Laboratoire Léon Brillouin

We propose a model giving the conformation of a star shaped polymer by taking into account the radial variation of the monomer concentration ϕ( r). For an isolated star when increasing r (at the centre of the star r = 0), the variation of ϕ (r) is first given by a constant value (r < f 1/2 l) then has a (r/l)-1 variation (for f1/2 / < r < f1/2 ν-1 /) and finally a (r/l)-4/3 variation (for r > f1/2 ν-1 l); wh ere f is the number of branches, N the number of monomers in a branch and ν and l are the excluded volume and the length associated to a monomer. For all these cases, it is shown that the size of a branch is always larger than that of a linear polymer made of N monomers. Beyond the overlapping concentration the star conformation is obtained from two characteristic lengths essentially : χ(c ) a radius inside which the branches of the other stars do not penetrate, this radius defines a domain where the conformation of a star is similar to that of an isolated one. Beyond χ(c) the interpenetration of branches is characterized by a screening length ξ(c) very similar to that found for semi-dilute solutions of linear polymers. For all these regimes the variation of the size of a star is predicted as a function of N, f, v and c.

Water is an essential participant in the stability, structure, dynamics, and function of proteins and other biomolecules. Thermodynamically, changes in the aqueous environment affect the stability of biomolecules. Structurally, water participates chemically in the catalytic function of proteins and nucleic acids and physically in the collapse of the protein chain during folding through hydrophobic collapse and mediates binding through the hydrogen bond in complex formation. Water is a partner that slaves the dynamics of proteins, and water interaction with proteins affect their dynamics. Here we provide a review of the experimental and computational advances over the past decade in understanding the role of water in the dynamics, structure, and function of proteins. We focus on the combination of X-ray and neutron crystallography, NMR, terahertz spectroscopy, mass spectroscopy, thermodynamics, and computer simulations to reveal how water assist proteins in their function. The recent advances in computer simulations and the enhanced sensitivity of experimental tools promise major advances in the understanding of protein dynamics, and water surely will be a protagonist.

Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.

The parent compound of the giant magnetoresistance Mn-perovskite, ${\mathrm{LaMnO}}_{3}$, has been studied by thermal analysis and high-resolution neutron-powder diffraction. The orthorhombic $\mathrm{Pbnm}$ structure at room temperature is characterized by an antiferrodistorsive orbital ordering due to the Jahn-Teller effect. This ordering is evidenced by the spatial distribution of the observed Mn-O bond lengths. ${\mathrm{LaMnO}}_{3}$ undergoes a structural phase transition at ${T}_{\mathrm{JT}}\ensuremath{\approx}750$ K, above which the orbital ordering disappears. There is no change in symmetry although the lattice becomes metrically cubic on the high-temperature side. The ${\mathrm{MnO}}_{6}$ octahedra become nearly regular above ${T}_{\mathrm{JT}}$ and the thermal parameter of oxygen atoms increases significantly. The observed average cubic lattice is probably the result of dynamic spatial fluctuations of the underlying orthorhombic distortion.

Extensive and high-quality quasi-elastic incoherent neutron scattering data were obtained for water in the temperature range extending from room temperature down to -20 \ifmmode^\circ\else\textdegree\fi{}C in the supercooled state. The analysis generally confirms findings of our previous experiment [S. H. Chen, J. Teixeira, and R. Nicklow, Phys. Rev. A 26, 3477 (1982)], but in particular three new results have been obtained: (a) two relaxation times are clearly identified, which are related to the short-time and intermediate-time diffusion of water molecules, respectively, and their temperature dependence has been determined; (b) one of these relaxation times is associated with jump diffusion of the protons, and the temperature dependence of the jump length has been qualitatively determined; (c) the Q dependence of the scattering intensity integrated over the quasi-elastic region gives a Debye-Waller factor which is temperature independent.

Bismuth ferrite, BiFeO3, is the only known room-temperature magnetic ferroelectric material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric order parameters are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarization reorientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than in thin films where the cycloid is absent.

One of the leading issues in high-T(c) superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

Hydrophobically modified maghemite (γ-Fe(2)O(3)) nanoparticles were encapsulated within the membrane of poly(trimethylene carbonate)-b-poly(l-glutamic acid) (PTMC-b-PGA) block copolymer vesicles using a nanoprecipitation process. This formation method gives simple access to highly magnetic nanoparticles (MNPs) (loaded up to 70 wt %) together with good control over the vesicles size (100-400 nm). The simultaneous loading of maghemite nanoparticles and doxorubicin was also achieved by nanoprecipitation. The deformation of the vesicle membrane under an applied magnetic field has been evidenced by small angle neutron scattering. These superparamagnetic hybrid self-assemblies display enhanced contrast properties that open potential applications for magnetic resonance imaging. They can also be guided in a magnetic field gradient. The feasibility of controlled drug release by radio frequency magnetic hyperthermia was demonstrated in the case of encapsulated doxorubicin molecules, showing the viability of the concept of magneto-chemotherapy. These magnetic polymersomes can be used as efficient multifunctional nanocarriers for combined therapy and imaging.

