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International audience; The lightness of the space and aerospace structures causes their vulnerability to vibrations. The cold temperatures do not allow using polymer materials. Active and semi-active control using piezos embedded on the structure can be used efficiently instead of polymers but they induce energy consumption. Friction damping is less efficient but it does not depend on temperature and it is energetically passive. Unfortunately its efficiency depends on the vibration amplitude as well as on the tightening force. The damping is very low for the lowest amplitudes and the largest tightening loads and increase up to an optimal value. This optimum damping is adjustable thanks to the tightening force. The purpose of our work is to evaluate the efficiency of the control of the tightening force in bolted joints to reduce the vibration of the assembled structure. In the first part, we present a original setup and a very detailed design of experiments that highlights the optimal sets of parameter in order to get a good control of the vibrations according to the frequency and the magnitude of the load. To conclude, we propose to share experimental data with the attendees for further discussions.

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half a flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.

The problem of the nuclear relaxation of nuclei of spin I = 3/2 in the presence of strong quadrupole coupling is treated by means of the fictitious-spin (FS) formalism. Considering the magnetic hyperfine field as the only source of spin - lattice relaxation, we employed the FS method to obtain both the multiexponential decay law of the spin - lattice relaxation and the so-called -contribution to the transverse relaxation . As an example we discuss the determination of the spin - spin relaxation rate of the planar copper in the high-temperature superconductor.

A full account is given of an NMR investigation of planar oxygen in the normal state (T>T c ) of two characteristic superconducting Y-Ba-Cu-O single crystals: YBa 2 Cu 3 O 6.52 , representing the underdoped «60-K phase,» and YBa 1.92 Sr 0.08 Cu 3 O 7 representing the overdoped «90-K phase.» (We argue that Sr impurities in the latter crystal do not significantly change the behavior of the system.) Data are analyzed within the framework of the Mila-Rice spin Hamiltonian and using inelastic-neutron-scattering results, in order to obtain detailed and consistent information on the antiferromagnetic fluctuations (AFF) in this system

17 O and 63 Cu NMR results obtained in three single crystals of YBa 2 Cu 3 O 6+x , YBa 1.92 Sr 0.08 Cu 3 O 7 (T c =89 K) which is overdoped, and YBa 1.93 Sr 0.07 Cu 3 O 6.92 (T c =91 K) and YBa 2 Cu 3 O 6.52 (T c =59 K) which are underdoped, show that the static spin susceptibility and (T 1 T) -1 for Cu(2) and O(2,3) have completely different temperature dependence according to the regime and irrespective of T c .The principal characteristic of the underdoped regime is the opening of a pseudogap in the magnetic excitations well above T c ,supporting the phase diagram based on the spin-charge separation.The contribution of the antiferromagnetic fluctuations at the O(2,3)sites have been observed in YBa 1.92 Sr 0.08 Cu 3 O 7 ,in good agreement with the inelastic-neutron scattering results.

We compare the 63,65Cu NMR data at δ = 0 and 0 < δ ⩽ 0.35 in single crystal samples of YBa2Cu3O7−δ. The lack of δ-dependence in the c component of the magnetic hyperfine shift (MHS) and in the nuclear spin-lattice relaxation rate is associated with the existence of localized Cu (3d) holes. For δ ≠ 0 samples, in addition to the variation of the number of free charge carriers, the system is perturbed by the presence of oxygen vacancies acting as local defects. In the concentration range (0 < δ < 0.1), the 2D character of the oxygen p-holes seems to be reinforced. This may change the coupling between localized Cu (3d) and mobile O (2p) holes, as reflected in the temperature dependence of antiferromagnetic fluctuations of Cu spins. The data on MHS show a strong suppression of its spin component, even for δ = 0.1.

