7,301 research outputs found
A frequency-adjustable electromagnet for hyperthermia measurements on magnetic nanoparticles
We describe a low-cost and simple setup for hyperthermia measurements on
colloidal solutions of magnetic nanoparticles (ferrofluids) with a
frequency-adjustable magnetic field in the range 5-500 kHz produced by an
electromagnet. By optimizing the general conception and each component (nature
of the wires, design of the electromagnet), a highly efficient setup is
obtained. For instance, in a useful gap of 1.1 cm, a magnetic field of 4.8 mT
is generated at 100 kHz and 500 kHz with an output power of 3.4 W and 75 W,
respectively. A maximum magnetic field of 30 mT is obtained at 100 kHz. The
temperature of the colloidal solution is measured using optical fiber sensors.
To remove contributions due to heating of the electromagnet, a differential
measurement is used. In this configuration the sensitivity is better than 1.5
mW at 100 kHz and 19.3 mT. This setup allows one to measure weak heating powers
on highly diluted colloidal solutions. The hyperthermia characteristics of a
solution of Fe nanoparticles are described, where both the magnetic field and
the frequency dependence of heating power have been measured
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
Functional approach for pairing in finite systems: How to define restoration of broken symmetries in Energy Density Functional theory ?
The Multi-Reference Energy Density Functional (MR-EDF) approach (also called
configuration mixing or Generator Coordinate Method), that is commonly used to
treat pairing in finite nuclei and project onto particle number, is
re-analyzed. It is shown that, under certain conditions, the MR-EDF energy can
be interpreted as a functional of the one-body density matrix of the projected
state with good particle number. Based on this observation, we propose a new
approach, called Symmetry-Conserving EDF (SC-EDF), where the breaking and
restoration of symmetry are accounted for simultaneously. We show, that such an
approach is free from pathologies recently observed in MR-EDF and can be used
with a large flexibility on the density dependence of the functional.Comment: proceeding of the conference "Many body correlations from dilute to
dense Nuclear systems", Paris, February 201
Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles
We report on transport properties of millimetric super-lattices of CoFe
nanoparticles surrounded by organic ligands. R(T)s follow R(T) =
R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s
follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a
universal curve when shifted by a voltage proportional to the temperature.
Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a
strong voltage-dependence is observed. Its amplitude only depends on the
magnetic field/temperature ratio. Its origin is attributed to the presence of
paramagnetic states present at the surface or between the nanoparticles. Below
1.8 K, this high-field magnetoresistance abruptly disappears and inverse
tunnelling magnetoresistance is observed, the amplitude of which does not
exceed 1%. At this low temperature, some samples display in their I(V)
characteristics abrupt and hysteretic transitions between the Coulomb blockade
regime and the conductive regime. The increase of the current during these
transitions can be as high as a factor 30. The electrical noise increases when
the sample is near the transition. The application of a magnetic field
decreases the voltage at which these transitions occur so magnetic-field
induced transitions are also observed. Depending on the applied voltage, the
temperature and the amplitude of the magnetic field, the magnetic-field induced
transitions are either reversible or irreversible. These abrupt and hysteretic
transitions are also observed in resistance-temperature measurements. They
could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64,
041302 (R), 2001] or could also be interpreted as a true phase transition
between a Coulomb glass phase to a liquid phase of electrons
Magnetic hyperthermia in single-domain monodisperse FeCo nanoparticles: Evidences for Stoner-Wohlfarth behaviour and large losses
We report on hyperthermia measurements on a colloidal solution of 15 nm
monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic
field display a sharp increase followed by a plateau, which is what is expected
for losses of ferromagnetic single-domain NPs. The frequency dependence of the
coercive field is deduced from hyperthermia measurement and is in quantitative
agreement with a simple model of non-interacting NPs. The measured losses (1.5
mJ/g) compare to the highest of the literature, though the saturation
magnetization of the NPs is well below the bulk one.Comment: 14 pages, 3 figure
Description of Pairing correlation in Many-Body finite systems with density functional theory
Different steps leading to the new functional for pairing based on natural
orbitals and occupancies proposed in ref. [D. Lacroix and G. Hupin,
arXiv:1003.2860] are carefully analyzed. Properties of quasi-particle states
projected onto good particle number are first reviewed. These properties are
used (i) to prove the existence of such a functional (ii) to provide an
explicit functional through a 1/N expansion starting from the BCS approach
