A. Sazonov, H. Thoma, R. Dutta, M. Meven, A. Gukasov, R. Fittipaldi, V. Granata, T. Masuda, B. Nafradi, and V. Hutanu
A detailed investigation of Ba2MnGe2O7 was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. In the paramagnetic state at 10 K, polarized neutron diffraction was applied to reveal the components of the susceptibility tensor. The crystal and magnetic structures below the antiferromagnetic transition temperature of TN≈4K were determined using unpolarized neutron diffraction. These data imply no structural phase transition from 10 K down to 2.5 K and are well described within the tetragonal space group P¯421m. We found that in zero magnetic field the magnetic space group is either Ccmc21 or Pc212121 with antiferromagnetic order along the  or  direction, respectively, while neighboring spins along the  axis are ordered antiferromagnetically. A noncollinear spin arrangement due to small canting within the ab plane is allowed by symmetry and observed experimentally. The ordered moment is found to be 3.24(3) μB/Mn2+ at 2.5 K and the temperature-field dependent magnetic phase diagram is mapped out by macroscopic magnetization. Distinct differences between the magnetic structure of Ba2MnGe2O7 as compared to those of Ba2CoGe2O7 and Ca2CoSi2O7 are discussed.
Maëva Almeida, Daniel Dudzinski, Catherine Amiel, Jean-Michel Guigner,Sylvain Prévost, Clémence Le Coeur and Fabrice Cousin
Here, we describe the behavior of mixtures of stearic acid (SA) and its hydroxylated counterpart 12-hydroxystearic acid (12-HSA) in aqueous mixtures at room temperature as a function of the 12-HSA/SA mole ratio R. The morphologies of the self-assembled aggregates are obtained through a multi-structural approach that combines confocal and cryo-TEM microscopies with small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS) measurements, coupled with rheology measurements. Fatty acids are solubilized by an excess of ethanolamine counterions, so that their heads are negatively charged. A clear trend towards partitioning between the two types of fatty acids is observed, presumably driven by the favorable formation of a H-bond network between hydroxyl OH function on the 12th carbon. For all R, the self-assembled structures are locally lamellar, with bilayers composed of crystallized and strongly interdigitated fatty acids. At high R, multilamellar tubes are formed. The doping via a low amount of SA molecules slightly modifies the dimensions of the tubes and decreases the bilayer rigidity. The solutions have a gel-like behavior. At intermediate R, tubes coexist in solution with helical ribbons. At low R, local partitioning also occurs, and the architecture of the self-assemblies associates the two morphologies of the pure fatty acids systems: they are faceted objects with planar domains enriched in SA molecules, capped with curved domains enriched in 12-HSA molecules. The rigidity of the bilayers is strongly increased, as well their storage modulus. The solutions remain, however, viscous fluids in this regime.
I. V. Golosovsky, I. A. Kibalin, A. Gukasov, A. G. Roca, A. López-Ortega, M. Estrader, M. Vasilakaki, K. N. Trohidou, T. C. Hansen, I. Puente-Orench, E. Lelièvre-Berna, J. Nogués
Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials.
Özge Azeri, Dennis Schönfeld, Bin Dai, Uwe Keiderling, Laurence Noirez and Michael Gradzielski
Block copolymers synthesized via Atom Transfer Radical Polymerization from alkyl acrylate and t-butyl acrylate and the subsequent hydrolysis of the t-butyl acrylate to acrylic acid were systematically varied with respect to their hydrophobic part by the variation in the alkyl chain length and the degree of polymerisation in this block. Depending on the architecture of the hydrophobic part, they had a more or less pronounced tendency to form copolymer micelles in an aqueous solution. They were employed for the preparation of IPECs by mixing the copolymer aggregates with the polycations polydiallyldimethylammonium chloride (PDADMAC) or q-chit. The IPEC structure as a function of the composition was investigated by Static Light and Small Angle Neutron Scattering. For weakly-associated block copolymers (short alkyl chain), complexation with polycation led to the formation of globular complexes, while already existing micelles (long alkyl chain) grew further in mass. In general, aggregates became larger upon the addition of further polycation, but this growth was much more pronounced for PDADMAC compared to q-chit, thereby leading to the formation of clusters of aggregates. Accordingly, the structure of such IPECs with a hydrophobic block depended largely on the type of complexing polyelectrolyte, which allowed for controlling the structural organisation via the molecular architecture of the two oppositely charged polyelectrolytes.
