Evolution of the structure of amorphous ice - from low-density amorphous (LDA) through high-density amorphous (HDA) to very high-density amorphous (VHDA) ice

We report results of molecular dynamics simulations of amorphous ice for pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by pressure-induced amorphization of Ih ice at T=80 K is annealed to T=170 K at various pressures to allow for relaxation. Upon increase of pressure, relaxed amorphous ice undergoes a pronounced change of structure, ranging from the low-density amorphous ice (LDA) at p=0, through a continuum of HDA states to the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The main part of the overall structural change takes place within the HDA megabasin, which includes a variety of structures with quite different local and medium-range order as well as network topology and spans a broad range of densities. The VHDA represents the limit to densification by adapting the hydrogen-bonded network topology, without creating interpenetrating networks. The connection between structure and metastability of various forms upon decompression and heating is studied and discussed. We also discuss the analogy with amorphous and crystalline silica. Finally, some conclusions concerning the relation between amorphous ice and supercooled water are drawn.Comment: 11 pages, 12 postscript figures. To be published in The Journal of Chemical Physic

Strongly Tunable Anisotropic Thermal Transport in MoS2 by Strain and Lithium Intercalation: First--Principles Calculations

The possibility of tuning the vibrational properties and the thermal conductivity of layered van der Waals materials either chemically or mechanically paves the way to significant advances in nanoscale heat management. Using first-principles calculations we investigate the modulation of heat transport in MoS2 by lithium intercalation and cross-plane strain. We find that both the in-plane and cross-plane thermal conductivity (kr, kz) of MoS2 are extremely sensitive to both strain and electrochemical intercalation. Combining lithium intercalation and strain, the in-plane and cross-plane thermal conductivity can be tuned over one and two orders of magnitude, respectively. Furthermore, since kr and kz respond in different ways to intercalation and strain, the thermal conductivity anisotropy can be modulated by two orders of magnitude. The underlying mechanisms for such large tunability of the anisotropic thermal conductivity of \Mos are explored by computing and analyzing the dispersion relations, group velocities, relaxation times and mean free paths of phonons. Since both intercalation and strain can be applied reversibly, their stark effect on thermal conductivity can be exploited to design novel phononic devices, as well as for thermal management in MoS2-based electronic and optoelectronic systems

Decisive role of nuclear quantum effects on surface mediated water dissociation at finite temperature

Water molecules adsorbed on inorganic substrates play an important role in several technological applications. In the presence of light atoms in adsorbates, nuclear quantum effects (NQE) influence properties of these systems. In this work, we explore the impact of NQE on the dissociation of water wires on stepped Pt(221) surfaces. By performing ab initio molecular dynamics simulations with van der Waals corrected density functional theory, we note that several competing minima for both intact and dissociated structures are accessible at finite temperatures, making it important to assess whether harmonic estimates of the quantum free energy are sufficient to determine the relative stability of the different states. We perform ab initio path integral molecular dynamics (PIMD) in order to calculate these contributions taking into account conformational entropy and anharmonicities at finite temperatures. We propose that when when adsorption is weak and NQE on the substrate are negligible, PIMD simulations can be performed through a simple partition of the system, resulting in considerable computational savings. We calculate the contribution of NQE to the free energies, including anharmonic terms. We find that they result in an increase of up to 20% of the quantum contribution to the dissociation free energy compared to harmonic estimates. We also find that the dissociation has a negligible contribution from tunneling, but is dominated by ZPE, which can enhance the rate by three orders of magnitude. Finally we highlight how both temperature and NQE indirectly impact dipoles and the redistribution of electron density, causing work function to changes of up to 0.4 eV with respect to static estimates. This quantitative determination of the change in work function provides a possible approach to determine experimentally the most stable configurations of water oligomers on the stepped surfaces

Multimedia digital solutions from image and range based models for ancient landscapes communication

