Laboratoire Léon Brillouin

UMR12 CEA-CNRS, Bât. 563 CEA Saclay

91191 Gif sur Yvette Cedex, France

+33-169085241 llb-sec@cea.fr

BD diffusons les neutrons

Faits marquants scientifiques 2024

23 mai 2024

Influence of Dy3+ environment on magnetic anisotropy and magnetocaloric effect in Dy3⁢B2⁢C3⁢O12 (B=In, Sc, Te ; C=Ga, Al, Li) garnets
F. Damay, S. Petit, D. Sheptyakov, C. V. Colin, E. Suard, S. Rols, J. Embs, U. Stuhr, D. Bounoua, O. Demortier, and C. Decorse, Phys. Rev. B 109 (2024) 014419
 


In the framework of the general phase diagram published recently for a pseudospin S = 1/2 on a hyperkagome network [Kibalin et al., Phys. Rev. Res. 2, 033509 (2020)], four Dy3B2C3O12 garnets with different species on the B and C sites, namely, Dy3Sc2Al3O12 (DyScAl), Dy3Te2Li3O12 (DyTeLi), Dy3Sc2Ga3O12 (DyScGa), and Dy3In2Ga3O12 (DyInGa), have been studied by neutron powder diffraction and inelastic neutron scattering, to investigate the impact of B and C substitutions on the Dy3+ dodecahedral oxygen environment and crystal field, and thus on the Dy3+ spin magnetic anisotropy. In the four compounds, the magnetic ground state is a multiaxis antiferromagnetic order similar to that of Dy3Al5O12, characterized by an ordered moment close to the theoretical maximum, similar to 6 mu B. The gap between the ground state and the first excited crystal electric field level varies from similar to 3.2 meV (DyInGa) up to 11.2 meV (DyScAl), confirming the rather strong Ising character of the Dy3+ moment in those garnets. For a better understanding of the impact of the distortion of the oxygen environment on the Dy3+ magnetic anisotropy, point-charge calculations were then performed, mapping a magnetic anisotropy diagram versus a wide range of distortions. Results show that Ising-like behaviors mostly predominate, associated with large gx, gy, or gz Lande values, whose stable area is determined by the shape of the DyO8 cage. Magnetic entropy calculations confirm that the magnetocaloric effect increases sharply near those boundaries and validate the use of dysprosium garnets for low temperature, low magnetic field adiabatic demagnetization refrigeration cryostats.

01 mai 2024

Salah Bouazizi, Salah Nasr & Marie-Claire Bellissent-Funel

MD simulation and analysis of the pair correlation functions, self-diffusion coefficients and orientational correlation times in aqueous KCl solutions at different temperatures and concentrations,
S. Bouazizi, S. Nasr and M.-C. Bellissent-Funel, J Solution Chem (2024) https://doi.org/10.1007/s10953-024-01366-8

Abstract : In this study, we investigate some structural and dynamical properties of aqueous KCl solutions at different temperatures and concentrations. We study a 1.6 mol·kg–1 aqueous KCl solution at five temperatures and five concentrations at ambient conditions only. Molecular dynamics simulations with the flexible SPC water model were conducted to characterize all partial pair correlation functions, the velocities auto-correlation ones, and the dielectric constants. The analysis of the water pair correlation functions shows a disruption of the H-bond network and a decrease of the oxygen-hydrogen coordination number as temperature or salt concentration increases. The increase of each parameter favors the exchange of molecules between the first and the second hydration shells. Ions pair correlation functions show principally that the fraction of K+-Cl contact ion pairs increases and that of separated ion pairs decreases with increasing temperature or concentration. For all particles, the values of the calculated self-diffusion coefficients rise with temperature and fall with salt concentration. The self-diffusion coefficients of K+ and Cl tend to towards each other at high concentration. Temperature or salt concentration causes a drop in the dielectric constant. For all studied temperatures or salt concentrations, the calculated ratio of the orientational correlation times τ12 for the OH vector indicates that the motion of water molecules can be accounted for by an angular jumps model.

