STEM study of Li4 Ti5 O12 anode material modified with Ag nanoparticles.

Comprehensive scanning transmission electron microscopy (STEM) analysis of Li4 Ti5 O12 (LTO) powder modified by deposited Ag nanoparticles was performed. Nanocomposite powders with Ag content of 1 wt.%, 4 wt.%, 10 wt.% were fabricated in a chemical process from suspensions of Ag and LTO. Apart from the STEM results, the presence of pure silver on the surface of the ceramic powder was confirmed by XRD and XPS analyses. The silver particles deposited on the LTO particles were characterized using the EDS mapping technique. The quantified results of the EDS mapping showed a relatively homogenous distribution of silver nanoparticles on the powder surface for every metal content. The mean diameter of the nanoparticles deposited on the LTO powder was about 4 nm in all cases. An increase in the Ag content during chemical surface modification did not cause changes in the microstructure. Focusing on an analysis of the metallic nanoparticles on the ceramic powder, electron tomography was used as an investigative technique. A very precise analysis of three-dimensional nanostructures is desirable for a comprehensive analysis of complex materials. The quantified analysis of the Ag nanoparticles visualized using electron tomography confirmed the results of the size measurements taken from the two-dimensional EDS maps.

Self-assembled quantum dots in a nanowire system for quantum photonics.

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Growth of ultrathin epitaxial Fe/MgO spin injector on (0, 0, 1) (Ga, Mn)As.

We have grown an ultrathin epitaxial Fe/MgO bilayer on (Ga, Mn)As by e-beam evaporation in UHV. The system structure has been investigated by high resolution transmission electron microscopy (TEM) experiments which show that the Fe and MgO films, covering completely the (Ga, Mn)As, grow with the epitaxial relationship Fe[100](001) [parallel] MgO[110](001) [parallel] (Ga,Mn)As[110](001). The magnetic reversal process, studied by the magneto-optical Kerr effect (MOKE) at room temperature, demonstrates that the iron is ferromagnetic and possesses a cubic anisotropy, confirming the epitaxy relationship found with TEM. Resistivity measurements across the barrier display a non-Ohmic behavior characterized by cubic conductance as a function of the applied voltage suggesting tunneling-dominated transport across the barrier.

Electric field control of magnetic properties and electron transport in BaTiO3-based multiferroic heterostructures.

In this paper, we report on a purely electric mechanism for achieving the electric control of the interfacial spin polarization and magnetoresistance in multiferroic tunneling junctions. We investigate micrometric devices based on the Co/Fe/BaTiO3/La0.7Sr0.3MnO3 heterostructure, where Co/Fe and La0.7Sr0.3MnO3 are the magnetic electrodes and BaTiO3 acts both as a ferroelectric element and tunneling barrier. We show that, at 20 K, devices with a 2 nm thick BaTiO3 barrier present both tunneling electroresistance (TER = 12 ± 0.1%) and tunneling magnetoresistance (TMR). The latter depends on the direction of the BaTiO3 polarization, displaying a sizable change of the TMR from -0.32 ± 0.05% for the polarization pointing towards Fe, to -0.12 ± 0.05% for the opposite direction. This is consistent with the on-off switching of the Fe magnetization at the Fe/BaTiO3 interface, driven by the BaTiO3 polarization, we have previously demonstrated in x-ray magnetic circular dichroism experiments.

Reconstruction of human immunodeficiency virus (HIV) sub-epidemics in Italian regions.

