X-ray Absorption Linear Dichroism at the Ti K-edge of TiO2 anatase single crystal

Anatase TiO2 (a-TiO2) exhibits a strong X-ray absorption linear dichroism with the X-ray incidence angle in the pre-edge, the XANES and the EXAFS at the titanium K-edge. In the pre-edge region the behaviour of the A1-A3 and B peaks, originating from the 1s-3d transitions, is due to the strong $p$-orbital polarization and strong $p-d$ orbital mixing. An unambiguous assignment of the pre-edge peak transitions is made in the monoelectronic approximation with the support of ab initio finite difference method calculations and spherical tensor analysis in quantitative agreement with the experiment. It is found that A1 is mostly an on-site 3d-4p hybridized transition, while peaks A3 and B are non-local transitions, with A3 being mostly dipolar and influence by the 3d-4p intersite hybridization, while B is due to interactions at longer range. Finally, peak A2 which was previously assigned to a transition involving pentacoordinated titanium atoms exhibits a quadrupolar angular evolution with incidence angle. These results pave the way to the use of the pre-edge peaks at the K-edge of a-TiO2 to characterize the electronic structure of related materials and in the field of ultrafast XAS where the linear dichroism can be used to compare the photophysics along different axes.Comment: 43 pages, 19 figure

The mechanical relaxation study of polycrystalline MgCNi3

The mechanical relaxation spectra of a superconducting and a non-superconducting MgCNi3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300 K labeled as P1 and 125 K as P2) for the superconducting sample. For the non-superconducting one, the position of P1 shifts to 250 K, while P2 is almost completely depressed. It is found that the peak position of P2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P2 to the carbon atom jumping among the off-center positions. And further we expect that the behaviors of carbon atoms maybe correspond to the normal state crossovers around 150 K and 50 K observed by many other experiments.Comment: 4 figure

Creep, Relaxation and Viscosity Properties for Basic Fractional Models in Rheology

The purpose of this paper is twofold: from one side we provide a general survey to the viscoelastic models constructed via fractional calculus and from the other side we intend to analyze the basic fractional models as far as their creep, relaxation and viscosity properties are considered. The basic models are those that generalize via derivatives of fractional order the classical mechanical models characterized by two, three and four parameters, that we refer to as Kelvin-Voigt, Maxwell, Zener, anti-Zener and Burgers. For each fractional model we provide plots of the creep compliance, relaxation modulus and effective viscosity in non dimensional form in terms of a suitable time scale for different values of the order of fractional derivative. We also discuss the role of the order of fractional derivative in modifying the properties of the classical models.Comment: 41 pages, 8 figure

Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature

We present high-resolution capacitance dilatometry studies from 5 - 500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O-ordering are found for x ~ 6.95, which disappear below a kinetic glass-like transition near room temperature. The kinetics at this glass transition is governed by an energy barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O-ordering in the Y123 system is particularly sensitive to uniaxial pressure (stress) along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR

Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate

GPS observations of coseismic deformation following the May 20 and 29, 2012, Emilia seismic events (northern Italy): data, analysis and preliminary models

In May-July 2012, a seismic sequence struck a broad area of the Po Plain Region in northern Italy. The sequence in- cluded two ML >5.5 mainshocks. The first one (ML 5.9) oc- curred near the city of Finale Emilia (ca. 30 km west of Ferrara) on May 20 at 02:03:53 (UTC), and the second (ML 5.8) occurred on May 29 at 7:00:03 (UTC), about 12 km south- west of the May 20 mainshock (Figure 1), near the city of Mirandola. The seismic sequence involved an area that ex- tended in an E-W direction for more than 50 km, and in- cluded seven ML ≥5.0 events and more than 2,300 ML >1.5 events (http://iside.rm.ingv.it). The focal mechanisms of the main events [Pondrelli et al. 2012, Scognamiglio et al. 2012, this volume] consistently showed compressional kinematics with E-W oriented reverse nodal planes. This sector of the Po Plain is known as a region charac- terized by slow deformation rates due to the northwards mo- tion of the northern Apennines fold-and-thrust belt, which is buried beneath the sedimentary cover of the Po Plain [Pi- cotti and Pazzaglia 2008, Toscani et al. 2009]. Early global po- sitioning system (GPS) measurements [Serpelloni et al. 2006] and the most recent updates [Devoti et al. 2011, Bennett et al. 2012] recognized that less than 2 mm/yr of SW-NE short- ening are accommodated across this sector of the Po Plain, in agreement with other present-day stress indicators [Mon- tone et al. 2012] and known active faults [Basili et al. 2008]. In the present study, we describe the GPS data used to study the coseismic deformation related to the May 20 and 29 mainshocks, and provide preliminary models of the two seismic sources, as inverted from consensus GPS coseismic deformation fields

Long‐Term Monitoring and Characterization of Soil Radon Emission in a Seismically Active Area

Abstract Radon (222Rn) is a radioactive gas, which originates everywhere in rocks of Earth's crust through a process of natural decay of other radioactive elements. The monitoring of soil radon can be useful to address a multitude of geological and environmental issues. However, some difficulties arise when trying to use radon as earthquake precursor because the earthquake‐related anomaly cannot be easily and univocally discriminated from other anomalies of different origin. To explore the relationship between soil radon emission and seismicity, a local network is operating in southeastern Sicily (Italy). Its peculiarities are the uniform geological condition of the monitoring sites and the long data record with simultaneous measures of radon and the main climate variables. In this paper, we applied continuous wavelet transformation to a ≈ 3.5 years long soil radon time series to detect periodic variations. Results indicate the occurrence of cycles at annual, semiannual, and monthly periodicity, which are ascribable to the effects of the climatic‐environmental parameters. The periodic components have been modeled and the signal conveniently filtered. We show as the characterization of the long‐term behavior of radon signals is essential to recognize anomalies in radon emission, which can be related to geological/environmental phenomena. The methodology proposed in this work provides a reliable characterization of the radon time series and can be applied at various spatial/temporal scales, depending on the scientific objectives to be achieved. This approach can also represent the base for further analysis like the investigation of the modulation between the periodic components and short‐term forecasts

Visualization of the Multi-Center Ultrafast Response in Photoexcited Spinel Co3O4

Femtosecond optical and X-ray measurements show excitation-specific ultrafast relaxation processes in spinel Co3O4 thin films. Upon ligand-to-metal and metal-to-metal charge transfer, the system decays through different energy pathways characterized by different coherent and incoherent responses