07 avril 2024

Nina Královič-Kanjaková, Ali Asi Shirazi, Lukáš Hubčík, Mária Klacsová, Atoosa Keshavarzi, Juan Carlos Martínez, Sophie Combet, José Teixeira, and Daniela Uhríková

The use of an exogenous pulmonary surfactant (EPS) to deliver other relevant drugs to the lungs is a promising strategy for combined therapy. We evaluated the interaction of polymyxin B (PxB) with a clinically used EPS, the poractant alfa Curosurf (PSUR). The effect of PxB on the protein-free model system (MS) composed of four phospholipids (diC16:0PC/16:0–18:1PC/16:0–18:2PC/16:0–18:1PG) was examined in parallel to distinguish the specificity of the composition of PSUR. We used several experimental techniques (differential scanning calorimetry, small- and wide-angle X-ray scattering, small-angle neutron scattering, fluorescence spectroscopy, and electrophoretic light scattering) to characterize the binding of PxB to both EPS. Electrostatic interactions PxB–EPS are dominant. The results obtained support the concept of cationic PxB molecules lying on the surface of the PSUR bilayer, strengthening the multilamellar structure of PSUR as derived from SAXS and SANS. A protein-free MS mimics a natural EPS well but was found to be less resistant to penetration of PxB into the lipid bilayer. PxB does not affect the gel-to-fluid phase transition temperature, Tm, of PSUR, while Tm increased by ∼+ 2 °C in MS. The decrease of the thickness of the lipid bilayer (dL) of PSUR upon PxB binding is negligible. The hydrophobic tail of the PxB molecule does not penetrate the bilayer as derived from SANS data analysis and changes in lateral pressure monitored by excimer fluorescence at two depths of the hydrophobic region of the bilayer. Changes in dL of protein-free MS show a biphasic dependence on the adsorbed amount of PxB with a minimum close to the point of electroneutrality of the mixture. Our results do not discourage the concept of a combined treatment with PxB-enriched Curosurf. However, the amount of PxB must be carefully assessed (less than 5 wt % relative to the mass of the surfactant) to avoid inversion of the surface charge of the membrane

https://doi.org/10.1021/acs.langmuir.3c03746

15 mars 2024

Universality of q=1/2 orbital magnetism in the pseudogap phase of the high-Tc superconductor YBa2Cu3O6+x
Dalila Bounoua, Yvan Sidis, Martin Boehm, Paul Steffens, Toshinao Loew, Lin Shan Guo, Jun Qian, Xin Yao, and Philippe Bourges, Phys. Rev. B 108 (2023) 214408.

Several decades of debate have centered around the nature of the enigmatic pseudogap state in high-temperature superconducting copper oxides. Recently, we reported polarized neutron diffraction measurements that suggested the existence of a magnetic texture bound to the pseudogap phase [Bounoua et al. Commun. Phys. 5, 268 (2022)]. Such a magnetic texture is likely to involve the spontaneous appearance of loop currents within the CuO2 unit cells, which give birth to complex correlated patterns. In the underdoped YBa2Cu3O6.6, the magnetic structure factor of such an orbital magnetic texture gives rise to two distinct magnetic responses at q=0 and q=1/2. As this pattern alters the lattice translation invariance, such a state of matter could contribute to an instability of the Fermi surface. Here, we report polarized neutron scattering measurements on a nearly optimally doped high-quality single crystal of YBa2Cu3O6.9 that exhibits the same q=1/2 magnetism and a weakly overdoped YBa2Cu3O7 sample where this signal is no longer sizable. The in-plane and out-of-plane magnetic neutron scattering intensities in YBa2Cu3O6.9 (at q=1/2) and YBa2Cu3O6.85 (at q=0), reported previously, display the same temperature-dependent hallmarks. The magnitudes of both q=0 and q=1/2 magnetic signals further exhibit the same trends upon doping in YBa2Cu3O6+x, confirming that they are likely intertwined.

https://doi.org/10.1103/PhysRevB.108.214408

06 mars 2024

Explorer la matière nécessite des faisceaux sondes de lumière (photons lasers, rayons X…), de neutrons, d'électrons, voire d'atomes, de molécules ou d'ions. Chaque type de faisceau interagit de façon très spécifique avec la matière : les rayons X sont fortement diffusés par les éléments lourds, tandis que les neutrons présentent le grand avantage d'être fortement diffusés par les éléments légers et sensibles au magnétisme.

Les faisceaux de neutrons sont ainsi devenus un outil analytique important des scientifiques dans les domaines aussi divers que la physique et la chimie de la matière condensée, la science des matériaux et de la matière molle, les sciences de la vie ou encore la géoscience. Ils sont aussi utilisés comme sonde dans de nombreux domaines industriels, l’identification de matériaux, la radiographie, la validation de résistance aux rayonnements, la qualification d’assemblages métallurgiques…

 

 


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