Christiane Alba-Simionesco , Patrick Judeinstein , Stéphane Longeville, Oriana Osta , Florence Porcher , Frédéric Caupin , and Gilles Tarjus

The proneness of water to crystallize is a major obstacle to understanding its putative exotic behavior in the supercooled state. It also represents a strong practical limitation to cryopreservation of biological systems. Adding some concentration of glycerol, which has a cryoprotective effect preventing, to some degree, water crystallization, has been proposed as a possible way out, provided the concentration is small enough for water to retain some of its bulk character and/or for limiting the damage caused by glycerol on living organisms. Contrary to previous expectations, we show that, in the “marginal” glycerol molar concentration ≈ 18%, at which vitrification is possible with no crystallization on rapid cooling, water crystallizes upon isothermal annealing even below the calorimetric glass transition of the solution. Through a time-resolved polarized neutron scattering investigation, we extract key parameters, size and shape of the ice crystallites, fraction of water that crystallizes, and crystallization time, which are important for cryoprotection, as a function of the annealing temperature. We also characterize the nature of the out-of-equilibrium liquid phases that are present at low temperature, providing more arguments against the presence of an isocompositional liquid–liquid transition. Finally, we propose a rule of thumb to estimate the lower temperature limit belowwhich water crystallization does not occur in aqueous solutions.

https://doi.org/10.1073/pnas.2112248119

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.

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 (R_g) and molecular weight (M_w) 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

N. Martin, M. Deutsch, T. C. Hansen, M. T. Fernandez-Diaz, L. N. Fomicheva, A. V. Tsvyashchenko, and I. Mirebeau

In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds Mn0.86Co0.14Ge and Mn0.9Rh0.1Ge, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures (≈19 GPa, to be compared with ≈13 GPa). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys.

DOI : 10.1103/PhysRevB.100.060401

A. S. Sukhanov, Praveen Vir, A. S. Cameron, H. C. Wu, N. Martin, S. Mühlbauer, A. Heinemann, H. D. Yang, C. Felser, and D. S. Inosov

The cubic chiral helimagnets with the P213 space group represent a group of compounds in which the stable skyrmion-lattice state is experimentally observed. The key parameter that controls the energy landscape of such systems and determines the emergence of a topologically nontrivial magnetic structure is the Dzyaloshinskii-Moriya interaction (DMI). Chemical substitution is recognized as a convenient instrument to tune the DMI in real materials and has been successfully utilized in studies of a number of chiral magnets, such as MnSi, FeGe, MnGe, and others. In our study, we applied small-angle neutron scattering to investigate how chemical substitution influences the skyrmionic properties of an insulating helimagnet Cu2OSeO3 when Cu ions are replaced by either Zn or Ni. Our results demonstrate that the DMI is enhanced in the Ni-substituted compounds (Cu,Ni)2OSeO3, but weakened in (Cu,Zn)2OSeO3. The observed changes in the DMI strength are reflected in the magnitude of the spin-spiral propagation vector and the temperature stability of the skyrmion phase.

https://doi.org/10.1103/PhysRevB.100.184408

N. Martin, I. Mirebeau, C. Franz, G. Chaboussant, L. N. Fomicheva, and A. V. Tsvyashchenko

We study the helimagnetic ground state of the MnGe cubic alloy using small-angle neutron scattering and a high-resolution method, the so-called MIEZE spectroscopy. Upon cooling below the Néel temperature TN = 170(5) K, we observe the proliferation of long-wavelength gapless spin fluctuations, concomitant with a continuous evolution of the helical correlation length. These fluctuations disappear at Tcom = 32(5) K when the helical period becomes commensurate with the lattice. We propose to describe this intermediate phase as a soliton lattice, promoting nonlinear collective modes, or phasons, over a large temperature interval. We discuss the possible relevance of our results to the previously observed magnetotransport anomalies.

https://doi.org/10.1103/PhysRevB.99.100402

Marguerite Léang, Didier Lairez, Fabrice Cousin, Frédérique Giorgiutti-Dauphiné, Ludovic Pauchard, Lay-Theng Lee

During evaporative drying of a colloidal dispersion, the structural behavior at the air-dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying-interface constitutes the region of initial drying-stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8 and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying-front of the dispersion with a macroscopic drying surface, and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation-diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin, and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk, and an increase in the degree of ordering with particle size.

https://pubs.acs.org/doi/pdfplus/10.1021/acs.langmuir.8b03772

M. Deutsch, W. Peng, P. Foury-Leylekian, V. Balédent, S. Chattopadhyay, M. T. Fernandez-Diaz, T. C. Hansen, A. Forget, D. Colson, M. Greenblatt, M.-B. Lepetit, S. Petit, and I. Mirebeau

The magnetic structure of TbMn2O5 and DyMn2O5 multiferroics has been studied by high-pressure neutron diffraction in a large pressure range up to 6.6 GPa. In both cases, we observe a pressure-induced commensurate magnetic phase with propagation vector ( 1/2 0 1/2 ), growing with pressure at the expense of the ambient pressure phases. Being previously observed in YMn2O5 and PrMn2O5, this phase is most likely a generic feature of the RMn2O5 multiferroic family. A simple model is proposed to explain qualitatively the emergence of this pressure-induced phase. Differences between TbMn2O5 and DyMn2O5 behaviors at ambient and low pressures provide clues on the interaction scheme.

