Non-equilibrium electron and phonon dynamics in metals under femtosecond laser pulses

Different models for relaxation dynamics of electrons and phonons in a thin metal film heated by femto-pico second laser pulses have been discussed. The traditional two-temperature approach reveals to be inaccurate due to deviations of electrons and phonons from Fermi-Dirac and Bose-Einstein distributions, respectively. Coupled Boltzmann kinetic equations have been adapted for the quantum statistics to study the energy distribution of electrons and phonons in metals. Theoretical details have been discussed and a new solution method has been proposed overcoming numerical problems and improving stability, allowing the study of the dynamics until the complete relaxation. Numerical results have been compared with photoemission spectroscopy experimental data.     

Molecular-scale structure in fluid-gel patterned bilayers: stability of interfaces and transmembrane distribution

Variations in two-dimensional membrane structures on the molecular length scale are considered to have an effect on the mechanisms by which living cell membranes maintain their functionality. We created a molecular model of a patterned bilayer to asses the static and dynamic variations of membrane lateral and transbilayer distribution in two-component lipid bilayers on the molecular level. We study DSPC (distearoylphosphatidylcholine) nanometer domains in a fluid DLPC (dilauroylphosphatidylcholine) background. The system exhibits coexisting fluid and gel phases and is studied on a microsecond time scale. We characterize three different kinds of patterns: symmetric domains, asymmetric domains, and symmetric-asymmetric domains. Preferred bilayer configurations on the nanoscale are those that minimize the hydrophobic mismatch. We find nanoscale patterns to be dynamic structures with mainly lateral and rotational diffusion affecting their stability on the microsecond time scale.     

Using nucleation rates to determine the interfacial line tension of symmetric and asymmetric lipid bilayer domains

This work presents a novel method for experimentally quantifying interfacial line tension, which can be readily applied to study a wide variety of different lipid mixtures exhibiting phase coexistence. The method combines AFM imaging of lipid domain nucleation with classical nucleation theories. The results, using symmetric and asymmetric domains, permit the prediction of key physical parameters (critical nuclei size and nucleation rate) in multicomponent bilayer systems with implications toward understanding the dynamic nature of submicrometer domains (i.e., lipid rafts) in cell membranes.     

Phase and mixing behavior in two-component lipid bilayers: a molecular dynamics study in DLPC/DSPC mixtures

Phase and mixing behavior of dilauroylphosphatidylcholine (DLPC)/distearoylphosphatidylcholine (DSPC) lipid mixtures are studied by molecular dynamics simulations with use of a coarse-grained model over a wide range of concentrations. The results reveal that phase transformations from the fluid to the gel state can be followed over a microsecond time scale. The changes in structure suggest regions of phase coexistence allowing us to outline the entire phase diagram for this lipid mixture using a molecular based model. We show that simulations yield good agreement with the experimental phase diagram. We also address the effect of macroscopic phase separation on the determination of the transition temperature, different leaflet composition, and finite size effects. This study may have implications on lateral membrane organization and the associated processes dependent on these membrane regions on different time and length scales.     

Activity and stereoselectivity of Ru-based catalyst bearing a fluorinated imidazolinium ligand

Grubbs II generation catalyst (3), bearing a fluorinated imidazolinium ligand, was investigated in cross metathesis (CM), ring closing metathesis (RCM) and ring opening polymerization metathesis (ROMP) for a variety of substrates. Kinetic studies showed reduced stability of the catalyst in methylene chloride following the first 15 minutes of reaction preventing a higher efficiency despite the very high activity. Beneficial solvent effects on the catalyst stability were observed by performing RCM in C₆F₆.      

Experimental measurement of thermophysical properties of oxide-water nano-fluids down to ice-point

