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Search Results: 1 - 10 of 461935 matches for " A. Forget "
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The modulated antiferromagnetic structures in multiferroic FeVO4: a 57Fe M?ssbauer spectroscopy investigation
D. Colson,A. Forget,P. Bonville
Physics , 2014,
Abstract: We present a 57Fe M\"ossbauer spectroscopy study of the two incommensurate magnetic phases in the multiferroic material FeVO4. We devise lineshapes appropriate for planar elliptical and collinear modulated magnetic structures and show that they reproduce very well the M\"ossbauer spectra in FeVO4, in full qualitative agreement with a previous neutron diffraction study. Quantitatively, our spectra provide precise determinations of the characteristics of the elliptical and modulated structures which are in good agreement with the neutron diffraction results. We find that the hyperfine field elliptical modulation persists as T goes to 0, which we attribute to an anisotropy of the hyperfine interaction since a moment modulation is forbidden at T=0 for a spin only ion like Fe3+.
Investigation of the magnetic fluctuations in Tb$_2$Sn$_2$O$_7$ ordered spin ice by high resolution energy-resolved neutron scattering
I. Mirebeau,H. Mutka,P. Bonville,A. Apetrei,A. Forget
Physics , 2008, DOI: 10.1103/PhysRevB.78.174416
Abstract: We have studied magnetically frustrated Tb$_2$Sn$_2$O$_7$ by neutron diffraction and high resolution energy-resolved neutron scattering. At 0.1 K, we observe short range magnetic correlations with a typical scale of 4 \AA, close to the near neighbor distance between Tb$^{3+}$ ions. %(3.686 \AA), This short range order coexists with ferromagnetic correlations and long range spin ice order at the scales of 18 and 190 \AA, respectively. Spin dynamics was investigated at a time scale down to 10$^{-9}$s, by energy-resolved experiments on a backscattering spectrometer. We observe a freezing of the spin dynamics for all length-scales, with a strong slowing down of the spin fluctuations when long range order settles in. We discuss the spin fluctuations remaining in the ground state in comparison with previous data obtained by muon spectroscopy.
Different effects of Ni and Co substitution on the transport properties of BaFe2As2
A. Olariu,F. Rullier-Albenque,D. Colson,A. Forget
Physics , 2010, DOI: 10.1103/PhysRevB.83.054518
Abstract: We report resistivity and Hall effect results on Ba(Fe1-xNix)2As2 and compare them with those in Ba(Fe1-xCox)2As2. The Hall number RH is negative for all x values from 0.01 to 0.14, which indicates that electron carriers dominate the transport both in the magnetic and paramagnetic regime. We analyse the data in the framework of a two-band model. Without any assumption on the number of carriers, we show that the electron resistivity can be estimated with good accuracy in the low temperature paramagnetic range. Although the phase diagrams of the two families are very similar with respect to the extra electrons added in the system, we find that the transport properties differ in several aspects. First, we evidence that the contribution of holes to the transport is more important for Ni doping than for Co doping. Secondly, Ni behaves as a stronger scatterer for the electrons, as the increase of the residual electron resistivity rho/x is about four times larger for Ni than for Co in the most doped samples.
Self-referenced spectral interferometry for ultra-short infrared pulse characterization
Trisorio A.,Grabielle S.,Divall M.,Forget N.
EPJ Web of Conferences , 2013, DOI: 10.1051/epjconf/20134112002
Abstract: Self-Referenced Spectral Interferometry is used for single shot pulse characterization over the 0.9-2.5 μm spectral range with a single spectrometer and an optimized optical setup. We characterize sub-55 fs pulses from 1.4 μm to 2 μm and broadband 2.5-cycle pulses at 1.65 μm (13 fs FWHM).
Vitamin D deficiency in a cohort of HIV-infected patients: clinical analysis
D Wilmes,P Forget,A Vincent,L Belkhir
Journal of the International AIDS Society , 2012, DOI: 10.7448/ias.15.6.18205
Abstract: Purpose of the study: Observational studies have noted very high rates of low serum 25-hydroxyvitamin D [25(OH)D3] levels in both general and HIV-infected populations. In HIV-infected patients, low 25(OH)D3 levels are secondary to a combination of usual risk factors and HIV-specific risk factors, like antiretroviral therapy [1]. The objective of our study is to analyse the magnitude of vitamin D deficiency or insufficiency and the role of various factors such as age, sex, ethnicity, season, and antiretroviral medications in our cohort of HIV-infected patients. Methods: We prospectively collected data on 25-hydroxyvitamin D levels sampled between January 2009 and June 2011 from our cohort of 930 HIV-infected patients. Vitamin D dosage was performed using immunoassay (‘Diasorin’ - Saluggia, Italy). We divided vitamin D levels into 3 categories: 25-hydroxyvitamin D levels <20 mg/nl were considered deficient, insufficient between 20 and 29 ng/ml. Levels ≥30 ng/ml were defined as normal [2]. Data on demographic features (age, ethnicity, season, heterosexuality vs homosexuality), clinical features and laboratory findings (CD4 cell count, viral load, HAART, BMI) were collected from patients’ medical records using our institutional database ‘Medical explorer v3r9, 2009’. Summary of results: Overall, 848 patients were included in our study (Table 1). Low levels of serum 25(OH)D3 were seen in 89.3% of the study population, from which 69.5% were deficient and 19.8% were insufficient. On univariate analysis, female sex, high BMI, black African, heterosexuality, undetectable viral load and antiretroviral treatment were all predictors of vitamin D deficiency and insufficiency. Treatment with efavirenz and tenofovir were the most associated with low vitamin D levels. On multivariate analysis (multiple linear regression model) only female sex (OR=1.14; 95% CI 0.84–0.96; p<0.001), dosage during winter months (OR=1.14; 95% CI 1–1.15; p<0.05) and HAART (OR=1.12; 95% CI 1.04–1.19; p=0.002) were identified as independent risk factors of low 25(OH)D3 levels. Conclusion: Vitamin D deficiency is frequent in HIV-infected populations (69.5%). Patients on antiretroviral therapy are at higher risk of vitamin D deficiency. In our cohort, black women and dosage during winter were also independent risk factors for low vitamin D levels.
