Frustration géométrique et nouveaux états quantiques de spins dans les composés vanadates fluorés à géométrie kagomé

The search for quantum liquid state is a very active field in condensed matter research. In two dimensions, the antiferromagnetic spin 1/2 kagome lattice seems to be the most able to stabilize such a ground state. Indeed, from recent theoretical investigations, we are now quite sure that this model has a quantum spin liquid ground state. However, we still do not know its nature, in particular the nature of its correlations. They could be short ranged with a gap in the excitation spectrum, or long ranged with a gapless excitation spectrum. On the experimental side, only few materials exist and only one possesses a geometrically perfect lattice, the Herbertsmithite. All the experiments that have been done on this compound reveal a gapless spin liquid state along with deviations to the spin 1/2 Heisenberg hamiltonian which could be responsible of the gap closure.This thesis deals with the experimental study, mainly by NMR and µSR, of new vanadium based kagomé compounds which are part of a newly synthesized family, the kagome fluoride vanadates. The material that we studied the most is a spin 1/2 kagomé compound based on V4+, (NH4)2[C7H14N][V7O6F18] (DQVOF). The magnetic model of this compound can be decomposed in two rather independent parts, trimerized kagome planes and quasi paramagnetic V3+ ions. The µSR studies, showing the absence of frozen moment down to 20 mK, reveal a spin liquid ground state in DQVOF. The heat capacity and the NMR experiments point out a gapless behavior despite trimerization and likely weak Dzyaloshinskii Moriya interactions. Our results demonstrate that the gapless ground state, whether intrinsic or due to deviation to the ideal hamiltonian, is a rather robust characteristic of kagome materials.Furthermore, we studied another compound of this family, (NH4)2[C2H8N][V3F12] (DDVF), which magnetic lattice is made of uncoupled kagomé planes based on V3+ (S = 1). The lattice shows large deviations to the ideal kagomé and the thermodynamic experiments and the µSR studies reveal a magnetic transition to a frozen state at 10 K with a long distance order which is effective only below 6 K.L’étude de l’état fondamental liquide de spins est un des domaines très actif de la recherche en matière condensée. Le réseau le plus à même de stabiliser un tel état fondamental semble être, à deux dimensions, le réseau kagomé de spins antiferromagnétiques 1/2. Il y a à présent un consensus théorique sur le fait que ce modèle stabilise un état fondamental liquide de spin. Cependant, la nature de cet état est encore inconnue, notamment la nature des corrélations. Nous ne savons toujours pas si ces dernières sont à courte portée avec un gap dans le spectre d’excitations, ou si elles sont à plus longue portée avec un spectre d’excitations sans gap. D’un point de vue expérimental il n’existe que très peu de matériaux et seul l’Herbertsmithite présente un réseau kagomé de spins 1/2 géométriquement parfait. Les différentes études réalisées sur ce composé pointent toutes vers un état liquide de spin sans gap mais révèlent aussi des déviations à l’hamiltonien de Heisenberg qui pourraient être responsables de la fermeture de ce gap.Cette thèse traite de l’étude expérimentale principalement par RMN et µSR de nouveaux composés kagomé à base de vanadium faisant partie d’une famille récemment synthétisée, les vanadates fluorés à géométrie kagomé. Le matériau que nous avons le plus étudié est un composé à réseau kagomé de spins 1/2 à base de V4+, (NH4)2[C7H14N][V7O6F18] (DQVOF). Le modèle magnétique de ce composé peut être décomposé en deux sous systèmes presque indépendants, des plans kagomé trimérisés isolés et des ions V3+ quasi paramagnétiques. Les études de µSR démontrent une absence de gel magnétique jusqu’à 20 mK donc un état liquide de spins dans DQVOF. Les études de chaleur spécifique et de RMN dévoilent un comportement liquide de spin sans gap malgré la trimérisation du réseau et la faible valeur supposée de l’interaction Dzyaloshinskii Moriya. Nos résultats montrent finalement que l’absence de gap, intrinsèque ou due à des déviations à l’hamiltonien idéal, est une caractéristique robuste des matériaux kagomé. Nous avons de plus étudié un second matériau de cette famille, (NH4)2[C2H8N][V3F12] (DDVF), dont le réseau magnétique est formé par des plans kagomé découplés entre eux à base de V3+ (S = 1). Ce réseau présente de fortes distorsions par rapport au réseau idéal et les expériences thermodynamiques et de µSR mettent en évidence une transition magnétique vers un état gelé à 10 K avec une mise en ordre à longue distance qui s’effectue à 6 K uniquement