Céline JAUDOIN, Isabelle GRILLO, Fabrice COUSIN, Maria GEHRKE, Malika OULDALI, Ana-Andreea ARTENI, Luc PICTON, Christophe RIHOUEY, Fanny SIMELIERE, Amélie BOCHOT, Florence AGNELY
Mixtures of hyaluronic acid (HA) with liposomes lead to hybrid colloid–polymer systems with a great interest in drug delivery. However, little is known about their microstructure. Small angle neutron scattering (SANS) is a valuable tool to characterize these systems in the semi-dilute entangled regime (1.5% HA) at high liposome concentration (80 mM lipids). The objective was to elucidate the influence of liposome surface (neutral, cationic, anionic or anionic PEGylated), drug encapsulation and HA concentration in a buffer mimicking biological fluids (37 °C). First, liposomes were characterized by SANS, cryo-electron microscopy, and dynamic light scattering and HA by SANS, size exclusion chromatography, and rheology. Secondly, HA-liposome mixtures were studied by SANS. In HA, liposomes kept their integrity. Anionic and PEGylated liposomes were in close contact within dense clusters with an amorphous organization. The center-to-center distance between liposomes corresponded to twice their diameter. A depletion mechanism could explain these findings. Encapsulation of a corticoid did not modify this organization. Cationic liposomes formed less dense aggregates and were better dispersed due to their complexation with HA. Liposome surface governed the interactions and microstructure of these hybrid systems.
Michal Swierczewski, Fabrice Cousin, Ewa Banach, Arnulf Rosspeintner, Latevi Max Lawson Daku, Abolfazl Ziarati, Rania Kazan, Gunnar Jeschke, Raymond Azoulay, Lay-Theng Lee, Thomas Bürgi
A bidentate chiral dithiol (diBINAS) is utilised to bridge Au25 nanoclusters to form oligomers. Separation by size allows the isolation of fractions that are stable thanks to the bidentate nature of the linker. The structure of the products is elucidated by small-angle X-ray scattering and calculated using density functional theory. Additional structural details are studied by diffusion-ordered nuclear magnetic resonance spectroscopy, transmission electron microscopy and matrix-assisted laser desorption/ionization time of flight mass spectrometry. Significant changes in the optical properties are analysed by UV-Vis and fluorescence spectroscopies, with the latter demonstrating a strong emission enhancement. Furthermore, the emergent chiral characteristics are studied by circular dichroism. Due to the geometry constraints of the nanocluster assemblies, diBINAS can be regarded as a templating molecule, taking a step towards the directed self-assembly of metal clusters.
Sophie Combet, Françoise Bonneté, Stéphanie Finet, Alexandre Pozza, Christelle Saade, Anne Martel, Alexandros Koutsioubas, Jean-Jacques Lacapère, Biochimie (2022) in press.
The translocator protein (TSPO) is a ubiquitous transmembrane protein of great pharmacological interest thanks to its high affinity to many drug ligands. The only high-resolution 3D-structure known for mammalian TSPO was obtained by NMR for the mouse mTSPO in DPC detergent only in presence of the high-affinity PK 11195 ligand. An atomic structure of free-ligand mTSPO is still missing to better understand the interaction of ligands with mTSPO and their effects on the protein conformation.
Michal Swierczewski, Alexis Chenneviere, Lay-Theng Lee, Plinio Maroni, Thomas Bürgi
Langmuir-Blodgett (LB) technique allows the deposition of gold nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers but kinetically disfavors the formation of the fourth layer. LB films of gold nanoclusters stabilized by thiolated ligand, Au38(SC2H4Ph)24, were prepared at a range of surface pressures in the bilayer/trilayer regime and their internal structure was analyzed with X-ray Reflectivity (XRR) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). Bimodal atomic force microscopy (AFM) imaging was used to quantify the elastic modulus, which can be correlated with the topography at the same point on the surface. Nanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20-25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.
I. Mirebeau et N. Martin, J. Appl. Cryst. (2022). 55, 1173-1183
The application is discussed of neutron methods to the study of reentrant spin glasses (RSGs), close to the transition towards a `canonical' spin glass (SG). The focus is on two emblematic systems, namely Au1−xFex and amorphous a-Fe1−xMnx. A set of experimental results is presented to highlight their peculiar static and dynamic properties. The role of small-angle neutron scattering (SANS) is stressed as an important tool to unravel the structure of these complex systems over mesoscopic length scales. Finally, recent SANS results performed under an applied magnetic field in the region of the RSG → SG transition are presented. They show that vortex-like defects are present in the RSG region up to the critical line and vanish in the SG region. These defects, which develop only in a ferromagnetic medium, could be a key feature to probe the emergence of long-range magnetic order.