This paper presents the measurement of the thermal conductivity and the dynamic viscosity of Al₂O₃-water (1-4% particle volume fraction) and TiO₂-water (1-6% particle volume fraction) nano-fluids carried out at atmospheric pressure in the temperature range from 1 to 40 °C, which is particularly interesting for the application of nano-fluids as thermal medium in refrigeration and air-conditioning.The thermal conductivity measurement was performed by using a Transient Hot Disk TPS 2500S apparatus instrumented with a 7577 probe (2.001 mm in radius) having a maximum uncertainty (k = 2) lower than ±5.0% of the reading. The dynamic viscosity measurement and the rheological analysis were carried out by a rotating disc type rheometer Haake Mars II instrumented with a single cone probe (60 mm in diameter and 1° angle) having a maximum uncertainty (k = 2) lower than ±5.0% of the reading.The thermal conductivity measurements of the tested nano-fluids show a great sensitivity to particle volume fraction and temperature and a weak sensitivity to cluster average size: TiO₂-water and Al₂O₃-water nano-fluids show a thermal conductivity enhancement (with reference to pure water) from −2 to 16% and from −2 to 23% respectively.TiO₂-water and Al₂O₃-water nano-fluids exhibit a Newtonian behaviour in all the investigated ranges of temperature and nano-particle volume fraction. The relative viscosity shows a great sensitivity to particle volume fraction and cluster average size and no sensitivity to temperature: TiO₂-water and Al₂O₃-water nano-fluids show a dynamic viscosity increase with respect to pure water from 17 to 210% and from 15 to 150% respectively.Al₂O₃-water nano-fluid seems to be more promising as thermal medium than TiO₂-water nano-fluid, particularly at low thermal level (between ambient temperature and ice point) where TiO₂-water is not suitable showing worse performance than pure water.Present experimental measurements were compared both with available measurements carried out by different researchers and computational models for thermophysical properties of suspensions.     

Hydrogen bond networks in water and methanol with varying interaction strengths

Metropolis Monte Carlo simulations of hydrogen-bonded liquids (water and methanol) were performed with the well tested effective pair potentials TIP5P and OPLS. The Coulomb contribution for the interaction potential was damped by a factor η varied from 1 to 0.49 for water and 1 to 0.15 for methanol. As a result, the networks formed by the hydrogen-bonded molecules presented interesting properties as a function of η, including small-world patterns and percolation transitions. These complex networks were analyzed by local (clustering coefficients, average degrees), semi-global (path lengths) and global (spectral densities) properties, and islands statistics. From these properties, small-world behavior was found for η in the range 0.60-0.75 for both liquids, interestingly independent of the molecular structure of the liquid. Phase transition behavior was observed for the average degrees and the clustering coefficient curves with critical values at 0.55 for water and 0.34 for methanol. Macroscopic properties such as mass density and vaporization enthalpy were also parametrically dependent on η and they presented phase transition behavior that coincides with the critical values obtained from the topological analysis. This is probably the first time that such phase transitions are observed for these quantities and shows a direct relation between macroscopic properties and topological features of hydrogen bond networks.     

On the hydrogen bond networks in the water-methanol mixtures: topology, percolation and small-world

Statistical mechanics based topological analysis and island (or cluster) statistics were used to study the hydrogen bond (H-bond) networks in the water-methanol mixtures with the following methanol mole fractions (x(m)): 0.00, 0.10, 0.20, 0.25, 0.28, 0.30, 0.32, 0.36, 0.38, 0.42, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00. NPT-Monte Carlo simulations were performed at room conditions using the TIP5P model potential for water and united-atoms (OPLS) for methanol to generate the H-bond networks. We have found evidence for non-ideal behavior of mixtures with x(m) ≈ 0.3. Several structural and topological properties present strong dependence with the mixture composition. Island statistics indicate a change from the percolated to non-percolate regime at x(m) ≈ 0.5. Statistical analysis of the islands' nature (homo-clusters: same type of molecules × hetero-clusters: two types of molecules) yields a preferential formation of homo-clusters that quantifies the local composition and preferential solvation ("microimmiscibility"). The topology of the hydrogen bond networks was characterized by local (clustering coefficients, average degrees), semi-global (path lengths) and global (spectral densities) properties. Small-world patterns (highly clustered and small path lengths) appear for x(m) in the range 0.40-0.70, and the momenta in the spectral densities correlate quite well with previous analysis based on rings, chains and branched chains topologies. It also seems that small quantities of methanol in water cause disruption of the continuous fully connected H-bond networks formed by water molecules.     

Quaternion algebras, Heegner points and the arithmetic of Hida families

Given a newform f, we extend Howard??s results on the variation of Heegner points in the Hida family of f to a general quaternionic setting. More precisely, we build big Heegner points and big Heegner classes in terms of compatible families of Heegner points on towers of Shimura curves. The novelty of our approach, which systematically exploits the theory of optimal embeddings, consists in treating both the case of definite quaternion algebras and the case of indefinite quaternion algebras in a uniform way. We prove results on the size of Neková???s extended Selmer groups attached to suitable big Galois representations and we formulate two-variable Iwasawa main conjectures both in the definite case and in the indefinite case. Moreover, in the definite case we propose refined conjectures à la Greenberg on the vanishing at the critical points of (twists of) the L-functions of the modular forms in the Hida family of f living on the same branch as f.