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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.

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 .

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).

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.

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

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.

We report /sup 63,65/Cu NMR spectra and nuclear spin-lattice relaxation data for YBa/sub 2/Cu/sub 3/O/sub 7/(T/sub c/ = 91 K) and YBa/sub 2/Cu/sub 3/O/sub 6.75/(T/sub c/ = 5/sub 7/ K) single crystals between 45 and 300 K. An analysis of the spin-lattice relaxation rates gives evidence for localized electronic spins on the planar Cu(2) sites. In YBa/sub 2/Cu/sub 3/O/sub 6.75/ both fourfold and twofold oxygen-coordinated Cu(1) are observed; the latter experience the same quadrupolar coupling (..nu../sub Q/approx. =31.5 MHz) as the Cu(2) sites but a nearly zero magnetic hyperfine shift and a much slower relaxation rate (d/sup 10/ configuration).

We have performed 17O NMR in YBa2Cu3O7 oriented powder samples in the temperature range 10–360 K for the orientation H0∥c and H0⊥c. The Electric Field Gradient (EFG) and Magnetic Hyperfine Shift (MHS) tensors have been determined for the four oxygen sites in the structure. The isotropic (axial) part of the MHS tensor were found to be equal to 0.21 (0.08)% for the O(2,3) site in the CuO2 plane and to 0.04(0.05)% for the bridging oxygen O(4). Both were found temperature independent in the normal state. Below Tc, they vanish to zero and the data for the MHS Kcc[O(2,3)] show that this decrease is faster than expected from BCS theory. The coupling between the localized spins at the Cu (2) sites and the mobile oxygen p-holes is discussed in the light of these results.

We have studied the 93Nb NMR in single crystals of (NbSe4)10/3I (1.8 mg) and (NbSe4)3I (20 mg). In the first compound, the onset of the CDW has been followed through the temperature dependence of the (1/2,−1/2) transition. Using computer simulation of the lineshapes, we have determined the temperature dependence of the order parameter. Comparison of the spin-lattice relaxation rate in (NbSe4)3I and (NbSe4)10/3I, indicate the fluctuations of the the pinned CDW are the dominant mechanism.

We report 63 Cu NMR nuclear spin-lattice relaxation rates (NSLRR) and magnetic hyperfine shift (K cc ) of copper Cu(2) in the CuO 2 planes of YBa 2 Cu 3 O 7−δ as a function of temperature and δ. The independence of K cc and NSLRR (above 120 K) on δ associated with the existence of localized Cu(3d) holes. We comment on the difference in the NSLRR between the δ = 0 and δ ≠ 0 samples which appear below 120 K. It is interpreted as an indirect consequence of the changes in the oxygen 2p electronic band caused by the introduction of oxygen vacancies in the system.

We present the temperature dependence of the Magnetic Hyperfine Shift tensors of 17 O nuclei for the sites within the CuO 2 plane (site O(I)) and in the BaO, LaO, SrO plane (O(II) site) in the compounds YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.65 , La 1.85 Sr 0.15 CuO 4 and Bi 2 Sr 2 CaCu 2 O 8 . For O(I) sites, the isotropic part of the tensor reflects the polarisation of the Oxygen 2s orbitals by the neigboring Cu d-holes and the temperature dependence of the susceptibility. We show that in YBa 2 Cu 3 O 6.65 , the spins of the oxygen p-holes have their own degree of freedom above 120 K.