Faits marquants scientifiques 2021

Eni Kume and Laurence Noirez

In the conventional picture, the temperature of a liquid bath in the quiescent state is uniform down to thermal fluctuation length scales. Here we examine the impact of a low-frequency shear mechanical field (hertz) on the thermal equilibrium of polypropylene glycol and liquid water away from any phase transition confined between high-energy surfaces. We show the emergence of both cooling and heating shear waves of several tens of micrometers widths varying synchronously with the applied shear strain wave. The thermal wave is stable at low strain amplitude and low frequency while thermal harmonics develop by increasing the frequency or the strain amplitude. The liquid layer behaves as a dynamic thermoelastic medium challenging the extension of the fluctuation–dissipation theorem to nonequilibrium fluids. This view is in agreement with recent theoretical models predicting that liquids support shear elastic waves up to a finite propagation length scale of the order the thermal wave.

https://doi.org/10.1021/acs.jpcb.1c04362

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.

https://doi.org/10.1021/acs.jpcb.1c06791

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

Jaehong Jeong, Benjamin Lenz, Arsen Gukasov, Xavier Fabrèges, Andrew Sazonov, Vladimir Hutanu, Alex Louat, Dalila Bounoua, Cyril Martins, Silke Biermann, Véronique Brouet, Yvan Sidis, and Philippe Bourges, Phys. Rev. Lett. 125 (2020) 097202.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr₂IrO₄ is particularly in the spotlight because of the so-called j_eff = 1/2 state consisting of a quantum superposition of the three local t_2g orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state.

Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_eff = 1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

https://doi.org/10.1103/PhysRevLett.125.097202.

Dalila Bounoua, Lucile Mangin-Thro, Jaehong Jeong, Romuald Saint-Martin, Loreynne Pinsard-Gaudart, Yvan Sidis & Philippe Bourges, Communications Physics 3, (2020) 123.

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate Sr_14−xCa_xCu₂₄O₄₁ hosts a very rich phase diagram where, upon hole doping, the system exhibits a spin liquid state ending to an intriguing ordered magnetic state at larger Ca content.

Using polarized neutron diffraction, we report here the existence of short range magnetism in this material for two Ca contents, whose origin cannot be ascribed to Cu spins. This magnetism develops exclusively within the two-leg ladders with a diffraction pattern at forbidden Bragg scattering, which is the hallmark of loop current-like magnetism breaking both time-reversal and parity symmetries.

Our discovery shows local discrete symmetry breaking in a one dimensional spin liquid system as theoretically predicted. It further suggests that a loop current-like phase could trigger the long range magnetic order reported at larger doping in two-leg ladder cuprates.

https://doi.org/10.1038/s42005-020-0388-1.

Alessio Zaccone and Laurence Noirez, J. Phys. Chem. Lett.12 (2021) 1, 650–657.

Liquids confined to sub-millimeter scales have remained poorly understood. One of the most striking effects is the large elasticity revealed using good wetting conditions, which grows upon further decreasing the confinement length, L. These systems display a low-frequency shear modulus in the order of 1–10³ Pa, contrary to our everyday experience of liquids as bodies with a zero low-frequency shear modulus.

While early experimental evidence of this effect was met with skepticism and abandoned, further experimental results and, most recently, a new atomistic theoretical framework have confirmed that liquids indeed possess a finite low-frequency shear modulus G′, which scales with the inverse cubic power of confinement length L. We show that this law is universal and valid for a wide range of materials (liquid water, glycerol, ionic liquids, non-entangled polymer liquids, isotropic liquids crystals). Open questions and potential applications in microfluidics mechanochemistry, energy, and other fields are highlighted.

https://doi.org/10.1021/acs.jpclett.0c02953.

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

Virginie Vergnat, Benoît Heinrich, Michel Rawiso, René Muller, Geneviève Pourroy and Patrick Masson

Embedding nanoparticles (NPs) with organic shells is a way to control their aggregation behavior. Using polymers allows reaching relatively high shell thicknesses but suffers from the difficulty of obtaining regular hybrid objects at gram scale. Here, we describe a three-step synthesis in which multi-gram NP batches are first obtained by thermal decomposition, prior to their covalent grafting by an atom transfer radical polymerization (ATRP) initiator and to the controlled growing of the polymer shell. Specifically, non-aggregated iron oxide NPs with a core principally composed of γ-Fe₂O₃ (maghemite) and either polystyrene (PS) or polymethyl methacrylate (PMMA) shell were elaborated. The oxide cores of about 13 nm diameter were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). After the polymerization, the overall diameter reached 60 nm, as shown by small-angle neutron scattering (SANS). The behavior in solution as well as rheological properties in the molten state of the polymeric shell resemble those of star polymers. Strategies to further improve the screening of NP cores with the polymer shells are discussed.

https://doi.org/10.3390/nano11092453

Jaehong Jeong, Benjamin Lenz, Arsen Gukasov, Xavier Fabrèges, Andrew Sazonov, Vladimir Hutanu, Alex Louat, Dalila Bounoua, Cyril Martins, Silke Biermann, Véronique Brouet, Yvan Sidis, and Philippe Bourges, Phys. Rev. Lett. 125 (2020) 097202.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr₂IrO₄ is particularly in the spotlight because of the so-called j_eff = 1/2 state consisting of a quantum superposition of the three local t_2g orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state.

Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_eff = 1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

https://doi.org/10.1103/PhysRevLett.125.097202.