(iii) to give a compact form of the functional summing up all orders in the
expansion. The functional is benchmarked in the case of the picked fence
pairing Hamiltonian where even and odd systems, using blocking technique are
studied, at various particle number and coupling strength, with uniform and
random single-particle level spacing. In all cases, a very good agreement is
found with a deviation inferior to 1% compared to the exact energy.Comment: 14 pages, 9 figure
A large- PNJL model with explicit Z symmetry
A PNJL model is built, in which the Polyakov-loop potential is explicitly
Z-symmetric in order to mimic a Yang-Mills theory with gauge group
SU(). The physically expected large- and large- behaviours of the
thermodynamic observables computed from the Polyakov-loop potential are used to
constrain its free parameters. The effective potential is eventually
U(1)-symmetric when is infinite. Light quark flavours are added by using
a Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (the PNJL model),
and the different phases of the resulting PNJL model are discussed in 't
Hooft's large- limit. Three phases are found, in agreement with previous
large- studies. When the temperature is larger than some deconfinement
temperature , the system is in a deconfined, chirally symmetric, phase for
any quark chemical potential . When however, the system is in a
confined phase in which chiral symmetry is either broken or not. The critical
line , signalling the restoration of chiral symmetry, has the same
qualitative features than what can be obtained within a standard PNJL
model.Comment: To appear in Phys Rev
Comparaison de techniques d’identification des Erwinia et des Pseudomonas responsables de la pourriture molle
Trois méthodes, soit la caractérisation physiologique, l'utilisation de systèmes miniaturisés d'identification (API 20E, API NFT et Biolog) et l'analyse du profil électrophorétique des protéines sécrétées, ont été expérimentées afin de déterminer une technique précise et rapide d'identification des Pseudomonas et des Erwinia responsables de la pourriture molle. L'analyse des patrons électrophorétiques des protéines sécrétées est une méthode très efficace pour identifier les différentes espèces pectinolytiques de Pseudomonas fluorescents. Le système Biolog reconnaît efficacement le P. marginalis et le P. viridiflava. Le système API NFT est efficace pour l'identification du P. marginalis, du P. viridiflava et du P. syringae. C'est le système API 20E qui s'est avéré le plus efficace pour l'identification des Erwinia. L'électrophorèse des protéines sécrétées et le système API NFT permettent une identification rapide et efficace des Pseudomonas, tandis que pour les Erwinia, seul le système API20E est performant.Three methods, namely physiological characterization, the use of miniaturized identification Systems (API 20E, API NFT, and Biolog) and the analysis of the electrophoretic profile of proteins secreted by the bacteria, were used in order to determine a precise and quick identification technique for the Pseudomonas and Erwinia species causing soft rots in several plant species. The analysis of the electrophoretic profile of the proteins secreted is a very efficient method of identification for the various pectinolytic species of fluorescent Pseudomonas. The Biolog System accurately recognizes P. marginalisand P. viridiflava. The API NFT System is efficient for the identification of P. marginalis, P. viridiflava and P. syringae. The API 20E System was found to be the most reliable System for identifying Erwinia species. The electrophoresis of proteins secreted and the API NFT system allow a quick and efficient identification of Pseudomonas species, while for Erwinia species, only the API 20E System is efficient
Out of equilibrium transport through an Anderson impurity: Probing scaling laws within the equation of motion approach
We study non-equilibrium electron transport through a quantum impurity
coupled to metallic leads using the equation of motion technique at finite
temperature T. Assuming that the interactions are taking place solely in the
impurity and focusing in the infinite Hubbard limit, we compute the out of
equilibrium density of states and the differential conductance G_2(T,V) to test
several scaling laws. We find that G_2(T,V)/G_2(T,0) is a universal function of
both eV/T_K and T/T_K, being T_K the Kondo temperature. The effect of an in
plane magnetic field on the splitting of the zero bias anomaly in the
differential conductance is also analyzed. For a Zeeman splitting \Delta, the
computed differential conductance peak splitting depends only on \Delta/T_K,
and for large fields approaches the value of 2\Delta . Besides the traditional
two leads setup, we also consider other configurations that mimics recent
experiments, namely, an impurity embedded in a mesoscopic wire and the presence
of a third weakly coupled lead. In these cases, a double peak structure of the
Kondo resonance is clearly obtained in the differential conductance while the
amplitude of the highest peak is shown to decrease as \ln(eV/T_K). Several
features of these results are in qualitative agreement with recent experimental
observations reported on quantum dots.Comment: 9 pages, 7 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
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