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Univ. Paris-Saclay

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.

https://doi.org/10.1002/smll.202005954

Une équipe réunissant des chercheurs du SPEC, de l’IPhT et de l’ENS-Paris a montré l’absence de transition de Gardner dans un verre moléculaire archétypique – le glycerol - jusqu’à une température de 10 K. Cette transition, déjà identifiée dans d’autres systèmes vitreux (granulaires vibrés ou colloïdes) pourrait être la clef permettant de relier la façon dont ces systèmes amorphes se figent – lorsqu’ils se forment - à leurs propriétés mécaniques lorsqu’ils sont solides. On peut ainsi entrevoir un début d’explication à la différence entre matériaux amorphes mécaniquement fragiles ou non [1].

Liu Chang; Hu Wenxian; Jiang Hanqiu; Liu Guoming; Han Charles; Sirringhaus Henning; Boué François; Wang Dujin

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.

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.

https://doi.org/10.1021/acs.macromol.0c01646

A. K. Nayak, V. Kumar, T. Ma, P. Werner, E. Pippel, R. Sahoo, F. Damay, U. K. Rößler, C. Felser and S. S. P. Parkin,  Nature 548, 561566.

Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch and Néel skyrmions. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures. Direct imaging by Lorentz transmission electron microscopy shows field-stabilized antiskyrmion lattices and isolated antiskyrmions from 100 kelvin to well beyond room temperature, and zero-field metastable antiskyrmions at low temperatures. These results enlarge the family of magnetic skyrmions and pave the way to the engineering of complex bespoke designed skyrmionic structures.

Un isolant topologique est un matériau isolant en volume, qui présente cependant un caractère conducteur via des états électroniques de surface. Comme ces états sont fortement polarisés en spin et robustes par rapport au désordre cristallin, ces matériaux présentent un grand intérêt potentiel pour l'électronique de spin. Une telle situation est réalisée dans certains semi-conducteurs à petit gap à base de bismuth, comme Bi2Te3 ou Bi2Se3, ou de mercure, comme HgCdTe. Or, dans ces mêmes matériaux, la conduction de surface est généralement masquée par celle de volume, induite par la présence de défauts de croissance.

L'équipe du LSI montre qu'il est possible de réduire cette conductivité de volume en compensant le dopage du matériau par irradiation électronique de haute énergie, ouvrant ainsi la voie à une possible restauration d'une conduction dominée par les états de surface.

 

En matière condensée, les corrélations électroniques sous-tendent un grand nombre de phénomènes fondamentaux encore inexpliqués, dont certains peuvent déboucher sur de nouvelles applications (stockage de l'information, composants magnétiques, électronique de spin,…). Les systèmes étudiés sont très divers : systèmes supraconducteurs à haute température critique, manganites à magnétorésistance géante, systèmes multiferroïques, composés à fermions lourds, etc... Les couplages entre les degrés de libertés électroniques, magnétiques et de réseau sont à la base de tous ces phénomènes physiques.

Dans certains de ces systèmes contenant des ions magnétiques, la coexistence de différents types de couplages entre  ces ions peut donner naissance à des états magnétiques et électroniques très particuliers. C'est le cas de la nouvelle famille de composés CeM2Al10 (M : métal de transition Fe, Os, Ru). Les mesures de diffusion neutronique dans le composé CeRu2Al10 mettent en évidence la possibilité de faire coexister deux états magnétiques normalement antagonistes : un ordre magnétique à longue distance et un état appelé "isolant Kondo" qui tend à supprimer tout magnétisme. Ces mesures sont des éléments importants contribuant à la compréhension de  l'origine microscopique des propriétés magnétiques et électroniques de ces systèmes.

 

 

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