BACKGROUND: The spread of human immunodeficiency virus (HIV) infections is likely to consist of sub-epidemics in local areas and/or risk groups. Small-area risk group specific analyses may thus be a suitable means of better understanding and controlling the epidemic course. METHODS: An age, period, and cohort back-calculation method was used to reconstruct region-specific epidemics of HIV infection. The HIV infection incidence rates were estimated for individual Italian regions by using as denominator the specific risk category population (i.e. intravenous drug user population [IDU], homosexual/ bisexual population [MSWM], and Italian general population for heterosexual contacts [HST]). Incidence rates obtained in this way represent the risk of HIV infection conditional to belonging to the specific sub-group of the population. RESULTS: The HIV epidemic is heterogeneous in terms of gender, risk category and region. The highest risk of HIV infection has been estimated in the Lombardia region (North-West area) among men belonging to the IDU category in 1985. In recent years, a trend of decrease in HIV has been estimated, especially among IDU and MSWM. For the HST category, some regions have shown trends of great increase, particularly on the island of Sardegna and the regions of Puglia and Sicilia (Southern Italy). In 1987, most HIV infections were observed among IDU (39-90%), while in 1992 the MSWM and HST categories made the greatest contribution to the HIV epidemic. CONCLUSIONS: The results stress the idea of sub-epidemics rather than a single epidemic affecting the entire country. Some Southern regions emerge as areas in which the spread of HIV infections, although still at a rather low level, should cause considerable concern, particularly the trend of new infections by heterosexual transmission involving the general population. Detailed information on levels and trends of HIV infection epidemics at the local level are essential for surveillance purposes and for planning health care facilities, and can highlight areas in which preventive measures can be effective.

Use of AIDS surveillance data to describe subepidemic dynamics.

BACKGROUND: Official reports on AIDS surveillance mainly consist of absolute numbers of AIDS cases or cumulative incidence rates. More detailed analyses focusing on the clusters of subepidemics within Italy seemed necessary for a better understanding and more accurate description of the epidemic. METHODS: Age-specific AIDS incidence rates were calculated with reference to resident population by sex, calendar time and geographical area. Age-standardized incidence rates, with the Italian resident population in 1990 as standard, were used to present time trends and geographical distributions. All analyses were repeated for injecting drug users, homosexual/bisexual men, heterosexual contacts and individuals with other or undetermined risk factors. RESULTS: Annual incidence rates for AIDS in Italy increased over the study period. The highest rates were observed in the North and in Sardinia, while Southern regions showed generally lower rates. This heterogeneity was more evident when examining small geographical areas (i.e. provinces). Epidemics in some of the smaller provinces, such as Imperia and Livorno (Northwestern port towns), were shown to be important in that they greatly affect AIDS incidence rates in the regions in which they are located. CONCLUSIONS: According to our analysis, the crude presentation of data from the Italian AIDS Registry is not adequate for understanding the national spread of the AIDS epidemic in terms of several local subepidemics, which may differ by size, temporal trend, and risk group composition. Classifying cases according to their place of residence, which we considered as a good proxy of the place of life, was fundamental for correctly locating these subepidemics. Furthermore, the use of age-standardized rates allowed for unbiased comparisons between regions whose population may have a different age structure and dynamics.

[Sorin-Pericarbon pericardial bioprosthesis. A 4-year follow-up study]. / Bioprothèses péricardiques de Sorin-Péricarbon. Suivi à 4 ans.

Between January 1986 and December 1986, 106 Sorin-Pericarbon (new prostheses made of bobine pericardium) valves were implanted by our group: the object of this paper is to report the medium-term results of this new bioprosthesis. The probability of degeneration-free function at 4 years was 96.5 +/- 3.7% in patients with aortic valve replacement; the probability of reoperation-free survival for all patients was 95.2 +/- 3.3% and 96.5 +/- 3.7% for patients with aortic valve replacement with this prosthesis. The results of this new Sorin-Pericarbon bioprosthesis at medium-term are satisfactory and need to be confirmed at long-term by further follow-up.

Electric control of magnetism at the Fe/BaTiO3 interface.

Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO3 system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO3 dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO3 system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO3 polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO3 produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices.

Multivariate statistical analysis as a tool for the segmentation of 3D spectral data.

