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Defect-induced magnetism and transport phenomena in epitaxial oxides
This work focuses on the impact of defects, intrinsic or artificially introduced, on the
functional properties of thin, epitaxial oxide films. In the first part, the origin of the ferromagnetic properties of Mn-doped and undoped zinc oxide is studied. The deposition conditions are found to have a significant impact on the structural, transport and
magnetic properties of the thin films. Combining x-ray magnetic circular dichroism and magnetometry experiments, it is established that the transition metal dopants (i.e. Mn) have no influence on the ferromagnetic nature of the zinc oxide, but that localised magnetic moments on intrinsic defects are in fact responsible for the ferromagnetic behaviour. A relation between strain (related to defect concentration) and magnetisation is established.
In the second part of this dissertation, artificially introduced defects are employed in order to discover the fundamental conduction mechanism behind the two-dimensionally conductive LaAlO3/SrTiO3 interface. All experiments, from varying deposition temperature, to oxygen pressure, to laser fluence or to the insertion of (doped) perovskite layers, point towards a structurally governed conduction mechanism, although the exact details are still unclear. Distinct transitions in the resistance versus temperature curves are observed at different values than the bulk phase transformation temperature. These transitions form the boundaries of different conduction modes, with tendencies towards non-Fermi-liquid behaviour observed in certain two-dimensionally conducting samples in limited temperature regimes. By optimising the (defect) structure at the interface, i.e. by introducing a single unit cell of (La0.5,Sr0.5)TiO3 or SnTiO3, it is shown that the sheet carrier density can be dramatically enhanced, up to an order of magnitude higher than unmodified LaAlO3/SrTiO3 interfaces with a value of 1e14 cm−2 at 200 K. Finally, attempts at functionalising the conductive heterointerface by doping and inserting (anti)ferromagnetic layers are made
Chaotic thermohaline convection in low-porosity hydrothermal systems
Fluids circulate through the Earth's crust perhaps down to depths as great as 5^15 km, based on oxygen isotope
systematics of exhumed metamorphic terrains, geothermal fields, mesozonal batholithic rocks and analysis of obducted
ophiolites. Hydrothermal flows are driven by both thermal and chemical buoyancy; the former in response to the
geothermal gradient and the latter due to differences in salinity that appear to be ubiquitous. Topographically driven
flows generally become less important with increasing depth. Unlike heat, solute cannot diffuse through solid matrix.
As a result, temperature perturbations advect more slowly than salinity fluctuations by the factor P, but diffuse more
rapidly by the factor U/D and are so smoothed out more efficiently. Here, P is porosity, while U and D denote the
thermal and chemical molecular diffusivity, respectively. Double-advective instabilities may play a significant role in
solute and heat transport in the deep crust where porosities are low. We have studied the stability and dynamics of the
flow as a function of P and thermal and chemical buoyancy, for situations where mechanical dispersion of solute
dominates over molecular diffusion in the fluid. In the numerical experiments, a porous medium is heated from below
while solute provides a stabilizing influence. For typical geological parameters, the thermohaline flow appears
intrinsically chaotic. We attribute the chaotic dynamical behavior of the flow to a dominance of advective and
dispersive chemical transfer over the more moderate convective heat transfer, the latter actually driving the flow. Fast
upward advective transport and lateral mixing of solute leads to formation of horizontal chemical barriers at depth.
These gravitationally stable interfaces divide the domain in several layers of distinct composition and lead to
significantly reduced heat flow for thousands of years. The unsteady behavior of thermochemical flow in low-porosity
regions has implications for heat transport at mid-ocean ridges, for ore genesis, for metasomatism and metamorphic
petrology, and the diagenetic history of sediments in subsiding basins. ß 1999 Elsevier Science B.V. All rights
reserved
Using Atom-Probe Tomography to Understand ZnO∶Al=SiO2=Si Schottky Diodes
We use electronic transport and atom-probe tomography to study ZnO∶Al/SiO[subscript 2]/Si Schottky diodes on lightly doped n- and p-type Si. We vary the carrier concentration in the ZnO∶Al films by 2 orders of magnitude, but the Schottky barrier height remains nearly constant. Atom-probe tomography shows that Al segregates to the interface, so that the ZnO∶Al at the junction is likely to be metallic even when the bulk of the ZnO∶Al film is semiconducting. We hypothesize that the observed Fermi-level pinning is connected to the insulator-metal transition in doped ZnO. This implies that tuning the band alignment at oxide/Si interfaces may be achieved by controlling the transition between localized and extended states in the oxide, thereby changing the orbital hybridization across the interface.United States. Dept. of Energy (EERE Postdoctoral Research Award)United States. Air Force Office of Scientific Research (Contract FA9550-12-1- 0189)National Science Foundation (U.S.) (Contract DMR-0952794)United States. Dept. of Energy (Bay Area Photovoltaic Consortium. Contract DE-EE0004946)National Science Foundation (U.S.) (Center for Nanoscale Systems. Contract ECS-0335765
New structure-performance relationships for surface-based lattice heat sinks
Heat sinks have manifold applications, from micro-electronics to nuclear fusion reactors. Their performance expectations will continue to increase in line with the power consumption and miniaturisation of technology. Additive manufacturing enables the creation of novel, compact heat sinks with greater surface-to-volume ratios and geometrical complexities than standard pin/fin arrays and pipes. Despite this, there has been little research into the use of high surface area lattice structures as heat sinks. Here, the hydraulic and thermal performance of five surface-based lattice structures were examined numerically. Computational fluid dynamics was used to create useful predictive models for pressure drop and volumetric heat transfer coefficients over a range of flow rates and volume fractions, which can henceforth be used by heat transfer engineers. The thermal performance of surface-based lattices was found to be heavily dependent on internal geometry, with structures capable of distributing thermal energy across the entire fluid volume having greater volumetric heat transfer coefficients than those with only localised areas of high heat transfer and low levels of fluid mixing
Candida dubliniensis candidemia in patients with chemotherapy-induced neutropenia and bone marrow transplantation.
