Raman modes in Pbca enstatite (Mg2Si2O6): an assignment by quantum mechanical calculation to interpret experimental results

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid Hamiltonian WC1LYP.[1] The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ~5 cm1. The quantum mechanical simulation allowed the assignment of Raman features to specific vibrational modes. This enabled to assess quantitatively the contributions of internal (tetrahedral stretching) and external (tetrahedral chains and M1 and M2 cations) vibrations. Moreover, the mass substitution of 56Fe for 24Mg in the M1 and M2 sites and of 30Si and 18O for the 28Si and 16O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate the major experimental peaks to specific structural vibrations, and to link the changes in crystal structure to those modes with pressure, temperature and composition. The results provide new clues to identify most suitable peaks for the investigation of the intracrystalline ordering of Fe and Mg in the M1 and M2 sites, and of Al in the tetrahedral and octahedral sites. Moreover we have been able to identify those peaks which are related to structural features, like tetrahedral bond distances

New molecular tool for a quick and easy detection of apple scab in the field

Venturia inaequalis, an agent of apple scab, is the most important pathogen of Malus x domestica. Control measures against this pathogen rely on intensive phytosanitary programs based on predictive models to identify the meteorological conditions conducive to the primary infection. The detection of the pathogen in field, both in naturally infected symptomatic and asymptomatic leaves, is desirable. Loop-mediated isothermal amplification (LAMP) assays are profitable molecular diagnostic tools for the direct detection of pathogens in field. A LAMP assay for V. inaequalis has been designed on the elongation factor 1-alpha sequence. The validation of the LAMP assay was carried out following the international EPPO standard PM 7/98 in terms of specificity, sensitivity, repeatability and reproducibility. Specificity testing was performed using target and non-target species, such as phylogenetically related Venturia species and other pathogens commonly found in apple, resulting in positive amplification only for the target with a time to positive ranging from 20 to 30 min. Sensitivity testing was performed with serial dilutions of DNA of the target and by artificial inoculation of young apple leaves. The reliability of the LAMP assay as an early-detection tool and its user-friendly application make it suitable for the diagnosis of apple scab in the field

Asynchronous Silent Programmable Matter Achieves Leader Election and Compaction

We study models and algorithms for Programmable Matter (PM), that is matter with the ability to change its physical properties (e.g., shape or optical properties) in a programmable fashion. PM can be implemented by assembling a system of weak self-organizing computational elements, called particles, that can be programmed via distributed algorithms to collectively achieve some global task. Recent advances in the production of nanotechnologies have rendered such systems increasingly possible in practice, thus triggering research interests from many areas of computer science. The most established models for PM assume that particles: are modeled as finite state automata; are all identical, executing the same algorithm based on local observation of the surroundings; live and operate in the cells of a hexagonal grid; can move from one cell to another by repeatedly alternating between a contracted state (a particle occupies one cell) and an expanded state (a particle occupies two neighboring cells). Given these elementary features, it is rather hard to design distributed algorithms even for basic tasks and, in fact, all existing solutions to solve fundamental problems via PM have resorted to endowing PM systems with various capabilities to overcome such hardness, thus assuming quite unrealistic features. In this paper, we move toward more realistic computational models for PM. Specifically, we first introduce, a new modeling approach that relaxes several assumptions used in previous ones. Second, we present a distributed algorithm to solve, in the model, a foundational primitive for PM, namely Leader Election. This algorithm works in O(n) rounds for all initial configurations of n particles that are both connected (i.e. particles induce a connected graph) and compact (i.e. without holes, that is no empty cells surrounded by particles occur). As usual in asynchronous contexts, a round is intended as the time within which all particles have been activated at least once. Third, we show that, if the initial configuration admits holes, it is impossible to achieve leader election while preserving connectivity. Finally, by slightly empowering the robots, we design an algorithm to handle initial configurations admitting holes that in O(n2) rounds solves the leader election problem while obtaining also compaction

Toward an accurate ab initio estimation of compressibility and thermal expansion of diamond in the [0, 3000 K] temperature and [0, 30 GPa] pressures ranges, at the hybrid HF/DFT theoretical level

The isothermal bulk modulus, together with its temperature dependence, and the thermal expansion of diamond at various pressures were calculated from first principles in the [0, 30 GPa] and [0, 3000 K] pressure and temperature ranges, within the limits of the quasi-harmonic approximation (QHA). The hybrid HF/DFT functional employed (WC1LYP) proved to be particularly effective in providing a very close agreement between the calculated and the available experimental data. In particular, the bulk modulus at 300 K was estimated to be 444.6 GPa (K' = 3.60); at the same temperature, the (volume) thermal expansion coefficient was 3.19×10-6 K-1. To the authors’ knowledge, among the theoretical papers devoted to the subject, the present one provides the most accurate thermo-elastic data in high-pressure and temperature ranges. Such data can confidently be used in the determination of the pressure of formation using the “elastic method” for minerals found as inclusions in diamonds (recently applied on different minerals included in diamonds), thus shedding light upon the genesis of diamonds in the Earth’s upper mantle

Measurement of strains in zircon inclusions by Raman spectroscopy

We have carried out ab initio hybrid Hartree-Fock/Density Functional Theory simulations to determine the structure and vibrational modes of zircon, ZrSiO4, as a function of different applied strains. The changes in phonon-mode wavenumbers are approximately linear in the unit-cell strains, and have been fitted to determine the components of the phonon-mode Grüneisen tensors of zircon which reproduce the change in measured Raman shifts with pressure. They can therefore be used to convert Raman shifts measured from zircon inclusions in metamorphic rocks into strains that in turn can be used to determine the metamorphic conditions at the time that the inclusion was trapped. Due to the strong anisotropy in the thermal pressure of zircon, the phonon-mode Grüneisen tensor is not able to reproduce the temperature-induced changes in Raman shifts. Because zircon inclusions are normally measured at room conditions this does not prevent the calculation of their entrapment conditions

Rendezvous of Two Robots with Constant Memory

We study the impact that persistent memory has on the classical rendezvous problem of two mobile computational entities, called robots, in the plane. It is well known that, without additional assumptions, rendezvous is impossible if the entities are oblivious (i.e., have no persistent memory) even if the system is semi-synchronous (SSynch). It has been recently shown that rendezvous is possible even if the system is asynchronous (ASynch) if each robot is endowed with O(1) bits of persistent memory, can transmit O(1) bits in each cycle, and can remember (i.e., can persistently store) the last received transmission. This setting is overly powerful. In this paper we weaken that setting in two different ways: (1) by maintaining the O(1) bits of persistent memory but removing the communication capabilities; and (2) by maintaining the O(1) transmission capability and the ability to remember the last received transmission, but removing the ability of an agent to remember its previous activities. We call the former setting finite-state (FState) and the latter finite-communication (FComm). Note that, even though its use is very different, in both settings, the amount of persistent memory of a robot is constant. We investigate the rendezvous problem in these two weaker settings. We model both settings as a system of robots endowed with visible lights: in FState, a robot can only see its own light, while in FComm a robot can only see the other robot's light. We prove, among other things, that finite-state robots can rendezvous in SSynch, and that finite-communication robots are able to rendezvous even in ASynch. All proofs are constructive: in each setting, we present a protocol that allows the two robots to rendezvous in finite time.Comment: 18 pages, 3 figure