Wen-Gen Zheng, Victor Balédent, Claire V. Colin, Françoise Damay, Jean-Pascal Rueff, Anne Forget, Dorothée Colson and Pascale Foury-Leylekian
It has been recently observed that a superconducting phase emerges under pressure in the Fe-based spin-ladders BaFe2X3 (X = S, Se). The low dimensionality of the Fe spin-ladders, which simplifies the elaboration of theoretical models, should help to understand the mechanism of superconductivity. We investigate here the frontier between magnetic and superconducting (SC) phases in BaFe2Se3 by performing challenging powder neutron diffraction (PND) and Fe Kβ x-ray emission spectroscopy (XES) under high pressure. We show that the ambient pressure ground state with a block-like magnetic order is destabilized under pressure. A pressure-induced antiferromagnetic stripe-like spin order, similar to the magnetic order of the parent superconductor BaFe2S3, is observed above 3-4 GPa. Our discovery shows that the stripe magnetic order is a key phase close to the SC dome and its particular magnetic fluctuations could be involved in the stabilization of superconductivity in Fe-based spin ladders.
Marion Grzelka, Iurii Antoniuk, Éric Drockenmuller, Alexis Chennevière, Liliane Léger, and Frédéric Restagno
The role of the polymer volume fraction, ϕ, on steady-state slippage and interfacial friction is investigated for semi-dilute polystyrene solutions in diethyl phthalate in contact with two solid surfaces. Significant slippage is evidenced for all samples, with slip lengths b obeying a power law dependence. Navier’s interfacial friction coefficient, k, is deduced from the slip length measurements and from independent measurements of the solution viscosity η. The observed scaling of k versus ϕ clearly excludes a molecular mechanism of friction based on the existence of a depletion layer. Instead, we show that the data of η(ϕ) and k(ϕ) are understood when taking into account the dependence of the solvent friction on ϕ. Two models based on the friction of blobs or of monomers on the solid surface well describe our data. Both point out that Navier’s interfacial friction is a semi-local phenomenon.
J.C.Riedl, M.Sarkar, T.Fiuza, F.Cousin, J.Depeyrot, E.Dubois,G.Mériguet,R.Perzynski and V.Peyre
Some of the most promising fields of application of ionic liquid-based colloids imply elevated temperatures. Their careful design and analysis is therefore essential. We assume that tuning the structure of the nanoparticle-ionic liquid interface through its composition can ensure colloidal stability for a wide temperature range, from room temperature up to 200 °C.
Emil A. Klahn, Andreas M. Thiel, Rasmus B. Degn, Iurii Kibalin, Arsen Gukassov, Claire Wilson, Angelos B. Canaj, Mark Murrie and Jacob Overgaard
We present the magnetic anisotropy of two isostructural pentagonal-bipyramidal complexes, [Ln(H2O)5(HMPA)2]I3·2HMPA (HMPA = hexamethylphosphoramide, Ln = Dy, Ho). Using ac magnetic susceptibility measurements, we find magnetic relaxation barriers of 600 K and 270 K for the Dy- and Ho-compounds, respectively. This difference is supported by polarized neutron diffraction (PND) measured at 5 K and 1 T which provides the first experimental evidence that the transverse elements in the magnetic anisotropy of the Ho-analogue are significant, whereas the Dy-analogue has a near-axial magnetic anisotropy with vanishing transverse contributions. The coordination geometries of the two complexes are highly similar, and we attribute the loss of strong magnetic axiality as expressed in the atomic susceptibility tensors from PND, as well as the smaller relaxation barrier in the Ho-complex compared to the Dy-complex, to the less favorable interaction of the pentagonal bipyramidal crystal field with the characteristics of the Ho(III) 4f-charge distribution.