The recently described species Candida dubliniensis has been recovered primarily from superficial oral candidiasis in HIV-infected patients. No clinically documented invasive infections were reported until now in this patient group or in other immunocompromised patients. We report three cases of candidemia due to this newly emerging Candida species in HIV-negative patients with chemotherapy-induced immunosuppression and bone marrow transplantation
Isolation and Immunocytochemical Characterization of Three Tachykinin-Related Peptides from the Mosquito, Culex salinarius
Three myotropic peptides belonging to the Arg-amide insect tachykinin family were isolated from whole-body extracts of the mosquito, Culex salinarius . The peptides, APSGFMGMR-NH 2 , APYGFTGMR-NH 2 and APSGFFGMR-NH 2 (designated culetachykinin I, II, and III) were isolated and purified on the basis of their ability to stimulate muscle contractions of isolated Leucophaea maderae hindgut. Biologically inactive methionine sulfoxides of two of the three peptides were isolated using an ELISA system based upon antiserum raised against APYGFTGMR-NH 2 and identified with mass spectrometry. Immunocytochemistry localized these peptides in cells in the brain, antennae, subesophageal, thoracic and abdominal ganglion, proventriculus and midgut. Nerve tracts containing these peptides were found in the median nerve of the brain, central body, nervi corpus cardiaci, cervical nerve, antennal lobe and on the surface of the midgut.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45417/1/11064_2004_Article_419298.pd
Transonic Dislocation Propagation in Diamond
The motion of line defects (dislocations) has been studied for over 60 years
but the maximum speed at which they can move is unresolved. Recent models and
atomistic simulations predict the existence of a limiting velocity of
dislocation motions between the transonic and subsonic ranges at which the
self-energy of dislocation diverges, though they do not deny the possibility of
the transonic dislocations. We use femtosecond x-ray radiography to track
ultrafast dislocation motion in shock-compressed single-crystal diamond. By
visualizing stacking faults extending faster than the slowest sound wave speed
of diamond, we show the evidence of partial dislocations at their leading edge
moving transonically. Understanding the upper limit of dislocation mobility in
crystals is essential to accurately model, predict, and control the mechanical
properties of materials under extreme conditions
Simultaneous Bright- and Dark-Field X-ray Microscopy at X-ray Free Electron Lasers
The structures, strain fields, and defect distributions in solid materials
underlie the mechanical and physical properties across numerous applications.
Many modern microstructural microscopy tools characterize crystal grains,
domains and defects required to map lattice distortions or deformation, but are
limited to studies of the (near) surface. Generally speaking, such tools cannot
probe the structural dynamics in a way that is representative of bulk behavior.
Synchrotron X-ray diffraction based imaging has long mapped the deeply embedded
structural elements, and with enhanced resolution, Dark Field X-ray Microscopy
(DFXM) can now map those features with the requisite nm-resolution. However,
these techniques still suffer from the required integration times due to
limitations from the source and optics. This work extends DFXM to X-ray free
electron lasers, showing how the photons per pulse available at these
sources offer structural characterization down to 100 fs resolution (orders of
magnitude faster than current synchrotron images). We introduce the XFEL DFXM
setup with simultaneous bright field microscopy to probe density changes within
the same volume. This work presents a comprehensive guide to the multi-modal
ultrafast high-resolution X-ray microscope that we constructed and tested at
two XFELs, and shows initial data demonstrating two timing strategies to study
associated reversible or irreversible lattice dynamics
Nanopillar spin filter tunnel junctions with manganite barriers.
The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.This work was partially supported by the ERC Advanced Integrators Grant “SUPERSPIN”. B.P. was funded by the Nehru Trust for Cambridge University and St John’s College. The TEM work at Texas A&M was supported by the U.S. National Science Foundation (NSF-DMR 0846504). The authors wish to thank Prof. J. Kumar (IIT Kanpur, India) for help in improving the manuscript.This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/nl500798
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