17 O NMR has been performed between 10 and 360 K on an oriented powder of YBa 2 Cu 3 O 6.65 ( T c =60 K). The temperature dependence of the magnetic hyperfine shift (MHS) tensors has been followed for the O(2,3) and O(4) sites. For O(4) nuclei, the MHS stays constant above T c , whereas for the O(2,3) sites it scales linearly with the macroscopic susceptibility χ m . This demonstrates that the origin of the temperature dependence of χ m is to be found within the CuO 2 plane. Two methods are presented to discriminate between the t - J model and two-band models: (a) the comparison of the temperature dependence of the isotropic K iso and the axial part K ax of the O(2,3) MHS, (b) a scaling between the experimentally measured products K 2 iso T 1 T in 17 O and 89 Y. From the second method, our results indicate the existence of an oxygen 2p-band with a density of states per spin direction of 0.11 to 0.19 eV -1 at -1 .

We study the effect of disorder and interactions on a recently proposed magnetic field induced localization mechanism. We show that both partially destroy the extreme confinement of the excitations occurring in the pure case and give rise to unusual behavior. We also point out the role of the edge states that allows for a propagation of the electrons in these systems.

We consider two interacting particles evolving in a one-dimensional periodic structure embedded in a magnetic field. We show that the strong localization induced by the magnetic field for particular values of the flux per unit cell is destroyed as soon as the particles interact. We study the spectral and the dynamical aspects of this transition., 4 pages, 5 EPS figures, minor misprints corrected

We report transport measurements on superconducting wire networks which provide the first experimental evidence of a new localization phenomenon induced by magnetic field on a 2D periodic structure. In the case of a superconducting wave function this phenomenon manifests itself as a depression of the network critical current and of the superconducting transition temperature at a half magnetic flux quantum per tile. In addition, the strong broadening of the resistive transition observed at this field is consistent with enhanced phase fluctuations due to this localization mechanism., Comment: 4 pages, 3 eps figures, final version with minor changes

The Glauber dynamics of various models (REM-like trap models, Brownian motion, BM model, Ising chain and SK model) is analyzed in relation with the existence of ageing. From a finite size Glauber matrix, we calculate a time $\tau_w(N)$ after which the system has relaxed to the equilibrium state. The case of metastability is also discussed. If the only non zero overlaps between pure states are only self-overlaps (REM-like trap models, BM model), the existence or absence of ageing depends only on the behavior of the density of eigenvalues for small eigenvalues. We have carried out a detailed numerical and analytical analysis of the density of eigenvalues of the REM-like trap models. In this case, we show that the behavior of the density of eigenvalues for typical trap realizations is related to the spectral dimension of the equivalent random walk model., Comment: 22 pages, 6 figures, accepted in Jour. Phys I (France)

17O and 63Cu NMR results obtained in three single crystals of YBa2Cu3O6+x, show that the static spin susceptibility and (T1T)−1 for Cu(2) and O(2,3) have completely different temperature dependence according to the regime — overdoped for x > 0.94, underdoped for x < 0.94. In the overdoped regime a Fermi liquid description seems to be correct, while in the underdoped regime a pseudo-gap in the magnetic excitations opens well above Tc, supporting the phase diagram based on the spin-charge separation. We also show that both neutron inelastic scattering results and Cu(2) and 0(2,3) NSLRR results can be explained using the same χ″(q, ω).

Using the adiabatic switching of interactions, we establish a condition for the existence of electronic quasiparticles in a Luttinger liquid. It involves a characteristic interaction strength proportional to the inverse square root of the system length. An investigation of the exact energy level separation probability distribution shows that this interaction scale also corresponds to a cross-over from the non interacting behaviour to a rather typical case for integrable systems, namely an exponential distribution. The level spacing statistics of a spin $1/2$, one branch Luttinger model are also analyzed, as well as the level statistics of a two coupled chain model., Comment: 22 pages, Latex