Incommensurate spin density wave in Co-doped BaFe2As2
P. Bonville,F. Rullier-Albenque,D. Colson,A. Forget
Physics , 2010, DOI: 10.1209/0295-5075/89/67008
Abstract: 57Fe Mossbauer spectroscopy measurements are presented in the underdoped Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~ 2 and 12K respectively). The spectral shapes in the so-called spin-density wave (SDW) phase are interpreted in terms of incommensurate modulation of the magnetic structure, and allow the shape of the modulation to be determined. In undoped BaFe2As2, the magnetic structure is commensurate, and we find that incommensurability is present at the lowest doping level (x=0.014). As Co doping increases, the low temperature modulation progressively loses its "squaredness" and tends to a sine-wave. The same trend occurs for a given doping level, as temperature increases. We find that a magnetic hyperfine component persists far above the SDW transition, its intensity being progressively tranferred to a paramagnetic component on heating.
Magnetic order in Tb$_2$Sn$_2$O$_7$ under high pressure: from ordered spin ice to spin liquid and antiferromagnetic order
I. Mirebeau,I. Goncharenko,H. Cao,A. Forget
Physics , 2009, DOI: 10.1103/PhysRevB.80.220407
Abstract: We have studied the Tb$_2$Sn$_2$O$_7$ frustrated magnet by neutron diffraction under isotropic pressure of 4.6 GPa, combined with uniaxial pressure of 0.3 GPa, in the temperature range 0.06 K$<$T$<$100 K. Magnetic order persists under pressure but the ordered spin ice structure stabilized at ambient pressure below 1.3 K partly transforms into an antiferromagnetic one. The long range ordered moment at 0.06 K is reduced under pressure, which is interpreted by a pressure induced enhancement of the spin liquid fluctuations. Above the ordering transition, short range spin correlations are affected by pressure, and ferromagnetic correlations are suppressed. The influence of pressure on the ground state is discussed considering both isotropic and stress effects.
Determination and Control of Oxygen Stoichiometry in the Cuprate Bi2sr2cuo6+D
F. Jean,D. Colson,G. Collin,N. Blanchard,A. Forget
Physics , 2003,
Abstract: The relatively low Tc of Bi2Sr2CuO6+D allows to study normal-state down to low temperatures and the non-substituted compound is intrinsically strongly overdoped. Hole concentration can be adjusted trough oxygen excess, but few data exist in the literature about the quantitative control of D. The synthesis, achieved in air by solid-state reaction, needs long-time annealing to obtain pure phase with stoichiometric cationic ratios. Thermogravimetric techniques were used to explore oxygen non-stoichiometry. Absolute oxygen content was determined by reduction with hydrogen, while the oxygen exchange was studied between 300 C and 670 C with different PO2. Oxygen excess varies between 0.14 and 0.18, with possibly two regimes of oxygen intercalation. These results are compared to Bi-2212.
Hall effect and resistivity study of the magnetic transition, carrier content and Fermi liquid behavior in Ba(Fe(1-x) Cox)2As2
F. Rullier-Albenque,D. Colson,A. Forget,H. Alloul
Physics , 2009, DOI: 10.1103/PhysRevLett.103.057001
Abstract: The negative Hall constant R_H measured all over the phase diagram of Ba(Fe(1-x)Cox)2As2 allows us to show that electron carriers always dominate the transport properties. The evolution of R_H with x at low doping (x<2%) indicates that important band structure changes happen for x<2% prior to the emergence of superconductivity. For higher x, an change of the electron concentration with T is required to explain the low T variations of R_H, while the electron scattering rate displays the T^2 law expected for a Fermi liquid. The T=0 residual scattering is affected by Co disorder in the magnetic phase, but is rather dominated by incipient disorder in the paramagnetic state.
Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs
F. Rullier-Albenque,D. Colson,A. Forget,H. Alloul
Physics , 2012, DOI: 10.1103/PhysRevLett.109.187005
Abstract: The resistivity, Hall effect and transverse magnetoresistance (MR) have been measured in low residual resistivity single crystals of LiFeAs. A comparison with angle resolved photoemission spectroscopy and quantum oscillation data implies that four carrier bands unevenly contribute to the transport. However the scattering rates of the carriers all display the T^2 behavior expected for a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2 variation permit us to extract the superconducting fluctuation contribution to the conductivity. Though below Tc the anisotropy of superconductivity is rather small, the superconducting fluctuations display a quasi ideal two-dimensional behavior which persists up to 1.4 Tc. These results call for a refined theoretical understanding of the multiband behavior of superconductivity in this pnictide.
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