Michal Swierczewski, Plinio Maroni, Alexis Chenneviere, Mohammad M. Dadras, Lay-Theng Lee, Thomas Bürgi
Nanoscale particles attract much attention due to their size-dependent optical, electrical and chemical properties. Of particular interest are ultrasmall metal nanoclusters which experience strong quantum confinement effect leading to profound changes in the atomic packing structure. The synthesis of these atomically precise metal clusters, typically with metal cores smaller than 2 nm in diameter, makes use of stabilizing functional molecules such as thiol ligands, hence deriving the common name – monolayer-protected clusters (MPCs). The next stage toward applications and at the same time a challenge in the field, is the manipulation and controlled organization of MPCs into two dimensional (2D) superlattices which would exhibit a collective response of the desired kind. Multiple examples of deposition techniques have been extensively studied, including droplet evaporation, spin-coating and chemical vapor deposition. However, a common drawback of all these methods is the failure to form large-scale structures of closely packed particles. Here, we study the formation and deposition of extended thin films of Au38(SC2H4Ph)24 nanoclusters onto solid supports by the Langmuir-Blodgett (LB) method. A combination of techniques, atomic force microscopy (AFM), high magnification transmission electron microscopy (TEM), X-ray reflectivity (XRR), and grazing incidence wide-angle X-ray scattering (GIWAXS) is applied to reveal the morphology and the degree of vertical and in-plane ordering of the transferred films. We find that while a degree of order is initially obtained between the clusters, temporal annealing of the compressed films successfully removes mesoscopic defects between islands of nanoclusters but that it does so at the cost of reducing the local order within the domains. To our knowledge, this is the first reported example of the deposition on an extended scale (several cm2) of ordered gold nanoclusters in the small size regime of 1 – 2 nm.
J. Wolanin, L. Michel, D. Tabacchioni, J. M. Zanotti, J. Peters, I. Imaz, B. Coasne, M. Plazanet, and C. Picard
With their strong confining porosity and versatile surface chemistry, zeolitic imidazolate frameworks—including the prototypical ZIF-8—display exceptional properties for various applications. In particular, the forced intrusion of water at high pressure (∼25 MPa) into ZIF-8 nanopores is of interest for energy storage. Such a system reveals also ideal to study experimentally water dynamics and thermodynamics in an ultrahydrophobic confinement. Here, we report on neutron scattering experiments to probe the molecular dynamics of water within ZIF-8 nanopores under high pressure up to 38 MPa. In addition to an overall confinement-induced slowing down, we provide evidence for strong dynamical heterogeneities with different underlying molecular dynamics. Using complementary molecular simulations, these heterogeneities are found to correspond to different microscopic mechanisms inherent to vicinal molecules located in strongly adsorbing sites (ligands) and other molecules nanoconfined in the cavity center. These findings unveil a complex microscopic dynamics, which results from the combination of surface residence times and exchanges between the cavity surface and center.
T. Fiuza, M. Sarkar, J. C. Riedl, A. Cebers, F. Cousin, G. Demouchy, J. Depeyrot, E. Dubois, F. Gelebart, G. Meriguet, R. Perzynski and V. Peyre
Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide – EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at F = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m1 ) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.
Christopher D. O’Neill, Gino Abdul-Jabbar, Didier Wermeille, Philippe Bourges, Frank Krüger, and Andrew D. Huxley
Quantum order by disorder revealed
Ferromagnets are ubiquitous in everyday life, present in household items ranging from fridge-magnets on a fridge door to the sensors and motors inside, while dozens can be found in an average car. As temperature is raised ferromagnetic order disappears, but more rarely the formation of more fascinating states can occur in which the uniform magnetic order is replaced by a static magnetic wave. One mechanism for this is known as quantum order by disorder; the wave (the order) forms because it has more low energy excited states (the disorder) available to it than the uniform state and this lowers its energy, offsetting the energy cost of producing the wave. In a recent paper appearing in Physical Review Letters such a static oscillation was achieved for the first time by applying a magnetic field at right angles to the easy axis moment in a ferromagnet, allowing the resulting increase in magnetic excitations to be clearly seen with neutron scattering. In the long term the creation of such magnetic oscillations with variable pitch could be useful, for example, to make magnetic diffraction gratings for spintronics.
Julie Wolanin, Jérôme Giraud, Claude Payre, Marianne Benoit, Claire Antonelli, Damien Quemener, Iliass Tahiri, Matthieu Vandamme, Jean-Marc Zanotti, and Marie Plazanet, Review of Scientific Instruments 92 (2021) 024106
In comparison to condensed matter, soft matter is subject to several interplaying effects (surface heterogeneities and swelling effect) that influence transport at the nanoscale. In consequence, transport in soft and compliant materials is coupled to adsorption and deformation phenomena. The permeance of the material, i.e., the response of the material to a pressure gradient, is dependent on the temperature, the chemical potential, and the external constraint.
