Motivated by predictions of a substantial contribution of the "buckling" vibration of the CuO 2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa 2 Cu 3 O 7 . The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below ∼ 105 K, but no corresponding anomaly at q = (0.3, 0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity.PACS numbers: 74.25. Kc, 74.72.Gh, 63.20.kd, 71.27.+a Research on the mechanism of high-temperature superconductivity in the cuprates has recently made substantial Two experimental strategies have been employed to test these theories and to quantify the strength of the different electron-boson coupling channels. The first approach has identified fingerprints of coupling to bosonic modes in the electronic spectral functions extracted from angle-resolved photoemission (ARPES), optical, and tunneling data, [1, 2] but these data are not specific enough to unequivocally discriminate between contributions from spin excitations and phonons. A second, complementary approach therefore targets corresponding anomalies in the bosonic exci- Much less information is available on the buckling mode, which has been predicted to favor d-wave superconductivity. [5-9] Raman scattering experiments on optimally doped cuprates have shown that the mode exhibits a superconductivity-induced softening of ∼ 1.5 % at wave vector q = 0. [22,23] However, q-dependent data are required for a comprehensive assessment of the contribution of the buckling phonon to the experimentally observed electron self-energy anomalies and to the d-wave pairing state. Such data can be collected by inelastic neutron scattering. Since the lattice dynamics is further complicated by structural disorder, prior neutron scattering work has focused on stoichiometric YBa 2 Cu 3 O 7 . [24,25] The results confirmed a modest softening of the buckling mode upon cooling below the superconducting transition temperature T c , which agrees with Raman scattering data [22,23] at q = 0 and decreases rapidly with increasing q, in accord with model calculations. [26,27] However, a broadening of the neutron profiles with a maximum amplitude at nonzero q [25] was not explained by these calculations. Since these experiments were carried out on twinned specimens, the in-plane anisotropy of these features could not be resolved.arXiv:1109.5511v1 [cond-mat.str-el]

Single-walled carbon nanotubes (SWNTs) were incorporated into precursor-derived ceramics made from a polysilazane. To improve the dispersion of the nanotubes in the liquid precursor and finally in the ceramic matrix, the SWNTs were chemically modified by (iodomethyl)trimethylsilane via a radical reaction. The functionalization degree of the modified SWNTs was determined to be 3 atom %. Microscopic investigation combined with viscosity measurements and thixotropy tests demonstrated that the functionalized SWNTs are more homogeneously dispersed in the liquid SWNT/polymer mixtures and the solid cross-linked precursor, as compared to pristine nanotubes. SWNT/Si-C-N ceramics with nanotube contents of up to 1 wt % were obtained through pyrolysis of cross-linked SWNT/polymer composites at 1000°C. The presence of intact nanotubes in these composites could be verified by scanning transmission electron microscopy. The high viscosity of the SWNT/polysilazane mixtures was identified as an important prerequisite for attaining good nanotube dispersion in the Si-C-N matrix.

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The present contribution reports two different approaches to achieve a good dispersion of single-walled carbon nanotubes (SWCNTs) in a precursor-derived Si-C-N matrix, which represents an important prerequisite for attaining a high-performance material. The first approach involves the use of SWCNTs covalently functionalized by disilazane groups, aiming at enhanced interfacial interaction between the nanotube surface and the matrix. Within the second approach, the effect of an electrical field applied in the cross-linking step during the nanocomposite synthesis was studied toward the task of dispersing and simultaneously aligning the SWCNTs in the Si-C-N matrix.