Path Integral Monte Carlo Calculation of Momentum Distribution in Solid 4He

We perform calculations of the momentum distribution n(k) in solid ⁴He by means of path integral Monte Carlo methods. We see that, in perfect crystals, n(k) does not depend on temperature T and that is different from the classical Gaussian shape of the Maxwell-Boltzmann distribution, even though these discrepancies decrease when the density of the system increases. In crystals presenting vacancies, we see that for T??0.75 K, n(k) presents the same behavior as in the perfect crystal, but, at lower T, it presents a peak when k??0.     

Development of natural fiber‐reinforced plastics (NFRP) based on biobased polyethylene and waste fibers from Posidonia oceanica seaweed

In the present study the valorization of wastes from Posidonia oceanica (PO) has been carried out in order to obtain a fully biobased composite material in combination with a biobased polyethylene obtained from sugar cane as matrix. Morphological analysis by scanning electron microscopy (SEM) of the fractured surfaces from impact tests has revealed a homogenous distribution of particles of PO, as a consequence, good balanced properties have been obtained for composites with PO contents in the 5–40 wt%. Thermal properties of composites have been studied through differential scanning calorimetry (DSC) and thermogravymetric analysis (TGA); the obtained results show an improvement on the thermal degradation. With regard to thermomechanical properties, dynamic mechanical analysis (DMA) results have shown a much enhanced storage modulus (G′) as the Posidonia oceanica content increases. Tensile tests have shown a remarkable increase in stiffness with tensile modulus values about 60% higher for composites with 40 wt% with regard to unfilled material. In a similar way, the flexural modulus is more than twice with regard the unloaded polyethylene. Shore D hardness confirms this improvement on mechanical properties and Charpy impact test shows values very similar to sample without PO, so that the intrinsic high impact energy absorption of HDPE is maintained in HDPE‐PO composites. The water uptake test determines that the water absorption percent does not exceed 8%, which is relatively low for a high immersion time (5 months), which guarantees a dimensional stability in lifetime for these composites. POLYM. COMPOS., 36:1378–1385, 2015. © 2014 Society of Plastics Engineers     

Isotopic Effects in Solid LiH and LiD at Very Low Temperature

The ground state of the ionic solids LiD and LiH is theoretically studied using the Variational Monte Carlo (VMC) method. Our main focus has been the calculation of relevant properties of the H⁻ and D⁻ ions using a fully quantum approach. In particular, we report results on their kinetic energies, mean-squared displacements, Einstein frequencies, radial distribution functions, and density profiles around the sites. The microscopic results obtained for both isotopes show corrections beyond trivial isotopic effects due to their quantum behavior. Finally, the VMC results are compared with predictions from self consistent average phonon (SCAP) theory at T = 0 K.PACS numbers: 61.12 −q, 67.80 Cx, 63.20 −e, 02.70 Ss     

Optimal heat release shaping in a reactivity controlled compression ignition (RCCI) engine

The present paper addresses the optimal heat release (HR) law in a single cylinder engine operated under reactivity controlled compression ignition (RCCI) combustion mode to minimise the indicated specific fuel consumption (ISFC) subject to different constraints including pressure related limits (maximum cylinder pressure and maximum cylinder pressure gradient). With this aim, a 0-dimensional (0D) engine combustion model has been identified with experimental data. Then, the optimal control problem of minimising the ISFC of the engine at different operating conditions of the engine operating map has been stated and analytically solved. To evaluate the method viability a data-driven model is developed to obtain the control actions (gasoline fraction) leading to the calculated optimal HR, more precisely to the optimal ratio between premixed and diffusive combustion. The experimental results obtained with such controls and the differences with the optimal HR are finally explained and discussed.     

An RTP/RTCP based approach for multimedia group and inter-stream synchronization

Most multimedia group and inter-stream synchronization techniques define or use proprietary protocols with new control messages. Many multimedia applications have been developed using RTP/RTCP as the standard for transmission of multimedia streams over IP networks. Instead of defining a new protocol, we propose the use of RTP/RTCP to provide synchronization. We take advantage of the feedback capabilities provided by RTCP and the ability to extend the protocol by extending and creating RTCP messages containing synchronization information. We have implemented our proposal and tested it in our University WAN. Our experiments have shown that network load resulting from synchronization is minimized and that asynchronies are within acceptable limits for multimedia applications.

Static Structure Factor and Static Response Function of Superfluid Helium 4: a Comparative Analysis

Extensive neutron scattering data on liquid helium 4 are available, and have been analyzed to give a number of physical quantities, e.g. static structure factor S(q), excitation energy, and roton linewidth. X-rays also give access to S(q). However, a comprehensive comparison between experimental data and theoretical results, including their dependence on pressure, is still lacking. The static response function χ(q) has been particularly overlooked, despite its fundamental role in theories of inhomogeneous helium. We present here a critical review about the strength of the main peaks of S(q) and χ(q). We include in the comparison the analysis of unpublished neutron data and new Monte-Carlo calculations of χ(q). We find a significant discrepancy between experiments, and suggest corrections which account for some of the differences. We give recommendations for the best values to use for S(q) and χ(q).     

Instability of Vacancy Clusters in Solid 4He

The behavior of pairs and clusters of vacancies in solid ⁴He crystals is studied with diffusion Monte Carlo method. We use a trial function suitable for describing solid ⁴He with long range order, arbitrary number of unoccupied sites and explicit Bose symmetry. It is found that vacancy clusters are unstable and collapse in all considered systems. We find no signature of stability of bound vacancy clusters of any size. Vacancies are found to exhibit weak attraction and a rough estimation of the binding energy is reported.     

A cross-technology benchmark for incremental graph queries

Software and Systems Modeling - To cope with the increased complexity of systems, models are used to capture what is considered the essence of a system. Such models are typically represented as a...