Therefore, the characterization of water dynamics in soft porous materials, which we address here, becomes much more complex. In this paper, the development of an original setup for scattering measurements of a radiation in the transmitted geometry in oedometric conditions is described. A specially designed cell enables a uniaxial compression of the investigated material, PIM-1 (Polymers of Intrinsic Microporosity), in the direction perpendicular to the applied hydraulic pressure gradient (up to 120 bars). High pressure boosting of the circulating water is performed with a commercially available high-pressure pump Karcher. This particular setup is adapted to the quasi-elastic neutron scattering technique, which enables us to probe diffusion and relaxation phenomena with characteristic times of 10−9 s–10−12 s. Moreover, it can easily be modified for other scattering techniques.
Chang Liu, Wenxian Hu, Hanqiu Jiang, Guoming Liu*, Charles C. Han, Henning Sirringhaus, François Boué, and Dujin Wang
The determination of intrinsic chain stiffness of conjugated polymers is challenging, in particular, for scattering techniques because of their strong light absorption and structural instability due to the complicated intra-/intermolecular interactions. In this work, the chain conformation and aggregation formation of a high charge mobility donor–acceptor polymer (DPPDTT) are systematically investigated by using small-angle neutron scattering (SANS) and static/dynamic light scattering (SLS/DLS). On the one hand, chloroform was chosen as a good solvent, in which SANS reveals a rod-like geometry with a radius of ∼15 Å. Once the absorption effect is properly accounted for, SLS shows a power law of 1 between the radius of gyration (Rg) and molecular weight (Mw) and a negative second virial coefficient (A2). On the other hand, 1,2-dichlorobenzene was chosen as a poor solvent, in which SANS, SLS/DLS, and atomic force microscopy (AFM) reveal a strong temperature-/concentration-dependent assembling behavior. The results provide a general picture of the multiscale assembly process of conjugated polymers.
K. Beauvois, V. Simonet, S. Petit, J. Robert, F. Bourdarot, M. Gospodinov, A.A. Mukhin, R. Ballou, V. Skumryev, and E. Ressouche
The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in Bi2Fe4O9 materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.
Understanding how spins orient and can be manipulated at very fast, pico and femtosecond scales has implications for ultra-fast, low-power data processing and storage applications.
An international research team, to which Jean-Eric Wegrowe of the Irradiated Solids Laboratory contributes, is experimentally demonstrating the intrinsic dynamics of inertial spin in ferromagnetic thin films, in the form of nutation of magnetization at a frequency of ~0.5 THz. This shows that the relaxation time of the angular momentum in ferromagnets is about ten picoseconds.
This work has been published in Nature Physics on September 28, 2020.
We show by X-ray and neutron small-angle scattering that gold nanoparticles with controlled sizes and morphologies can be obtained by the metallic reduction of AuCl4– ions trapped in 3D organic molds by X-ray radiolysis. The molds are spherical frozen micelles of polystyrene-b-poly(dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) block copolymer in acidic aqueous solution with a PS spherical core surrounded by a corona of PDMAEMA chains in good solvent. The behavior of micelles is controlled by the [AuCl4–]/[DMAEMA] ratio RAuCl4–/DMAEMA. At low gold concentration, AuCl4– ions condense on the positively charged DMAEMA moieties without changing the behavior of the PDMAEMA chains. At intermediate gold concentration, the ions induce a progressive contraction of the corona’s chains and dehydration of micelles. At large gold concentration, the corona becomes a fully dry phase loaded with gold ions, which induces micelle aggregation. Radiolysis of the solution by an intense X-ray beam produces different types of gold nanoparticles with respect to RAuCl4–/DMAEMA and irradiation time. At RAuCl4–/DMAEMA = 0.033, irradiation produces in the first step gold clusters in the micelle corona which in the second step merge to form nanoparticles of a similar size to that of the micelle. Conversely, at RAuCl4–/DMAEMA = 0.33, micelles do not operate as templates but only as nucleation zones and large nanoparticles grow outside the micelles.
Black phosphorus (BP) is a stack of monoatomic layers of phosphorus, bound together by Van der Waals forces. This 2d material is today attracting a great interest due to its widely tunable band gap, depending on the thickness of the material, the very high mobility of its carriers, for its application in field effect transistors (FETs), and the possible emergence of topologically protected states. It has been shown that electrostatic action or evaporation of alkali metals, as electron donors, can be successfully used for doping the PB from a semiconductor phase to a semi-metallic phase with Dirac cone transport channels (with linear dispersion).
