Stability conditions and positivity of invariants of fibrations

We study three methods that prove the positivity of a natural numerical invariant associated to $1-$parameter families of polarized varieties. All these methods involve different stability conditions. In dimension 2 we prove that there is a natural connection between them, related to a yet another stability condition, the linear stability. Finally we make some speculations and prove new results in higher dimension.Comment: Final version, to appear in the Springer volume dedicated to Klaus Hulek on the occasion of his 60-th birthda

Raman tensor analysis of hexagonal polyoxymethylene and its application to study the molecular arrangement in highly crystalline electrospun nanofibers

The orientation dependence in space of Raman-active vibrations in the hexagonal structure of polyoxymethylene (POM) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. The variation of polarized intensity for the A1 and the E1 vibrational modes with respect to the POM molecular orientation is systematically studied, from both theoretical and experimental viewpoints, according to the symmetry assignments of each vibrational mode. A set of working equations including the Raman selection rules associated with the A1 and the E1 modes and the orientation distribution function are explicitly formulated and validated by means of a least-square fitting procedure on experimental data. In addition, an approach based on the introduction of orientation distribution functions is applied to quantitatively assess and compare on a statistical base the molecular orientation of two different types of electrospun POM nanofibers. Copyright (c) 2012 John Wiley & Sons, Ltd

Contrasting pressure evolution of f-electron hybridized states in CeRhIn5 and YbNi3 Ga9 : An optical conductivity study

Optical conductivity [σ (ω)] of CeRhIn5 and YbNi3Ga9 have been measured at external pressures to 10 GPa and at low temperatures to 6 K. Regarding CeRhIn5, at ambient pressure the main feature in σ (ω) is a Drude peak due to free carriers. With increasing pressure, however, a characteristic midinfrared (mIR) peak rapidly develops in σ (ω), and its peak energy and width increase with pressure. These features are consistent with an increased conduction (c)- f electron hybridization at high pressure and show that pressure has tuned the electronic state of CeRhIn5 from very weakly to strongly hybridized ones. As for YbNi3Ga9, in contrast, a marked mIR peak is observed already at ambient pressure, indicating a strong c- f hybridization. At high pressures, however, the mIR peak shifts to lower energy and becomes diminished and seems to merge with the Drude component at 10 GPa. Namely, CeRhIn5 and YbNi3Ga9 exhibit some opposite tendencies in the pressure evolution of σ (ω) and electronic structure. These results are discussed in terms of the pressure evolution of c- f hybridized electronic states in Ce and Yb compounds, in particular in terms of the electron-hole symmetry often considered between Ce and Yb

Birefringence of interferential mirrors at normal incidence Experimental and computational study

In this paper we present a review of the existing data on interferential mirror birefringence. We also report new measurements of two sets of mirrors that confirm that mirror phase retardation per reflection decreases when mirror reflectivity increases. We finally developed a computational code to calculate the expected phase retardation per reflection as a function of the total number of layers constituting the mirror. Different cases have been studied and we have compared computational results with the trend of the experimental data. Our study indicates that the origin of the mirror intrinsic birefringence can be ascribed to the reflecting layers close to the substrate.Comment: To be published in Applied Physics

Observation of single collisionally cooled trapped ions in a buffer gas

Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of ~5 min (~1 min) are achieved using He (Ar) as a buffer gas at pressures in the range 8e-5 - 4e-3 torr. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. A. Minor text and figure change

Hydrotropism: analysis of the root response to a moisture gradient

Hydrotropism is a genuine response of roots to a moisture gradient to avoid drought. An experimental system for the induction of hydrotropic root response in petri dishes was designed by pioneering groups in the field. This system uses split agar plates containing an osmolyte only in a region of the plate in order to generate a water potential gradient. Arabidopsis seedlings are placed on the MS agar plate so that their root tips are near the junction between plain MS medium and the region supplemented with the osmolyte. This elicits a hydrotropic response in Arabidopsis roots that can be measured as the root curvature angle

Thermochemiluminescent peroxide crystals

Chemiluminescence, a process of transduction of energy stored within chemical bonds of ground-state reactants into light via high-energy excited intermediates, is known in solution, but has remained undetected in macroscopic crystalline solids. By detecting thermally induced chemiluminescence from centimeter-size crystals of an organic peroxide here we demonstrate direct transduction of heat into light by thermochemiluminescence of bulk crystals. Heating of crystals of lophine hydroperoxide to ~115 °C results in detectable emission of blue-green light with maximum at 530 nm with low chemiluminescent quantum yield [(2.1 ± 0.1) × 10 ‒7 E mol ‒1 ]. Spectral comparison of the thermochemiluminescence in the solid state and in solution revealed that the solid-state thermochemiluminescence of lophine peroxide is due to emission from deprotonated lophine. With selected 1,2-dioxetane, endoperoxide and aroyl peroxide we also establish that the thermochemiluminescence is common for crystalline peroxides, with the color of the emitted light varying from blue to green to red

Optical Conductivity and Electronic Structure of CeRu4Sb12 under High Pressure

Optical conductivity [s(w)] of Ce-filled skutterudite CeRu4Sb12 has been measured at high pressure to 8 GPa and at low temperature, to probe the pressure evolution of its electronic structures. At ambient pressure, a mid-infrared peak at 0.1 eV was formed in s(w) at low temperature, and the spectral weight below 0.1 eV was strongly suppressed, due to a hybridization of the f electron and conduction electron states. With increasing external pressure, the mid-infrared peak shifts to higher energy, and the spectral weight below the peak was further depleted. The obtained spectral data are analyzed in comparison with band calculation result and other reported physical properties. It is shown that the electronic structure of CeRu4Sb12 becomes similar to that of a narrow-gap semiconductor under external pressure.Comment: 8 pages, 9 figure

Turbulent flow at 190 m height above London during 2006-2008: A climatology and the applicability of similarity theory

Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature, standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ_i/u_* values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor height is above the mean roughness sublayer depth