A biaxial apparatus for the study of heterogeneous and intermittent strains in granular materials

We present an experimental apparatus specifically designed to investigate the precursors of failure in granular materials. A sample of granular material is placed between a latex membrane and a glass plate. A confining effective pressure is applied by applying vacuum to the sample. Displacement-controlled compression is applied in the vertical direction, while the specimen deforms in plane strain. A Diffusing Wave Spectroscopy visualization setup gives access to the measurement of deformations near the glass plate. After describing the different parts of this experimental setup, we present a demonstration experiment where extremely small (of order $10^{-5}$) heterogeneous strains are measured during the loading process

Electron transfer properties of mono- and diferrocenyl based Cu complexes attached as self-assembled monolayers on gold electrodes by "self-induced" electroclick

International audienceTwo new Cu complexes bearing a 6-ethynyl bis-(methyl-pyridyl) amine (6eBMPA) moiety, as an electroclickable function linked to a ferrocenyl-based triazolyl arm (ligands 3 and 4) have been synthetized and characterized by UV-Visible, EPR spectroscopies and cyclic voltammetry in acetonitrile. Two different spacer groups between the terminal ferrocene and the triazolyl group were inserted: an hexyl chain in the case of the complex Cu-3, an ethenyl-bridged diferrocenyl system for the complex Cu-4. The monoelectronic oxidation of the diferrocenyl species yields a stable mixed-valence complex. NIR-Visible spectroscopic studies show a moderate interaction between ferrocenyl units (class II according to the Robin-Day classification). The immobilization of these systems as SAMs on an azidoundecanethiol modified gold electrode has been successfully operated by using the "self-induced electroclick" procedure. The voltammetric characterization of the surface-tagged Cu complexes indicates that good surface coverage was achieved, with moderately fast electron-transfer reaction between the electrode and the redox active immobilized systems (k0(Cu) = 2-4 s−1, k0(Fc) = 20-90 s−1). Remarkably, the rate of charge transport is significantly controlled by the nature of the spacer on the ferrocenyl triazole arm

Persistent spin dynamics in the pressurized spin-liquid candidate YbMgGaO4_4

Single-crystal x-ray diffraction, density-functional band-structure calculations, and muon spin relaxation ($\mu$SR) are used to probe pressure evolution of the triangular spin-liquid candidate YbMgGaO$_4$. The rhombohedral crystal structure is retained up to at least 10 GPa and shows a nearly uniform compression along both in-plane and out-of-plane directions, whereas local distortions caused by the random distribution of Mg$^{2+}$ and Ga$^{3+}$ remain mostly unchanged. The $\mu$SR data confirm persistent spin dynamics up to 2.6 GPa and down to 250 mK with no change in the muon relaxation rate. Longitudinal-field $\mu$SR reveals power-law behavior of the spin-spin autocorrelation function, both at ambient pressure and upon compression.Comment: published versio

Type-I superconductivity in PdTe2_2 probed by μ\muSR

The Dirac semimetal PdTe$_2$ was recently reported to be a type-I superconductor with $T_c = 1.64$ K and a critical field $\mu_0H_c = 13.6$ mT. Since type-I superconductivity is unexpected for binary compounds, we have conducted muon spin rotation experiments to probe the superconducting phase on the microscopic scale via its intermediate state. For crystals with a finite demagnetization factor, $N$, the intermediate state forms in applied fields $(1-N)H_c < H_a < H_c$. We have carried out transverse field muon spin rotation measurements on a thin disk-like crystal with the field perpendicular to ($N_{\perp}=0.86$) and in the plane ($N_{\parallel}=0.08$) of the disk. By analysing the $\mu$SR signal we find that the volume fraction of the normal domains grows quasi-linearly with applied field at the expense of the Meissner domain fraction. This then provides solid evidence for the intermediate state and type-I superconductivity in the bulk of PdTe$_2$

