712 research outputs found
Magnetic anisotropy determination and magnetic hyperthermia properties of small Fe nanoparticles in the superparamagnetic regime
We report on the magnetic and hyperthermia properties of iron nanoparticles
synthesized by organometallic chemistry. They are 5.5 nm in diameter and
display a saturation magnetization close to the bulk one. Magnetic properties
are dominated by the contribution of aggregates of nanoparticles with respect
to individual isolated nanoparticles. Alternative susceptibility measurements
are been performed on a low interacting system obtained after eliminating the
aggregates by centrifugation. A quantitative analysis using the Gittleman s
model allow a determination of the effective anisotropy Keff = 1.3 * 10^5
J.m^{-3}, more than two times the magnetocristalline value of bulk iron.
Hyperthermia measurements are performed on agglomerates of nanoparticles at a
magnetic field up to 66 mT and at frequencies in the range 5-300 kHz. Maximum
measured SAR is 280 W/g at 300 kHz and 66 mT. Specific absorption rate (SAR)
displays a square dependence with the magnetic field below 30 mT but deviates
from this power law at higher value. SAR is linear with the applied frequency
for mu_0H=19 mT. The deviations from the linear response theory are discussed.
A refined estimation of the optimal size of iron nanoparticles for hyperthermia
applications is provided using the determined effective anisotropy value
Quantitative Cathodoluminescence Opens New Areas of Investigation in Semiconductor Research and Production
The increasing demand for new opto-electronics devices such as solar cells, laser diodes (LD), and high-brightness light-emitting diodes (HBLED), combined with the economic necessity to achieve lower energy consumption levels and higher device yields, is motivating researchers to develop new materials. The semiconductor industry is actively looking for alternatives to silicon, for example, to address new niche market applications in power devices. Constant efforts employed to reduce production costs are leading manufacturers to grow GaN on silicon substrate, creating new technical challenges, especially regarding the control of defect density on wafer. For all these reasons many studies are being initiated to improve understanding of the fundamental physical properties and behavior of compound semiconductor materials used in quantum wells, quantum dots and nanowire-like structures. Cathodoluminescence (CL) is a spectroscopy method that can generate reliable, quantitative, and stable data for research as well as prepare a basis for quality control during productio
Computer-Generated Ovaries to Assist Follicle Counting Experiments
Precise estimation of the number of follicles in ovaries is of key importance in the field of reproductive biology, both from a developmental point of view, where follicle numbers are determined at specific time points, as well as from a therapeutic perspective, determining the adverse effects of environmental toxins and cancer chemotherapeutics on the reproductive system. The two main factors affecting follicle number estimates are the sampling method and the variation in follicle numbers within animals of the same strain, due to biological variability. This study aims at assessing the effect of these two factors, when estimating ovarian follicle numbers of neonatal mice. We developed computer algorithms, which generate models of neonatal mouse ovaries (simulated ovaries), with characteristics derived from experimental measurements already available in the published literature. The simulated ovaries are used to reproduce in-silico counting experiments based on unbiased stereological techniques; the proposed approach provides the necessary number of ovaries and sampling frequency to be used in the experiments given a specific biological variability and a desirable degree of accuracy. The simulated ovary is a novel, versatile tool which can be used in the planning phase of experiments to estimate the expected number of animals and workload, ensuring appropriate statistical power of the resulting measurements. Moreover, the idea of the simulated ovary can be applied to other organs made up of large numbers of individual functional units
Phonon Mode Spectroscopy, Electron-Phonon Coupling and the Metal-Insulator Transition in Quasi-One-Dimensional M2Mo6Se6
We present electronic structure calculations, electrical resistivity data and
the first specific heat measurements in the normal and superconducting states
of quasi-one-dimensional M2Mo6Se6 (M = Tl, In, Rb). Rb2Mo6Se6 undergoes a
metal-insulator transition at ~170K: electronic structure calculations indicate
that this is likely to be driven by the formation of a dynamical charge density
wave. However, Tl2Mo6Se6 and In2Mo6Se6 remain metallic down to low temperature,
with superconducting transitions at Tc = 4.2K and 2.85K respectively. The
absence of any metal-insulator transition in these materials is due to a larger
in-plane bandwidth, leading to increased inter-chain hopping which suppresses
the density wave instability. Electronic heat capacity data for the
superconducting compounds reveal an exceptionally low density of states DEF =
0.055 states eV^-1 atom^-1, with BCS fits showing 2Delta/kBTc >= 5 for
Tl2Mo6Se6 and 3.5 for In2Mo6Se6. Modelling the lattice specific heat with a set
of Einstein modes, we obtain the approximate phonon density of states F(w).