DOI : http://dx.doi.org/10.1103/PhysRevB.98.024408

Structural characterization of nanoparticles formed by fluorinated poly(2-oxazoline)-based polyphiles,
Anna Riabtseva, Leonid I. Kaberov, Laurence Noirez, Vasyl Ryukhtin, Corinne Nardin, Bart Verbraeken, Richard Hoogenboom, Petr Stepanek, Sergey K. Filippov, European Polymer Journal 99 (2018) 518.

Abstract :

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.

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 D_2d 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.

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 $\mathrm{SrCu}{O}_2$ 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, $\mathrm{SrC}{u}_{0.99}{M}_{0.01}{O}_2$ (with $M=\mathrm{Zn}$ or Mg) and $S{r}_{0.99}L{a}_{0.01}\mathrm{Cu}{O}_2$ ($C{u}^{+}$ carrying $S=0$ created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of $\mathrm{SrCu}{O}_2$ upon doping, stemming from disruptions of the spin chains by quantum impurities.

Le théorème fluctuation-dissipation (TFD) est un des résultats les plus fondamentaux de la physique statistique. Il relie l'intensité des fluctuations thermiques d’un système en équilibre à sa réponse à une force extérieure. Il traduit le fait que l’énergie apportée par cette force sera dissipée en chaleur en raison du caractère erratique et fluctuant des interactions avec les composants microscopiques du système. Sa version la plus célèbre est l’équation d’Einstein qui relie le coefficient de diffusion D d’une particule Brownienne dans un fluide (D "mesure" les fluctuations thermiques) à la viscosité η du fluide (η est la "réponse" du fluide à une force s’exerçant sur un objet en mouvement).

Dans sa forme générale, ce théorème relie le spectre S_x(w) des fluctuations δx d’une observable x, à la partie dissipative de la susceptibilité χ”(w) qui traduit la réponse au champ conjugué à cette observable.

où ω/2π est la fréquence et T la température.

Le TFD s'applique aux systèmes à l’équilibre. Durant les trois dernières décades, son extension au cas hors d’équilibre a fait l’objet de nombreuses recherches expérimentales et théoriques [1]. Selon certains modèles, le TFD peut être conservé dans le cas hors d’équilibre à condition de remplacer la température T par une température effective T_eff supérieure, le rapport (T_eff / T) donnant une mesure de l’écart à l’équilibre du système. Pour tester cette prédiction, les verres sont des systèmes physiques idéaux car ils sont par nature hors d’équilibre et à dynamique lente. Parmi ces systèmes, un verre de spin est un ensemble de moments magnétiques désordonnés interagissant entre eux. Pour baisser l’énergie d’un spin donné, il ne suffit pas de le réorienter, mais il faut aussi réorienter ses voisins, puis les voisins des voisins, etc… Il faut donc un temps très long pour atteindre l’équilibre.

L’étude de la violation du TFD a concerné divers système vitreux jusqu’à ce jour, mais peu de résultats expérimentaux existent du fait de la difficulté à mesurer les faibles signaux de "bruit" à l'origine des fluctuations d’une observable. L’équipe MFD ('Magnétisme, Frustration et Désordre') de l'IRAMIS/SPEC dans le cadre d’une collaboration avec une équipe du LSI, a ainsi testé le théorème fluctuation-dissipation sur un verre de spin bien particulier : un ensemble de nanoparticules magnétiques (γ-Fe₂O₃) en suspension dans un liquide (ferrofluide) gelé car refroidi à basse température [2]. Chaque nanoparticule est monodomaine et porte un moment magnétique ~ 10⁴μ_B très supérieur aux moments magnétiques atomiques, formant ainsi un "verre de superspins". La température est choisie inférieure à la température de transition T_g = 67K où le système passe de l’état superparamagnétique (aimantation fluctuante des grains individuels, à l'équilibre) à l’état verre de superspins. Dans ce système, les interactions dipolaires entre moments magnétiques sont à l’origine des propriétés de verre de spin avec une dynamique lente et des effets de mémoire.

Depending on their fracture mode, materials are traditionally gathered into three distinct classes:

• (i) ductile materials that like metals deform plastically before their fracture
• (ii) quasi-brittle materials such as rock or concrete, where "non-visible" damage starts to accumulate through microcracking, up to coalescence that yields to catastrophic failure.
• (iii) brittle materials like oxide or polymer glasses... that deform elastically up to their fracture. This occurs through the successive breaking of atomic bonds at the crack tip. 

Experiments performed at IRAMIS-SPCSI show that a given material may, depending on the crack velocity, belong to two of these categories: Plexiglas^®, archetype of brittle material, is found to damage through the nucleation of microcracks like quasi-brittle materials, beyond a well defined crack speed.