Experimentalists and theoreticians at the Laboratory for Irradiated Solids (LSI) have thus studied the excited electronic states of black phosphorus after deposition of an increasing dose of alkaline atoms. The FemtoARPES (angle-resolved photoemission) experimental device at the LSI was used to monitor the band gap closure with unprecedented accuracy. This engineering of the electronic band structure would enable the design of devices with improved and optimized electronic and optoelectronic functionality.
de Oliveira-Silva, Rodrigo; Bélime, Agathe; Le Coeur, Clémence; Chennevière, Alexis; Helary, Arnaud; Cousin, Fabrice; Judeinstein, Patrick; Sakellariou, Dimitrios; Zanotti, Jean-Marc
In soft condensed matter, Small Angle Neutron Scattering (SANS) is a central tool to probe structures with characteristic sizes ranging from 1 to 100 nm. However, when used as a standalone technique, the dynamic properties of the sample are not accessible. Nuclear Magnetic Resonance (NMR) is a versatile technique which can easily probe dynamical information. Here, we report on the coupling of a low-field NMR system to a SANS instrument. We show that this original set-up makes it possible to obtain structural information and to simultaneously extract in situ on a same sample, long-range translational diffusion coefficient, T1T1 and T2T2 nuclear spin relaxation times. Such a feature is of major interest when a sample experiences a transient physical state or evolves rapidly. We illustrate the capabilities of alliancing these experimental methods by following the critical temperature-induced phase separation of a concentrated Poly(Methacrylic Acid) solution at its Lower Critical Solution Temperature. The characteristic size related to the domain growth of the polymer-rich phase of the gel is monitored by the evolution of the SANS spectra, while the dynamics of the sol phase (H2O and polymer) is simultaneously characterized by NMR by measuring T1T1, T2T2 and the diffusion coefficient. Great care has been taken to design a cell able to optimize the thermalization of the sample and in particular its equilibration time. Details are given on the sample cell specifically designed and manufactured for these experiments. The acquisition time needed to reach good signal-to-noise ratios, for both NMR and SANS, match: it is of the order of one hour. Altogether, we show that in situ low-field NMR/SANS coupling the NMR is meaningful and is a promising experimental approach.
M. Ruminy, S. Guitteny, J. Robert, L.-P. Regnault, M. Boehm, P. Steffens, H. Mutka, J. Ollivier, U. Stuhr, J. S. White, B. Roessli, L. Bovo, C. Decorse, M. K. Haas, R. J. Cava, I. Mirebeau, M. Kenzelmann, S. Petit, and T. Fennell
Tb2Ti2O7 presents an ongoing conundrum in the study of rare-earth pyrochlores. Despite the expectation that it should be the prototypical unfrustrated noncollinear Ising antiferromagnet on the pyrochlore lattice, it presents a puzzling correlated state that persists to the lowest temperatures. Effects which can reintroduce frustration or fluctuations are therefore sought, and quadrupolar operators have been implicated. One consequence of strong quadrupolar effects is the possible coupling of magnetic and lattice degrees of freedom, and it has previously been shown that a hybrid magnetoelastic mode with both magnetic and phononic character is formed in Tb2Ti2O7 by the interaction of a crystal field excitation with a transverse-acoustic phonon. Here, using polarized and unpolarized inelastic neutron scattering, we present a detailed characterization of the magnetic and phononic branches of this magnetoelastic mode, particularly with respect to their composition, the anisotropy of any magnetic fluctuations, and also the temperature dependence of the different types of fluctuation that are involved. We also examine the dispersion relations of the exciton branches that develop from the crystal field excitation in the same temperature regime that the coupled mode appears, and find three quasidispersionless branches where four are expected, each with a distinctive structure factor indicating that they are nonetheless cooperative excitations. We interpret the overall structure of the spectrum as containing four branches, one hybridized with the phonons and gaining a strong dispersion, and three remaining dispersionless.
