203 research outputs found
Normalization factors for magnetic relaxation of small particle systems in non-zero magnetic field
We critically discuss relaxation experiments in magnetic systems that can be
characterized in terms of an energy barrier distribution, showing that proper
normalization of the relaxation data is needed whenever curves corresponding to
different temperatures are to be compared. We show how these normalization
factors can be obtained from experimental data by using the
scaling method without making any assumptions about the nature of the energy
barrier distribution. The validity of the procedure is tested using a
ferrofluid of Fe_3O_4 particles.Comment: 5 pages, 6 eps figures added in April 22, to be published in Phys.
Rev. B 55 (1 April 1997
Epitaxial growth of Cu on Cu(001): experiments and simulations
A quantitative comparison between experimental and Monte Carlo simulation
results for the epitaxial growth of Cu/Cu(001) in the submonolayer regime is
presented. The simulations take into account a complete set of hopping
processes whose activation energies are derived from semi-empirical
calculations using the embedded-atom method. The island separation is measured
as a function of the incoming flux and the temperature. A good quantitative
agreement between the experiment and simulation is found for the island
separation, the activation energies for the dominant processes, and the
exponents that characterize the growth. The simulation results are then
analyzed at lower coverages, which are not accessible experimentally, providing
good agreement with theoretical predictions as well.Comment: Latex document. 7 pages. 3 embedded figures in separate PS files. One
bbl fil
A high-mobility two-dimensional electron gas at the heteroepitaxial spinel/perovskite complex oxide interface of {\gamma}-Al2O3/SrTiO3
The discovery of two-dimensional electron gases (2DEGs) at the
heterointerface between two insulating perovskite-type oxides, such as LaAlO3
and SrTiO3, provides opportunities for a new generation of all-oxide electronic
and photonic devices. However, significant improvement of the interfacial
electron mobility beyond the current value of approximately 1,000 cm2V-1s-1 (at
low temperatures), remains a key challenge for fundamental as well as applied
research of complex oxides. Here, we present a new type of 2DEG created at the
heterointerface between SrTiO3 and a spinel {\gamma}-Al2O3 epitaxial film with
excellent quality and compatible oxygen ions sublattices. This
spinel/perovskite oxide heterointerface exhibits electron mobilities more than
one order of magnitude higher than those of perovskite/perovskite oxide
interfaces, and demonstrates unambiguous two-dimensional conduction character
as revealed by the observation of quantum magnetoresistance oscillations.
Furthermore, we find that the spinel/perovskite 2DEG results from
interface-stabilized oxygen vacancies and is confined within a layer of 0.9 nm
in proximity to the heterointerface. Our findings pave the way for studies of
mesoscopic physics with complex oxides and design of high-mobility all-oxide
electronic devices.Comment: 25pages, 5 figure
Finite-Size and surface effects in maghemite nanoparticles: Monte Carlo simulations
Finite-size and surface effects in fine particle systems are investigated by
Monte Carlo simulation of a model of a -FeO (maghemite) single
particle. Periodic boundary conditions have been used to simulate the bulk
properties and the results compared with those for a spherical shaped particle
with free boundaries to evidence the role played by the surface on the
anomalous magnetic properties displayed by these systems at low temperatures.
Several outcomes of the model are in qualitative agreement with the
experimental findings. A reduction of the magnetic ordering temperature,
spontaneous magnetization, and coercive field is observed as the particle size
is decreased. Moreover, the hysteresis loops become elongated with high values
of the differential susceptibility, resembling those from frustrated or
disordered systems. These facts are consequence of the formation of a surface
layer with higher degree of magnetic disorder than the core, which, for small
sizes, dominates the magnetization processes of the particle. However, in
contradiction with the assumptions of some authors, our model does not predict
the freezing of the surface layer into a spin-glass-like state. The results
indicate that magnetic disorder at the surface simply facilitates the thermal
demagnetization of the particle at zero field, while the magnetization is
increased at moderate fields, since surface disorder diminishes ferrimagnetic
correlations within the particle. The change in shape of the hysteresis loops
with the particle size demonstrates that the reversal mode is strongly
influenced by the reduced atomic coordination and disorder at the surface.Comment: Twocolumn RevTex format. 19 pages, 15 Figures included. Submitted to
Phys. Rev.
Error bounds for monomial convexification in polynomial optimization
Convex hulls of monomials have been widely studied in the literature, and
monomial convexifications are implemented in global optimization software for
relaxing polynomials. However, there has been no study of the error in the
global optimum from such approaches. We give bounds on the worst-case error for
convexifying a monomial over subsets of . This implies additive error
bounds for relaxing a polynomial optimization problem by convexifying each
monomial separately. Our main error bounds depend primarily on the degree of
the monomial, making them easy to compute. Since monomial convexification
studies depend on the bounds on the associated variables, in the second part,
we conduct an error analysis for a multilinear monomial over two different
types of box constraints. As part of this analysis, we also derive the convex
hull of a multilinear monomial over .Comment: 33 pages, 2 figures, to appear in journa
Comparative (Meta)genomic Analysis and Ecological Profiling of Human Gut-Specific Bacteriophage φB124-14
Bacteriophage associated with the human gut microbiome are likely to have an important impact on community structure and function, and provide a wealth of biotechnological opportunities. Despite this, knowledge of the ecology and composition of bacteriophage in the gut bacterial community remains poor, with few well characterized gut-associated phage genomes currently available. Here we describe the identification and in-depth (meta)genomic, proteomic, and ecological analysis of a human gut-specific bacteriophage (designated φB124-14). In doing so we illuminate a fraction of the biological dark matter extant in this ecosystem and its surrounding eco-genomic landscape, identifying a novel and uncharted bacteriophage gene-space in this community. φB124-14 infects only a subset of closely related gut-associated Bacteroides fragilis strains, and the circular genome encodes functions previously found to be rare in viral genomes and human gut viral metagenome sequences, including those which potentially confer advantages upon phage and/or host bacteria. Comparative genomic analyses revealed φB124-14 is most closely related to φB40-8, the only other publically available Bacteroides sp. phage genome, whilst comparative metagenomic analysis of both phage failed to identify any homologous sequences in 136 non-human gut metagenomic datasets searched, supporting the human gut-specific nature of this phage. Moreover, a potential geographic variation in the carriage of these and related phage was revealed by analysis of their distribution and prevalence within 151 human gut microbiomes and viromes from Europe, America and Japan. Finally, ecological profiling of φB124-14 and φB40-8, using both gene-centric alignment-driven phylogenetic analyses, as well as alignment-free gene-independent approaches was undertaken. This not only verified the human gut-specific nature of both phage, but also indicated that these phage populate a distinct and unexplored ecological landscape within the human gut microbiome
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