2,281 research outputs found
Magnetic Properties of Mesoporous and Nano-particulate Metal Oxides
The magnetic properties of the first row transition metal oxides are wide and varied and have
been studied extensively since the 1930โs. Observations that the magnetic properties of these
material types change with the dimension of the sample have stimulated many theoretical and
experimental studies of the systems involved. As sample sizes decrease towards the nanoscale
long range crystallographic order is no longer possible. However, the application of mesoporous
silica samples as hard exo-templates to direct the formation of mesoporous metal oxides has
provided a new opportunity to explore the influence of scale of crystallographic order further.
These types of samples have pore systems running through the material on the mesoscale
(diameter between 2nm to 50nm) with pore walls truly in the nanoscale region (7nm to 9nm
thick) crystallographically ordered over large scale distances.
The work presented in this thesis presents magnetic and crystallographic studies of a variety
of the first row transition metal oxides from chromium to nickel in three dimensional mesoporous
forms predominantly using SQUID magnetometry and neutron powder diffraction. Rietveld
refinements of diffraction data from hematite and eskolaite (ยฎ-Fe2O3 and Cr2O3) show that
the samples have space groups identical to their bulk counterparts, however slight differences
in lattice parameters are observed. Refinement of magnetic properties has also been performed
and compared to magnetic property measurements. Of particular interest are results from
a mesoporous hematite which show suppression of a well defined first-order magnetic phase
transition (the Morin transition). This suppression has been studied extensively with neutron
powder diffraction and preliminary inelastic neutron spectroscopic measurements. Comparisons
with hematite nanoparticles which also show the suppression of the Morin transition can be
drawn. Parametric neutron powder diffraction studies on Co3O4 and NiO samples shows that
the Nรฉel ordering temperatures are lowered as the mesoporous structure is imposed. This too
was observed in eskolaite.
Other studies have been carried out on mesoporous alpha-MnO2 (magnetometry) and nanoscale
Li1+xMn2โxO4 (X-ray photo electron spectroscopy) with comparisons to their bulk counterparts
and finally nanoparticulate hausmannite Mn3O4 (magnetometry and muon spin relaxation)
which exhibits spin-glass type behaviour
Optimal Entanglement Enhancement for Mixed States
We consider the actions of protocols involving local quantum operations and
classical communication (LQCC) on a single system consisting of two separated
qubits. We give a complete description of the orbits of the space of states
under LQCC and characterise the representatives with maximal entanglement of
formation. We thus obtain a LQCC entanglement concentration protocol for a
single given state (pure or mixed) of two qubits which is optimal in the sense
that the protocol produces, with non-zero probability, a state of maximal
possible entanglement of formation. This defines a new entanglement measure,
the maximum extractable entanglement.Comment: Final version: to appear in Phys. Rev. Let
Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9
We show that the valence electrons of Ba3NaRu2O9, which has a quasi-molecular
structure, completely crystallize below 210 K. Using an extended Hubbard model,
we show that the charge ordering instability results from long-range Coulomb
interactions. However, orbital ordering, metal-metal bonding and formation of a
partial spin gap enforce the magnitude of the charge separation. The striped
charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition
with a quasi-degenerate charge-melted phase under photo-excitation at low
temperature. Our results establish a broad class of simple metal oxides as
models for emergent phenomena at the border between the molecular and solid
states.Comment: Minor changes, with supporting information. To appear in Phys. Rev.
Let
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
A system for de-identifying medical message board text
There are millions of public posts to medical message boards by users seeking support and information on a wide range of medical conditions. It has been shown that these posts can be used to gain a greater understanding of patientsโ experiences and concerns. As investigators continue to explore large corpora of medical discussion board data for research purposes, protecting the privacy of the members of these online communities becomes an important challenge that needs to be met. Extant entity recognition methods used for more structured text are not sufficient because message posts present additional challenges: the posts contain many typographical errors, larger variety of possible names, terms and abbreviations specific to Internet posts or a particular message board, and mentions of the authorsโ personal lives. The main contribution of this paper is a system to de-identify the authors of message board posts automatically, taking into account the aforementioned challenges. We demonstrate our system on two different message board corpora, one on breast cancer and another on arthritis. We show that our approach significantly outperforms other publicly available named entity recognition and de-identification systems, which have been tuned for more structured text like operative reports, pathology reports, discharge summaries, or newswire
Identifying Potential Adverse Effects Using the Web: A New Approach to Medical Hypothesis Generation
Medical message boards are online resources where users with a particular condition exchange information, some of which they might not otherwise share with medical providers. Many of these boards contain a large number of posts and contain patient opinions and experiences that would be potentially useful to clinicians and researchers. We present an approach that is able to collect a corpus of medical message board posts, de-identify the corpus, and extract information on potential adverse drug effects discussed by users. Using a corpus of posts to breast cancer message boards, we identified drug event pairs using co-occurrence statistics. We then compared the identified drug event pairs with adverse effects listed on the package labels of tamoxifen, anastrozole, exemestane, and letrozole. Of the pairs identified by our system, 75โ80% were documented on the drug labels. Some of the undocumented pairs may represent previously unidentified adverse drug effects
Identification of 34 Novel Proinflammatory Proteins in a Genome-Wide Macrophage Functional Screen
Signal transduction pathways activated by Toll-like Receptors and the IL-1 family of cytokines are fundamental to mounting an innate immune response and thus to clearing pathogens and promoting wound healing. Whilst mechanistic understanding of the regulation of innate signalling pathways has advanced considerably in recent years, there are still a number of critical controllers to be discovered. In order to characterise novel regulators of macrophage inflammation, we have carried out an extensive, cDNA-based forward genetic screen and identified 34 novel activators, based on their ability to induce the expression of cxcl2. Many are physiologically expressed in macrophages, although the majority of genes uncovered in our screen have not previously been linked to innate immunity. We show that expression of particular activators has profound but distinct impacts on LPS-induced inflammatory gene expression, including switch-type, amplifier and sensitiser behaviours. Furthermore, the novel genes identified here interact with the canonical inflammatory signalling network via specific mechanisms, as demonstrated by the use of dominant negative forms of IL1/TLR signalling mediators
Identification of Antigens Specific to Non-Tuberculous Mycobacteria: The Mce Family of Proteins as a Target of T Cell Immune Responses
The lack of an effective TB vaccine hinders current efforts in combating the TB pandemic. One theory as to why BCG is less protective in tropical countries is that exposure to non-tuberculous mycobacteria (NTM) reduces BCG efficacy. There are currently several new TB vaccines in clinical trials, and NTM exposure may also be relevant in this context. NTM exposure cannot be accurately evaluated in the absence of specific antigens; those which are known to be present in NTM and absent from M. tuberculosis and BCG. We therefore used a bioinformatic pipeline to define proteins which are present in common NTM and absent from the M. tuberculosis complex, using protein BLAST, TBLASTN and a short sequence protein BLAST to ensure the specificity of this process. We then assessed immune responses to these proteins, in healthy South Africans and in patients from the United Kingdom and United States with documented exposure to NTM. Low level responses were detected to a cluster of proteins from the mammalian cell entry family, and to a cluster of hypothetical proteins, using ex vivo ELISpot and a 6 day proliferation assay. These early findings may provide a basis for characterising exposure to NTM at a population level, which has applications in the field of TB vaccine design as well as in the development of diagnostic tests
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