|La métrologie (spectroscopie, mesures de temps ou de distances) ou encore la réalisation de réseaux optiques quantiques nécessitent des sources de photons uniques efficaces. Une équipe du SPEC à Saclay, en collaboration avec l'IQST d'Ulm en Allemagne, démontre expérimentalement une voie originale pour obtenir une source de photons microonde uniques, simple, efficace et brillante. Cette nouvelle voie exploite un régime de fort couplage lumière-matière rendu accessible grâce aux développements récents des circuits électriques quantiques.|
Souha Ben Mahmoud, Wafa Essafi, Annie Brûlet and François Boué
The chain conformation in sulfonated polystyrene PSSNa of a degree of sulfonation 0.34 ≤ f ≤ 1, i.e., of various hydrophobicity, is followed in mixtures of water and increasing content of tetrahydrofuran (THF), a good solvent of the hydrophobic polystyrene moieties (which improves the solvent quality of the mixture). This is achieved by measuring the chain form factor by small-angle neutron scattering using the zero average contrast method (ZAC). Polymer concentrations 0.17 and 0.34 M correspond in our case to the semidilute regime or its limit with dilute regime depending on the chain conformation. The main result is the monitoring with added THF of the pearl necklace conformation. This heterogeneous structure, made of wormlike chain parts (strings) and pearls, was observed formerly in water: when f decreases, the string contribution decreases, and the pearls size, characterized by a maximum in Kratky q2S1(q) representation, slightly increases. Here we see that in the presence of increasing content of added THF (i) the pearls contribution decreases, as expected, and (ii) their size does not change by more than 10% in most cases (30% at the most). Among different modeling, the most complete has been done following the pearl necklace models of Schweins, Huber et al. and Lages, Huber et al.; beyond the size and distribution of pearls, it addresses the radius of gyration, the correlation distance between spheres, weakly visible, and, importantly their number N. The values of Rg, as well as the modeling, suggest that while the sphere size varies a few, N decreases clearly with added good solvent. A link with the simulation of Liao et al. can be found. A second result, important in practice, is that all modeling of the scattering establishes that THF addition makes vanish an excess of low q scattering due to large compact spheres present in water. A third result is obtained after a “THF treatment” procedure, consisting of adding THF and then removing it by evaporation: (i) the spherical aggregates are washed out, and (ii) the pearl necklace conformation of the chain returns to the one in water solution before treatment. Therefore, the pearl necklace conformation of the hydrophobic polyelectrolyte in aqueous solution appears to be in an annealed equilibrium state resulting from hydrophobic attraction and electrostatic repulsion.
Béatrice Gillon, Albert Hammerschmied, Arsen Gukasov, Alain Cousson, Thomas Cauchy, Eliseo Ruiz, John A. Schlueter, Jamie L. Manson
We report neutron‐diffraction investigations of the quasi‐2D MnII(dca)2(pym)(H2O) (pym = N2C4H4) compound, where high‐spin MnII ions are bridged by dicyanamide anions, [N(CN)2]– (herein abbreviated dca). Inside the layers, Mn2+ ions are connected by single or double dca bridges. The magnetic phase diagram was established by neutron diffraction on a single crystal. In the low‐field phase, the MnII ions are antiferromagnetically ordered in the layers, with moments nearly parallel to the c axis, and the layers are antiferromagnetically coupled. The spin‐flop phase corresponds to ferromagnetic coupling between the antiferromagnetic layers, in which the MnII moments are nearly perpendicular to the c axis. The induced spin‐density distribution in the paramagnetic phase, determined by polarized neutron diffraction, visualizes the superexchange pathways through the dca ligands within the layers and through H bonding between neighboring layers. The theoretical spin density obtained by bidimensional periodic DFT calculations is compared with the experimental results. Furthermore, quantum Monte Carlo simulations have been performed to compare the DFT results with experimental susceptibility measurements.
We report on the self-assembly behavior of poly(2-methyl-2-oxazoline)–block–poly(2-octyl-2-oxazoline) comprising different terminal perfluoroalkyl fragments in aqueous solutions. As reported previously [Kaberov et al. (2017)] such polyphiles can form a plethora of nanostructures depending of the composition and on the way of preparation. Here we report, for the first time, detailed information on the internal structure of the nanoparticles resulting from the self-assembly of these copolymers. Small-angle neutron and X-ray scattering (SANS/SAXS) experiments unambiguously prove the existence of polymersomes, wormlike micelles and their aggregates in aqueous solution. It is shown that increasing content of fluorine in the poly(2-oxazoline) copolymers results in a morphological transition from bilayered or multi-layered vesicles to wormlike micelles for solutions prepared by direct dissolution.