Acquisition of three-dimensional (3D) spectral data is nowadays common using many different microanalytical techniques. In order to proceed to the 3D reconstruction, data processing is necessary not only to deal with noisy acquisitions but also to segment the data in term of chemical composition. In this article, we demonstrate the value of multivariate statistical analysis (MSA) methods for this purpose, allowing fast and reliable results. Using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) coupled with a focused ion beam (FIB), a stack of spectrum images have been acquired on a sample produced by laser welding of a nickel-titanium wire and a stainless steel wire presenting a complex microstructure. These data have been analyzed using principal component analysis (PCA) and factor rotations. PCA allows to significantly improve the overall quality of the data, but produces abstract components. Here it is shown that rotated components can be used without prior knowledge of the sample to help the interpretation of the data, obtaining quickly qualitative mappings representative of elements or compounds found in the material. Such abundance maps can then be used to plot scatter diagrams and interactively identify the different domains in presence by defining clusters of voxels having similar compositions. Identified voxels are advantageously overlaid on secondary electron (SE) images with higher resolution in order to refine the segmentation. The 3D reconstruction can then be performed using available commercial softwares on the basis of the provided segmentation. To asses the quality of the segmentation, the results have been compared to an EDX quantification performed on the same data.

Conditions for efficient on-chip magnetic bead detection via magnetoresistive sensors.

A commonly used figure of merit of magnetoresistive sensors employed to detect magnetic beads labeling biomolecules in lab-on-chip applications is the sensor sensitivity (S0) to external magnetic fields in the linear region of the sensor. In this paper we show that, in case of lock-in detection and bead excitation by a small AC magnetic field, S0 is not the good figure of merit to optimize. Indeed, the highest sensitivity to the magnetic beads is achieved biasing the sensor in the region of its characteristics where the product between the DC bias field and the second derivative of the resistance with respect to the magnetic field is maximum. The validity of this criterion, derived from a phenomenological model of bead detection, is proved in case of magnetic tunneling junction sensors detecting magnetic beads with 250nm diameter. This work paves the way to the development of a new generation of sensors properly designed to maximize the bead sensitivity.

Direct observation of a highly spin-polarized organic spinterface at room temperature.

Organic semiconductors constitute promising candidates toward large-scale electronic circuits that are entirely spintronics-driven. Toward this goal, tunneling magnetoresistance values above 300% at low temperature suggested the presence of highly spin-polarized device interfaces. However, such spinterfaces have not been observed directly, let alone at room temperature. Thanks to experiments and theory on the model spinterface between phthalocyanine molecules and a Co single crystal surface, we clearly evidence a highly efficient spinterface. Spin-polarised direct and inverse photoemission experiments reveal a high degree of spin polarisation at room temperature at this interface. We measured a magnetic moment on the molecule's nitrogen π orbitals, which substantiates an ab-initio theoretical description of highly spin-polarised charge conduction across the interface due to differing spinterface formation mechanisms in each spin channel. We propose, through this example, a recipe to engineer simple organic-inorganic interfaces with remarkable spintronic properties that can endure well above room temperature.

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices.

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.

The impact of the 1993 European revision of the AIDS case definition on back-calculation estimates: an application in Italy.

OBJECTIVES: The revision of the case definition for AIDS within a given observation period causes discontinuity points on the epidemic curve to which back-calculation procedures can be sensitive. The aim of this work is to characterize the impact of the 1993 European revision of AIDS case definition and to evaluate the degree of distortion by which back-calculation estimates obtained on Italian data are affected. METHODS: The back-calculation procedure used, is a generalization of standard methods for estimating and projecting the AIDS epidemic. Age at onset of HIV infection, incubation time distribution, susceptible population and competitive non-AIDS mortality rates for infected persons are included in the estimation process. RESULTS: Back-calculation estimates resulted to be sensitive to the discontinuity point that the 1993 revision of AIDS case definition induced on the epidemic curve. The resulting effect is a tendency to overestimate HIV incidence in recent years by 20-40%. The maximum effect was found using data with end of AIDS cases at December 1993. The overestimation is a transient effect, that is, it is expected to almost disappear in 1995-1996. CONCLUSIONS: The introduction of the 1993 European revision of AIDS case definition caused distortion of back-calculation estimates in Italy, leading to overestimate incidence and prevalence of HIV infection for recent years. It is believed that this effect has occurred in all countries that adopted the 1993 European revision of AIDS case definition, and its characterization is potentially useful for further changes of AIDS case definition in the future.