Electrochemical measurements of the kinetics of inhibition of two FeFe hydrogenases by O2 demonstrate that the reaction is partly reversible

International audienceThe mechanism of reaction of FeFe hydrogenases with oxygen has been debated. It is complex, apparently very dependent on the details of the protein structure, and difficult to study using conventional kinetic techniques. Here we build on our recent work on the anaerobic inactivation of the enzyme [Fourmond et al, Nat. Chem. 4 336 (2014)] to propose and apply a new method for studying this reaction. Using electrochemical measurements of the turnover rate of hydrogenase, we could resolve the first steps of the inhibition reaction and accurately determine their rates. We show that the two most studied FeFe hydrogenases, from Chlamydomonas reinhardtii and Clostridium acetobutylicum, react with O2 according to the same mechanism, despite the fact that the former is much more O2 sensitive than the latter. Unlike often assumed, both enzymes are reversibly inhibited by a short exposure to O2. This will have to be considered to elucidate the mechanism of inhibition, before any prediction can be made regarding which mutations will improve oxygen resistance. We hope that the approach described herein will prove useful in this respect

Evidence and modeling of mechanoluminescence in a transparent glass particulate composite

International audienceMechanoluminescence (ML) of a transparent alkali-phosphate glass composite with SrAl2O4:Eu, Dy particles is reported. Uniaxial compression experiments show the linear dependence of the mechanoluminescence intensity with the mechanical power. A theoretical model, based on the physics of delayed processes (in analogy of viscoelasticity), is proposed. This model accurately predicts the ML intensity changes induced by a complex mechanical loading and provides a convincing description of the mechanoluminescence response

Superconducting order parameter of the nodal-line semimetal NaAlSi

Nodal-line semimetals are topologically non-trivial states of matter featuring band crossings along a closed curve, i.e. nodal-line, in momentum space. Through a detailed analysis of the electronic structure, we show for the first time that the normal state of the superconductor NaAlSi, with a critical temperature of $T_{\rm c} \approx$ 7 K, is a nodal-line semimetal, where the complex nodal-line structure is protected by non-symmorphic mirror crystal symmetries. We further report on muon spin rotation experiments revealing that the superconductivity in NaAlSi is truly of bulk nature, featuring a fully gapped Fermi-surface. The temperature-dependent magnetic penetration depth can be well described by a two-gap model consisting of two $s$-wave symmetric gaps with $\Delta_1 =$ 0.6(2) meV and $\Delta_2 =$ 1.39(1) meV. The zero-field muon experiment indicates that time-reversal symmetry is preserved in the superconducting state. Our observations suggest that notwithstanding its topologically non-trivial band structure, NaAlSi may be suitably interpreted as a conventional London superconductor, while more exotic superconducting gap symmetries cannot be excluded. The intertwining of topological electronic states and superconductivity renders NaAlSi a prototypical platform to search for unprecedented topological quantum phases

A relationship between non-exponential stress relaxation and delayed elasticity in the viscoelastic process in amorphous solids: Illustration on a chalcogenide glass

International audienceInorganic glasses are viscoelastic materials since they exhibit, below as well as above their glass transition temperature, a viscoelas-tic deformation under stress, which can be decomposed into a sum of an elastic part, an inelastic (or viscous) part and a delayed elastic part. The delayed elastic part is responsible for the non-linear primary creep stage observed during creep tests. During a stress relaxation test, the strain, imposed, is initially fully elastic, but is transformed, as the stress relaxes, into an inelastic and a delayed elastic strains. For linear viscoelastic materials, if the stress relaxation function can be fitted by a stretched exponential function, the evolution of each part of the strain can be predicted using the Boltzmann superposition principle. We develop here the equations of these evolutions, and we illustrate their accuracy by comparing them with experimental evolutions measured on GeSe 9 glass fibers. We illustrate also, by simple equations, the relationship between any kind of relaxation function based on additive contribution of different relaxation processes and the delayed elastic contribution to stress relaxation: the delayed elasticity is directly correlated to the dispersion of relaxations times of the processes involved during relaxation