7,124 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
High Dimensional Classification with combined Adaptive Sparse PLS and Logistic Regression
Motivation: The high dimensionality of genomic data calls for the development
of specific classification methodologies, especially to prevent over-optimistic
predictions. This challenge can be tackled by compression and variable
selection, which combined constitute a powerful framework for classification,
as well as data visualization and interpretation. However, current proposed
combinations lead to instable and non convergent methods due to inappropriate
computational frameworks. We hereby propose a stable and convergent approach
for classification in high dimensional based on sparse Partial Least Squares
(sparse PLS). Results: We start by proposing a new solution for the sparse PLS
problem that is based on proximal operators for the case of univariate
responses. Then we develop an adaptive version of the sparse PLS for
classification, which combines iterative optimization of logistic regression
and sparse PLS to ensure convergence and stability. Our results are confirmed
on synthetic and experimental data. In particular we show how crucial
convergence and stability can be when cross-validation is involved for
calibration purposes. Using gene expression data we explore the prediction of
breast cancer relapse. We also propose a multicategorial version of our method
on the prediction of cell-types based on single-cell expression data.
Availability: Our approach is implemented in the plsgenomics R-package.Comment: 9 pages, 3 figures, 4 tables + Supplementary Materials 8 pages, 3
figures, 10 table
Quadruplexes In âDictyâ: Crystal Structure Of A Four-Quartet G-Quadruplex Formed By G-Rich Motif Found In The Dictyostelium Discoideum Genome
Guanine-rich DNA has the potential to fold into non-canonical G-quadruplex (G4) structures. Analysis of the genome of the social amoeba Dictyostelium discoideum indicates a low number of sequences with G4-forming potential (249â1055). Therefore, D. discoideum is a perfect model organism to investigate the relationship between the presence of G4s and their biological functions. As a first step in this investigation, we crystallized the dGGGGGAGGGGTACAGGGGTACAGGGG sequence from the putative promoter region of two divergent genes in D. discoideum. According to the crystal structure, this sequence folds into a four-quartet intramolecular antiparallel G4 with two lateral and one diagonal loops. The G-quadruplex core is further stabilized by a G-C WatsonâCrick base pair and a AâTâA triad and displays high thermal stability (Tm \u3e 90°C at 100 mM KCl). Biophysical characterization of the native sequence and loop mutants suggests that the DNA adopts the same structure in solution and in crystalline form, and that loop interactions are important for the G4 stability but not for its folding. Four-tetrad G4 structures are sparse. Thus, our work advances understanding of the structural diversity of G-quadruplexes and yields coordinates for in silico drug screening programs and G4 predictive tools
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 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
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
Stochastic mean-field dynamics for fermions in the weak coupling limit
Assuming that the effect of the residual interaction beyond mean-field is
weak and has a short memory time, two approximate treatments of correlation in
fermionic systems by means of Markovian quantum jump are presented. A
simplified scenario for the introduction of fluctuations beyond mean-field is
first presented. In this theory, part of the quantum correlations between the
residual interaction and the one-body density matrix are neglected and jumps
occur between many-body densities formed of pairs of states where and are
antisymmetrized products of single-particle states. The underlying Stochastic
Mean-Field (SMF) theory is discussed and applied to the monopole vibration of a
spherical Ca nucleus under the influence of a statistical ensemble of
two-body contact interaction. This framework is however too simplistic to
account for both fluctuation and dissipation. In the second part of this work,
an alternative quantum jump method is obtained without making the approximation
on quantum correlations. Restricting to two particles-two holes residual
interaction, the evolution of the one-body density matrix of a correlated
system is transformed into a Lindblad equation. The associated dissipative
dynamics can be simulated by quantum jumps between densities written as is a normalized Slater determinant. The
associated stochastic Schroedinger equation for single-particle wave-functions
is given.Comment: Enlarged version, 10 pages, 2 figure
Self-consistent spin-wave theory for a frustrated Heisenberg model with biquadratic exchange in the columnar phase and its application to iron pnictides
Recent neutron scattering studies revealed the three dimensional character of
the magnetism in the iron pnictides and a strong anisotropy between the
exchange perpendicular and parallel to the spin stripes. We extend studies of
the J1-J2-Jc Heisenberg model with S = 1 using self-consistent spin-wave
theory. A discussion of two scenarios for the instability of the columnar phase
is provided. The relevance of a biquadratic exchange term between in-plane
nearest neighbors is discussed. We introduce mean-field decouplings for
biquadratic terms using the Dyson-Maleev and the Schwinger boson
representation. Remarkably their respective mean-field theories do not lead to
the same results, even at zero temperature. They are gauged in the N'eel phase
in comparison to exact diagonalization and series expansion. The J1-J2-Jc model
is analyzed under the influence of the biquadratic exchange Jbq and a detailed
description of the staggered magnetization and of the magnetic excitations is
given. The biquadratic exchange increases the renormalization of the in-plane
exchange constants which enhances the anisotropy between the exchange parallel
and perpendicular to the spin stripes. Applying the model to iron pnictides, it
is possible to reproduce the spin-wave dispersion for CaFe2As2 in the direction
perpendicular to the spin stripes and perpendicular to the planes.
Discrepancies remain in the direction parallel to the spin stripes which can be
resolved by passing from S = 1 to S = 2. In addition, results for the dynamical
structure factor within the self-consistent spin-wave theory are provided.Comment: 18 pages, 12 figures. Updated version, several references adde
Surface induced magnetization reversal of MnP nanoclusters embedded in GaP
We investigate the quasi-static magnetic behavior of ensembles of
non-interacting ferromagnetic nanoparticles consisting of MnP nanoclusters
embedded in GaP(001) epilayers grown at 600, 650 and 700{\deg}C. We use a
phenomenological model, in which surface effects are included, to reproduce the
experimental hysteresis curves measured as a function of temperature (120-260
K) and direction of the applied field. The slope of the hysteresis curve during
magnetization reversal is determined by the MnP nanoclusters size distribution,
which is a function of the growth temperature. Our results show that the
coercive field is very sensitive to the strength of the surface anisotropy,
which reduces the energy barrier between the two states of opposite
magnetization. Notably, this reduction in the energy barrier increases by a
factor of 3 as the sample temperature is lowered from 260 to 120 K.Comment: 7 pages, 5 figure
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