The red long-lasting luminescence properties of the ZnGa2O4:Cr3+ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn1+xGa2–2x(Ge/Sn)xO4 (0 ≤ x ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to 71Ga solid state NMR studies. In contrast to ZnGa2O4:Cr3+ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn1+xGa2–2x(Ge/Sn)xO4. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps.

In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio c/a) have been scrutinized intensely. However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics. Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of BiFeO3 with a giant axial ratio (c/a=1.23) that is both ferroelectric and magnetic at room temperature. Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with c/a. We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases.

The authors discuss the physical nature of interactions between linear flexible chains in a solvent. They review the results obtained on the structure of polymer solutions, with the help of the observation techniques and conceptual approaches developed since 1965. Comparison between observed and predicted behaviour is made for values of the critical exponents and the universal amplitude.

As part of a general work on doped manganese perovskites, we have carried out detailed neutron-scattering experiments on powder and single crystals of the othorhombic phase of undoped LaMn${\mathrm{O}}_{3}$. The temperature dependence of the sublattice magnetization has been determined in the antiferromagnetic phase (${T}_{N}=139.5$ K), and the critical exponent is $\ensuremath{\beta}=0.28$, well below that corresponding to a pure three-dimensional Heisenberg antiferromagnet. We have measured the dispersion of the spin waves propagating in the highest symmetry directions solving the problems related to twinning. The whole spin wave spectrum is well accounted for with a Heisenberg Hamiltonian and a single ion anisotropy term responsible for the easy magnetization direction (b axis). This term induces a gap of 2.7 meV at low temperature in the spin wave dispersion curve. An important result is that the ferromagnetic exchange integral (${J}_{1}\ensuremath{\approx}0.83$ meV), coupling the spins within the ferromagnetic basal plane (a,b), is larger by a factor 1.4 than the antiferromagnetic exchange integral (${J}_{2}\ensuremath{\approx}\ensuremath{-}0.58$ meV) coupling spins belonging to adjacent Mn${\mathrm{O}}_{2}$ planes along c.

The magnetic structures of LiFePO4 and of its delithiated form FePO4 (triphylite, olivine group, space group Pnma) have been solved using neutron diffraction on polycrystalline samples. Both compounds show antiferromagnetic behavior below 52 K for LiFePO4 (Fe2+) and below 125 K for FePO4 (Fe3+), characterized by the appearance of extra peaks in the neutron diffraction patterns below the Néel temperature. These magnetic reflections are indexed with a propagation vector k = (0,0,0). The magnetic moments of the four iron atoms present in the unit cell are oriented along [010] for LiFePO4 and mostly along [100] (with a small component along [010]) for FePO4. The magnetic structures and the differences in the Néel temperatures are discussed in terms of super and super-super exchange intractions and of anisotropy of Fe2+. A comparison is made with other olivine compounds with similar cation distribution.

We report the observation of resolved $^{89}\mathrm{Y}$ NMR lines due to Zn nearest neighbors (nn) in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Zn}}_{\mathit{y}}$${)}_{3}$${\mathrm{O}}_{6.64}$. Comparison of the Curie dependence of their frequency shifts with susceptibility data allows us to conclude that the Zn induced local moments reside on the nn Cu orbitals and to explain the absence of resolvable nn lines for x=1. Analysis of the magnitude of the long-range spin-density oscillations monitored by the $^{89}\mathrm{Y}$ NMR width indicates that the reduction of ${\mathit{T}}_{\mathit{c}}$ might be connected with magnetic effects. For x=0.64, the low T increase of the spin-lattice relaxation rate of $^{89}\mathrm{Y}$ nuclei near Zn is shown to be governed by the local moment dynamic susceptibility.

${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ presents a first order structural transition at 290 K that was known to perturb the functioning as cathode in rechargeable Li batteries. We have solved the structure at 230 K and deciphered unambiguously the nature of this phase transition. The analysis of valence bond sums shows that the transition results from a partial charge ordering: two of the five Mn sites correspond to well-defined ${\mathrm{Mn}}^{4+}$ and the other three sites are close to ${\mathrm{Mn}}^{3+}$ ions. Charge ordering is accompanied by simultaneous orbital ordering due to the Jahn-Teller effect in ${\mathrm{Mn}}^{3+}$ ions. The microscopic details obtained from the structure are crucial for understanding the electron hopping persisting below the transition.