The transition metal bronzes are ternary compounds of generic formula AxTyOz, where A is usually an alkali metal. The bronzes have been of interest for a number of research fields, ranging from intercalation chemistry, fast ionic transport, to physics of low dimensional conductors. Those of them which exhibit low dimensional electronic properties have been recently reviewed by M. Greenblatt [1]. The existence of charge density wave (CDW) instabilities has been firmly demonstrated for the molybdenum bronzes [2] which include the quasi two dimensional purple bronzes A0.9M06O17 with A = Li, Na, K, Tl and the quasi one dimensional blue bronzes A0.3M0O3 with A = K, Rb, Tl. These latter are maybe the best studied among quasi one dimensional systems in which the charge density waves are depinned by a small electric field [3]. Compared to other systems, large crystals have been grown [4], enabling the determination of the crystalline structure and its deformations below the Peierls transition in detail [5]. The dynamic response to an electric field both at low and high frequencies has been thoroughly investigated [6]. The crystals grow with high purity and the CDW may be depinned at low electric fields so that various properties have been studied in the depinned state also. The microscopic methods, especially nuclear magnetic resonance, contributed considerably to the understanding of both the static and dynamic aspects. As we shall detail in this chapter, it provided an unambiguous test for the incommensurate plane wave nature of the CDW in a broad temperature range [7, 8, 9] and a direct proof for the collective motion [10, 11]. The phase velocity of the depinned wave was measured with high precision [12, 13]. Temporal fluctuations of the CDW phase have also been studied [13] but here a lot remains to be done.

Since the discovery of the high Tc superconducting oxides, the electronic structure of these compounds in the normal state (T > Tc) is still a matter of controversy. It was reported early on by Emery 1 that the holes introduced in these materials by doping were primarly going into the O(2p) orbitals, leaving exactly one d-hole at the Cu site (Cu2+) in the CuO2 plane. Neutron inelastic scattering 2 and La NQR 3 in La2-xSrxCuO4 as well as Cu NMR spectroscopy 4-6 and neutron inelastic scattering7,8 in YBa2Cu3O6+x, support this scheme. In YBa2Cu3O7 at least, however the temperature dependence of the Nuclear Spin Lattice Relaxation Rate (NSLRR)9 and the Magnetic Hyperfine Shift (MHS)10,11 of the Cu nuclei below Tc imply that these quasi- localized d-holes must be involved in the quasi-particles associated with superconductivity. Thus the nature of the coupling between these copper d-spins and the oxygen p-holes remains a central issue for the theoretical understanding of these high Tc materials 12-15.17O NMR in these oxides is one of the most powerful tool to address this problem16,23. In the framework of the so- called t-J model 12,14coupiing is so strong that a knowledge of the Cu(2) spin dynamical susceptibility χCu(q,ω) should allow one to predict all NMR observables at O(2,3) and Y sites in the crystal. In contrast, two spin fluids models imply 15 or strongly suggest 13 the existence of a second spin susceptibility associated with the oxygen p-holes χh(q,ω) which at least above some temperature T* > Tc should behave differently from χCu(q,ω).

Following the observation of local spin fluctuations on the Cu(2) site we discuss the possibility to distinguish an independent magnetic susceptibility on the O(2,3) site

NMR measurements of 63 Cu and 17 O are reported for two single crystals of YBa 2 Cu 3 O 6+ x ( x =0.52, 0.92) and compared with neutron diffraction measurements on the same or equivalent samples. Comparison with the model of Millis, Monien and Pines shows that it is not consistent with the measurements. The contribution from the antiferromagnetic fluctuations is not enough to explain the 63 Cu spin relaxation.

NMR measurements of 63 Cu and 17 O are reported for three single crystals of YBa 2 Cu 3 O 6+ x (x=0.52, 0.92 and 1). The samples are the same as or equivalent to those studied by neutron diffraction. Comparison between neutron and NMR results shows that the model of Millis, Monien and Pines is not consistent with experiments.

The anisotropy of the nuclear spin lattice relaxation rate T −1 1 of planar 17 O is reported for the “90 K” and “60 K” phases of YBCO single crystals. We show that the model of Millis Monien and Pines cannot account for the “60 K” phase data.