The paper presents the results of an interdisciplinary project which aimed at the dissemination of some archaeological remains producing multimedia contents from multisensor surveyed 3D data. The scope of this application pertained to the use of 3D detailed models as a base for some video-installations with the aim to arouse the visitors’ emotions and improve their museum experience. This work has been applied to the Arch of Augustus located in the archaeological site of Susa and to two ancient Roman marble statues, found in the city of Susa in 1802 and now displayed in the Archaeological Museum of Turin. The Arch of Augustus is in a remarkable state of conservation. Its decorated frieze tells about the peace between the Romans and the Celts but it is difficult to see for visitors at the ground level. A multisensor 3D survey, by means of laser scanning technique and photogrammetric method, made it possible to process a detailed 3D textured model, which provided the base for the creation of a life-size model to be placed in the Museum of Susa on which a designed didactic video map is projected, which explains the meaning of the frieze. The two statues, known as ‘busti loricati di Susa’ and representing two Roman emperors, were surveyed with a photogrammetric method with the aim of processing two 3D models representing the statues before the 19th century restoration, on the basis of archival sources. These models provided the base for a video installation for the museum which simulates a holographic projection and explains the different armour parts highlighting them in sequence. Nowadays modern metric survey technologies allow us to collect and process very detailed 3D models able to satisfy a wide variety of applications field, from specialized representation to didactic final uses in museums exhibitions

Photoelasticity of sodium silicate glass from first principles

Based on density-functional perturbation theory we have computed the photoelastic tensor of a model of sodium silicate glass of composition (Na$_2$O)$_{0.25}$(SiO$_2$)$_{0.75}$ (NS3). The model (containig 84 atoms) is obtained by quenching from the melt in combined classical and Car-Parrinello molecular dynamics simulations. The calculated photoelastic coefficients are in good agreement with experimental data. In particular, the calculation reproduces quantitatively the decrease of the photoelastic response induced by the insertion of Na, as measured experimentally. The extension to NS3 of a phenomenological model developed in a previous work for pure a-SiO$_2$ indicates that the modulation upon strain of other structural parameters besides the SiOSi angles must be invoked to explain the change in the photoelstic response induced by Na

Statistical equilibrium in simple exchange games I

Simple stochastic exchange games are based on random allocation of finite resources. These games are Markov chains that can be studied either analytically or by Monte Carlo simulations. In particular, the equilibrium distribution can be derived either by direct diagonalization of the transition matrix, or using the detailed balance equation, or by Monte Carlo estimates. In this paper, these methods are introduced and applied to the Bennati-Dragulescu-Yakovenko (BDY) game. The exact analysis shows that the statistical-mechanical analogies used in the previous literature have to be revised.Comment: 11 pages, 3 figures, submitted to EPJ

Germoplasma de manga no Brasil.

O Brasil ocupa atualmente o nono lugar entre os maiores produtores mundiais de manga, com 456 mil toneladas de frutos, o que corresponde a cerca de 2% do total da produção mundial (FAO, 1999). Esta produção, advem em grande parte, de plantios extensivos não comerciais, com elevadas perdas pós-colheita, que atende primordialmente a demanda interna, considerando um consumo per capita de 2,4 kg. A área colhida de manga no Brasil é de 63,5 mil ha, sendo que a maior concentração está na região Nordeste, com 45%, vindo logo a seguir a região Sudeste, com 41%. Porém, os maiores produtores, em ordem decrescente, são os estados de São Paulo, Minas Gerais, Pernambuco, Bahia e Ceará. A região Nordeste lidera também as exportações, com 88% do total da manga brasileira exportada, o que corresponde a US$ 28 milhões .Trabalhos apresentados no I Simpósio Latino Americano sobre Produção de Manga, 1999, Vitória da Conquista

Simulation of dimensionality effects in thermal transport

The discovery of nanostructures and the development of growth and fabrication techniques of one- and two-dimensional materials provide the possibility to probe experimentally heat transport in low-dimensional systems. Nevertheless measuring the thermal conductivity of these systems is extremely challenging and subject to large uncertainties, thus hindering the chance for a direct comparison between experiments and statistical physics models. Atomistic simulations of realistic nanostructures provide the ideal bridge between abstract models and experiments. After briefly introducing the state of the art of heat transport measurement in nanostructures, and numerical techniques to simulate realistic systems at atomistic level, we review the contribution of lattice dynamics and molecular dynamics simulation to understanding nanoscale thermal transport in systems with reduced dimensionality. We focus on the effect of dimensionality in determining the phononic properties of carbon and semiconducting nanostructures, specifically considering the cases of carbon nanotubes, graphene and of silicon nanowires and ultra-thin membranes, underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture Notes in Physics volume "Thermal transport in low dimensions: from statistical physics to nanoscale heat transfer" (S. Lepri ed.