Breather solutions are time-periodic and space-localized solutions of nonlinear dynamical systems. We show that the concept of anticontinuous limit, which was used before for proving an existence theorem on breathers and multibreather solutions in arrays of coupled nonlinear oscillators, can be used constructively as a high-precision numerical method for finding these solutions. The method is based on the continuation of breather solutions which trivially exist at the anticontinuous limit. It is quite universal and applicable to a wide class of nonlinear models which can be of arbitrary dimension, periodic or random, with or without a driving force plus damping, etc. The main advantage of our method compared with other available methods is that we can distinguish unambiguously the different breather (or multibreather) solutions by their coding sequence. Another advantage is that we can obtain the corresponding solutions whether they are linearly stable or not. These solutions can be calculated in their full domain of existence. We illustrate the techniques with examples of breather calculations in several models. We mostly consider arrays of coupled anharmonic oscillators in one dimension, but we also test the method in two dimensions. Our method allows us to show that the breather solution can be continued while its frequency enters the phonon band (it then superposes to a band edge phonon with a finite amplitude). We also test that our method works when introducing an extra time-periodic driving force plus damping. Our method is applied for the calculation of breathers in so-called Fermi - Pasta - Ulam (FPU) chains, that is, one-dimensional chains of atoms with anharmonic nearest-neighbour coupling without on-site potential. The breather and multibreather solutions are then obtained by continuation from the anticontinuous limit of an extended model containing an extra parameter. Finally, we show that we can also calculate `rotobreathers' in arrays of coupled rotators, which correspond to solutions with one or several rotators rotating while the remaining rotators are only oscillating. The linear stability analysis of the obtained time periodic solutions (Floquet analysis) of all these models will be done in a forthcoming paper.

We investigate the dispersion mechanisms of nanocomposites made of well-defined polymer (polystyrene, PS) grafted-nanoparticles (silica) mixed with free chains of the same polymer using a combination of scattering (SAXS/USAXS) and imaging (TEM) techniques. We show that the relevant parameter of the dispersion, the grafted/free chains mass ratio R tuned with specific synthesis process, enables to manage the arrangement of the grafted nanoparticles inside the matrix either as large and compact aggregates (R < 0.24) or as individual nanoparticles dispersion (R > 0.24). From the analysis of the interparticles structure factor, we can extract the thickness of the spherical corona of grafted brushes and correlate it with the dispersion: aggregation of the particles is associated with a significant collapse of the grafted chains, in agreement with the theoretical models describing the free energy as a combination of a mixing entropy term between the free and the grafted chains and an elastic term of deformation of the grafted brushes. At fixed grafting density, the individual dispersion of particles below the theoretical limit of R = 1 can be observed, due to interdiffusion between the grafted and the free chains but also to processing kinetics effects, surface curvature and chains poly dispersity. Mechanical analysis of nanocomposites show the appearance of a longer relaxation time at low frequencies, more pronounced in the aggregated case even without direct connectivity between the aggregates. Correlation between the local structure and the rheological behavior suggests that the macroscopic elastic modulus of the nanocomposite could be described mainly by a short-range contribution, at the scale of the interactions between grafted particles, without significant effect of larger scale organizations.

We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.

We reconsider the initial growth process of the unstable concentration fluctuations when a polymer blend of A and B chains (NA = NB) is rapidly quenched into the unstable region. We correct an earlier result and now predict (i) that the unstable wavelength 2π/? is on the order of R0 (ideal chain radius) for the case of strong segregation, and (ii) the corresponding growth rate is proportional to the melt reptation diffusion constant. These results are consistant with the phase separation kinetics observed in polystyrene–poly(vinyl methyl ether) blends by Nishi, Wang, and Kwei.

Cr2Ge2Te6 is a new layered material belonging to the lamellar ternary M2X2Te6 chalcogenides family (where M is a 3+ oxidation state metal and X2 a silicon or a germanium pair). It was synthesized from a stoichiometric mixture of the elements and heated in a sealed evacuated quartz tube for 20 d at 700 degrees C. The crystal symmetry is rhombohedral, of space group R3, with the following hexagonal cell parameters: a=b=0.68275(4) nm, c=2.05619(9) nm, V=0.830 1(1) nm3 and Z=3. The crystal structure of Cr2Ge2Te6 was solved using both X-ray single-crystal diffraction and neutron powder diffraction data. Growth defects were detected and investigated using high-resolution electron microscopy. The magnetic structure and properties of Cr2Ge2Te6 have been determined by neutron powder diffraction and susceptibility and magnetization measurements. Below 61 K, Cr2Ge2Te6 is ferromagnetic with spins aligned along the c axis of the cell ( mu (Cr3+)=2.80(4)mu B at 5 K). The thermal evolution of the magnetic moments below Tc is given. At room temperature, Cr2Ge2Te6 presents a rho =0.02 Omega cm resistivity. Above Tc, the thermal evolution of the resistivity can be fitted as rho =exp(A/kT), where A=0.2 eV. Band-structure and crystal orbitals of Cr2Ge2Te6 have been calculated using the extended Huckel method and indicate no gap but highly localized Cr 3d states, giving evidence of a hopping mechanism for Cr2Ge2Te6, electrical conductivity.