptive global EDF scheduling of stretched threads has a resource augmentation bound equal to (3+ √ 5)/2 for all tasksets with n < ϕ * m , where n is the number of tasks in the taskset and ϕ is the golden ratio 1

A Parallelizing Algorithm for Real-Time Tasks of Directed Acyclic Graphs Model

International audienceIn this paper, we consider parallel real-time tasks follow- ing a Directed Acyclic Graph (DAG) model. This task model is classical in embedded and industrial system applications. Each real-time task is defined by a set of subtasks under precedence constraints. With each subtask being associated a worst case execution time and a maximal degree of parallelism. We propose a parallelizing algorithm based on the critical path concept, in which we find the best parallelizing structure of the task according to the response time and the required number of processors, considering the worst case execution time of the subtasks

Internest sex-ratio variation and male brood survival in the ant Pheidole pallidula

Sex allocation in social insects has become a general model in tests of inclusive fitness theory, sex-ratio theory, and parent-offspring conflict. Several studies have shown that colony sex ratios are often bimodally distributed, with some colonies producing mainly females and others mainly males. Sex specialization may result from workers assessing their relatedness to male brood versus female brood, relative to the average worker-relatedness asymmetry in other colonies of their population. Workers then adjust the sex ratio in their own interest This hypothesis assumes that workers can recognize the sex of the brood in their colony and selectively eliminate males. We compared the primary sex ratio (at the egg stage) and secondary sex ratio (reproductive pupae and adults) of colonies in the ant Pheidole pallidula. There was a strong bimodal distribution of secondary sex ratios, with most colonies producing mainly reproductives of one sex. In contrast, there was no evidence of a bimodal distribution of primary sex ratios. The proportion of haploid eggs produced by queens was 0.35 in early spring and decreased to about 0.1 in summer. Male eggs also were present in virtually all field colonies sampled in July, although eggs laid at this time of year never give rise to males. All male brood is, therefore, selectively eliminated beginning in July and continue to be eliminated through the rest of the year. Finally, the population sex-ratio investment was female-biased. Together, these results are consistent with the hypothesis that workers control the secondary sex ratio by selectively eliminating male brood in about half the colonies, perhaps those with high relatedness asymmetry.[Behav Ecol 7: 292-298 (1996)

Improvement of schedulability bound by task splitting in partitioning scheduling

International audienceWe focus on the class of static-priority partitioning scheduling algorithm on multiprocessor. We are interested in improving the schedulability of these algorithms by splitting the tasks which cannot be successfully allocated on processors