Helium irradiation effects in polycrystalline Si, silica, and single crystal Si

Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin (≈55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change

Lamniform shark teeth from the Late Cretaceous of southernmost South America (Santa Cruz Province, Argentina)

Here we report multiple lamniform shark teeth recovered from fluvial sediments in the (Campanian-Maastrichtian) Cerro Fortaleza Formation, Santa Cruz Province, Argentina. This small tooth assemblage is compared to various lamniform sharks possessing similar dental morphologies, including Archaeolamna, Cretalamna, Dwardius, Dallasiella, and Cretodus. Although the teeth share numerous morphological features with the genus Archaeolamna, including a developed neck that maintains a relatively consistent width along the base of the crown, the small sample size and incomplete nature of these specimens precludes definitive taxonomic assignment. Regardless, the discovery of selachian teeth unique from those previously described for the region broadens the known diversity of Late Cretaceous South American sharks. Additionally, the discovery of the teeth in fluvial sandstone may indicate a euryhaline paleobiology in the lamniform taxon or taxa represented by this tooth assemblage.Fil: Schroeter, Elena R.. Drexel University; Estados UnidosFil: Egerton, Victoria M.. University Of Manchester; Reino UnidoFil: Ibiricu, Lucio Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; ArgentinaFil: Lacovara, Kenneth J.. Drexel University; Estados Unido

Attosecond tracking of light absorption and refraction in fullerenes

The collective response of matter is ubiquitous and widely exploited, e.g. in plasmonic, optical and electronic devices. Here we trace on an attosecond time scale the birth of collective excitations in a finite system and find distinct new features in this regime. Combining quantum chemical computation with quantum kinetic methods we calculate the time-dependent light absorption and refraction in fullerene that serve as indicators for the emergence of collective modes. We explain the numerically calculated novel transient features by an analytical model and point out the relevance for ultra-fast photonic and electronic applications. A scheme is proposed to measure the predicted effects via the emergent attosecond metrology.Comment: 11 pages, 3 figures, accepted in Phys. Rev.

Compositional analysis of InAs-GaAs-GaSb heterostructures by low-loss electron energy loss spectroscopy

As an alternative to Core-Loss Electron Energy Loss Spectroscopy, Low-Loss EELS is suitable for compositional analysis of complex heterostructures, such as the InAs-GaAs-GaSb system, since in this energy range the edges corresponding to these elements are better defined than in Core-Loss. Furthermore, the analysis of the bulk plasmon peak, which is present in this energy range, also provides information about the composition. In this work, compositional information in an InAs-GaAs-GaSb heterostructure has been obtained from Low-Loss EEL spectra

Residual disorder and diffusion in thin Heusler alloy films

Co2FeSi/GaAs(110) and Co2FeSi/GaAs(111)B hybrid structures were grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and X-ray diffraction. The films contained inhomogeneous distributions of ordered L2_1 and B2 phases. The average stoichiometry was controlled by lattice parameter measurements, however diffusion processes lead to inhomogeneities of the atomic concentrations and the degradation of the interface, influencing long-range order. An average long-range order of 30-60% was measured by grazing-incidence X-ray diffraction, i.e. the as-grown Co2FeSi films were highly but not fully ordered. Lateral inhomogeneities of the spatial distribution of long-range order in Co2FeSi were found using dark-field TEM images taken with superlattice reflections

Defect Structure of the High-Dielectric-Constant Perovskite Cacu3ti4o12

Using transmission electron microscopy (TEM) we studied CaCu3Ti4O12, an intriguing material that exhibits a huge dielectric response, up to kilohertz frequencies, over a wide range of temperature. Neither in single crystals, nor in polycrystalline samples, including sintered bulk- and thin-films, did we observe the twin domains suggested in the literature. Nevertheless, in the single crystals, we saw a very high density of dislocations with a Burger vector of [110], as well as regions with cation disorder and planar defects with a displacement vector 1/4[110]. In the polycrystalline samples, we observed many grain boundaries with oxygen deficiency, in comparison with the grain interior. The defect-related structural disorders and inhomogeneity, serving as an internal barrier layer capacitance (IBLC) in a semiconducting matrix, might explain the very large dielectric response of the material. Our TEM study of the structure defects in CaCu3Ti4O12 supports a recently proposed morphological model with percolating conducting regions and blocking regions.Comment: To be published in Physical Review B 21 pages, 8 figure

Robust and adjustable C-shaped electron vortex beams

Wavefront engineering is an important quantum technology, often applied to the production of states carrying orbital angular momentum (OAM). Here, we demonstrate the design and production of robust C-shaped beam states carrying OAM, in which the usual doughnut shaped transverse intensity structure of the vortex beam contains an adjustable gap. We find that the presence of the vortex lines in the core of the beam is crucial for maintaining the stability of the C-shape structure during beam propagation. The topological charge of the vortex core controls mainly the size of the C-shape, while its opening angle is related to the presence of vortex-anti-vortex loops. We demonstrate the generation and characterisation of C-shaped electron vortex beams, although the result is equally applicable to other quantum waves. C-shaped electron vortex beams have potential applications in nanoscale fabrication of planar split ring structures and three dimensional chiral structures as well as depth sensing and magnetic field determination through rotation of the gap in the C-shape

Electronic structure of undoped and potassium doped coronene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical gap upon potassium addition. Furthermore, our core level excitation data clearly signal filling of the conduction bands with electrons.Comment: 15 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:1102.328