Deconvolving the resistivity for the two superconductors then yields their
electron-phonon transport coupling function a^2F(w). In Tl2Mo6Se6 and
In2Mo6Se6, F(w) is dominated by an optical "guest ion" mode at ~5meV and a set
of acoustic modes from ~10-30meV. Rb2Mo6Se6 exhibits a similar spectrum;
however, the optical phonon has a lower intensity and is shifted to ~8meV.
Electrons in Tl2Mo6Se6 couple strongly to both sets of modes, whereas In2Mo6Se6
only displays significant coupling in the 10-18meV range. Although pairing is
clearly not mediated by the guest ion phonon, we believe it has a beneficial
effect on superconductivity in Tl2Mo6Se6, given its extraordinarily large
coupling strength and higher Tc compared to In2Mo6Se6.Comment: 16 pages, 13 figure
Structural effect of heavy ion irradiation on GdBaCuO ceramics
The influence of twin boundaries as sinks on defects induced by 480 keV Kr ion irradiation in GdBaCuO crystals was observed in situ at 40 and 300 K. The interaction of the dislocations with the twin boundaries followed on a video recording. A crystalline to amorphous transition was observed above a total fluence of ∼ 4 - 5 x 10^12 Kr/cm2. A comparison between orthorhombic (Os) crystals and a monoclinic structure (Ms) (close to Os and whose parameters were calculated) shows that the behaviour of irradiation-induced extended defects does not depend on a small initial deformation of the orthorhombic cell. In both case, an occasional orthorhombic (or monoclinic) to tetragonal phase transition only occurs when the amorphization process has begun
Prevalence of oropharyngeal beta-lactamase-producing Capnocytophaga spp. in pediatric oncology patients over a ten-year period
BACKGROUND: The aim of this study was to evaluate the prevalence of beta-lactamase-producing Capnocytophaga isolates in young children hospitalized in the Pediatric Oncology Department of Hôpital Sud (Rennes, France) over a ten-year period (1993–2002). METHODS: In neutropenic children, a periodic survey of the oral cavity allows a predictive evaluation of the risk of systemic infections by Capnocytophaga spp. In 449 children with cancer, 3,053 samples were collected by oral swabbing and plated on TBBP agar. The susceptibility of Capnocytophaga isolates to five beta-lactams was determined. RESULTS: A total of 440 strains of Capnocytophaga spp. were isolated, 309 (70%) of which were beta-lactamase producers. The beta-lactamase-producing strains were all resistant to cefazolin, 86% to amoxicillin, and 63% to ceftazidime. The proportion of strains resistant to third-generation cephalosporins remained high throughout the ten-year study, while susceptibility to imipenem and amoxicillin combined with clavulanic acid was always conserved. CONCLUSION: These results highlight the risk of antibiotic failure in Capnocytophaga infections and the importance of monitoring immunosuppressed patients and testing for antibiotic susceptibility and beta-lactamase production
Bridging Time Scales in Cellular Decision Making with a Stochastic Bistable Switch
Cellular transformations which involve a significant phenotypical change of
the cell's state use bistable biochemical switches as underlying decision
systems. In this work, we aim at linking cellular decisions taking place on a
time scale of years to decades with the biochemical dynamics in signal
transduction and gene regulation, occuring on a time scale of minutes to hours.
We show that a stochastic bistable switch forms a viable biochemical mechanism
to implement decision processes on long time scales. As a case study, the
mechanism is applied to model the initiation of follicle growth in mammalian
ovaries, where the physiological time scale of follicle pool depletion is on
the order of the organism's lifespan. We construct a simple mathematical model
for this process based on experimental evidence for the involved genetic
mechanisms. Despite the underlying stochasticity, the proposed mechanism turns
out to yield reliable behavior in large populations of cells subject to the
considered decision process. Our model explains how the physiological time
constant may emerge from the intrinsic stochasticity of the underlying gene
regulatory network. Apart from ovarian follicles, the proposed mechanism may
also be of relevance for other physiological systems where cells take binary
decisions over a long time scale.Comment: 14 pages, 4 figure
Large specific absorption rates in the magnetic hyperthermia properties of metallic iron nanocubes
We report on the magnetic hyperthermia properties of chemically synthesized
ferromagnetic 11 and 16 nm Fe(0) nanoparticles of cubic shape displaying the
saturation magnetization of bulk iron. The specific absorption rate measured on
16 nm nanocubes is 1690+-160 W/g at 300 kHz and 66 mT. This corresponds to
specific losses-per-cycle of 5.6 mJ/g, largely exceeding the ones reported in
other systems. A way to quantify the degree of optimization of any system with
respect to hyperthermia applications is proposed. Applied here, this method
shows that our nanoparticles are not fully optimized, probably due to the
strong influence of magnetic interactions on their magnetic response. Once
protected from oxidation and further optimized, such nano-objects could
constitute efficient magnetic cores for biomedical applications requiring very
large heating power
- …