In contrast, nanoparticles prepared by dialysis of a polymer solution in a non-selective organic solvent against water are characterized by SAXS method. The internal structure of the nanoparticles could be assessed by fitting of the scattering data, revealing complex core-double shell architecture of spherical symmetry. Additionally, long range ordering is identified for all studied nanoparticles due to the crystallization of the poly(2-octyl-2-oxazoline) segments inside the nanoparticles.
D. Bounoua, R. Saint-Martin, S. Petit, P. Berthet, F. Damay, Y. Sidis, F. Bourdarot, and L. Pinsard-Gaudart, Phys. Rev. B 95, 224429
The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates SrCuO2 have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, SrCu0.99M0.01O2 (with M=Zn or Mg) and Sr0.99La0.01CuO2 (Cu+ carrying S=0 created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of SrCuO2 upon doping, stemming from disruptions of the spin chains by quantum impurities.
Fractionalized excitations that emerge from a many-body system have revealed rich physics and concepts, from composite fermions in two-dimensional electron systems, revealed through the fractional quantum Hall effect, to spinons in antiferromagnetic chains and, more recently, fractionalization of Dirac electrons in graphene and magnetic monopoles in spin ice. Even more surprising is the fragmentation of the degrees of freedom themselves, leading to coexisting and a priori independent ground states.
This puzzling phenomenon was recently put forward in the context of spin ice, in which the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic monopole crystal. In a Nature Physics publication it is reported that, by means of neutron scattering measurements, such fragmentation occurs in the spin ice candidate Nd2Zr2O7. A spectacular coexistence of an antiferromagnetic order induced by the monopole crystallization and a fluctuating state with ferromagnetic correlations is observed. Experimentally, this fragmentation manifests itself through the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating.
The Editors of the journal Physical Review Letters recently awarded the label "Editor's suggestion" to an article from the Quantronics group of SPEC (Laboratory of Condensed Matter Physics) : "Phase Controlled Superconducting Proximity Effect Probed by Tunneling Spectroscopy, Phys. Rev. Lett. 100 (2008) 197002". By this label publishers aim to put forward a small number of items they regard as particularly clear and likely to attract readers outside their specialty.
This paper provides for the first time a neat, clear and complete overview of "the superconducting proximity effect." Such an effect occurs at the interface between metal superconductor (S) and metals with "normal" resistivity (N), in which superconductivity can locally expand within the normal metal and make it non-resistive.
LLB researchers have published several papers in 2008 in the prestigious journal Science [1-2] and Nature . These results show the full potential of the neutron diffraction techniques at the forefront of research on new materials.
1 Institut de Physique de Rennes, CNRS UMR 6251, Univ. Rennes 1, 35042 Rennes, France
2 CNRS, UMR 6251, IPR, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France.
3 Laboratoire Léon Brillouin, CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France
4 Institut Laue-Langevin, 38042 Grenoble Cedex 9, France.
5 Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA.
6 Facultad de Ciencias, Universidad del Pais Vasco, Apdo 644, Bilbao, Spain.
One way to probe the structure of matter at the atomic scale is to use radiation diffraction (such as X-rays, or the wave associated with particles: neutrons, electrons ...). The presence of a long-distance order is then characterized by the presence of diffraction peaks, forming an image reflecting the symmetry of the object. This led to the discovery in the recent decades of non-periodic materials, but still with long range order as evidenced by discrete peaks in their diffraction spectrum. Physicists represent this type of crystals as periodic crystals but in a super-space (with dimension 3 + d, which represent the 3 usual dimensions of space and the dimension d of the internal space).
The systems understudy are aperiodic supramolecular model systems consisting of an urea single crystal (host structure) and alkane molecules (invited molecules), whose length is determined by the number nC of carbon atoms in the molecule (nC>7). Crystalline plane of urea have a structure of hexagonal symmetry. Along the C perpendicular axis, the structure exhibits a double helix forming channels in which alkane molecules may be inserted. At room temperature, these molecules are ordered along the C axis of the urea lattice, but according to an aperiodic order.
Neutron diffraction spectrum along the C* axis. Crossing the first critical temperature Tc1 superstructure rods (h = 1/2) along C* appear that indicates a change in the symmetry of the system. Along the rods, incommensurate discrete diffraction peaks are characteristic of the new aperiodic ordered structure. This mesh doubling is completely original because the observed phase transition between the two incommensurate phases can be only properly described by means of the 4-dimensional space. A second structural phase appears below Tc2.