Anelastic spectroscopy studies of high-Tc superconductors

Two families of cuprates exhibiting high-Tc superconductivity, YBa2Cu3O6+x (YBCO) and La2xSrxCuO4+δ (LSCO), have been extensively studied by anelastic spectroscopy, by measuring the complex dynamic Young’s modulus E (ω, T ) = E′ + iE′′ of ceramic samples at frequencies of 0.5-20 kHz between 1 and 900 K. Results are also presented on oxygen vacancies in the ruthenocuprate RuSr2GdCu2O8−δ. The elastic energy loss curves as a function of temperature, Q−1 (ω, T ) = E′′/E′, contain peaks at the temperatures Tm such that ωτ(Tm) ≃ 1, where ω/2π is the measuring frequency and τ is the relaxation time of any microscopic process coupled to strain, like hopping of O atoms, tilting of O octahedra or fluctuations of the hole stripes. By measuring such anelastic spectra at different frequencies, it is therefore possible to selectively probe the dynamics of the various relaxation processes, precisely determining their characteristic times τ (T ). The reliability of the anelastic experiments and of the assignments to various microscopic mechanisms is also discussed. The richest anelastic spectra are those of LSCO, and have been studied in the whole range of Sr doping (0 < x < 0.2) and at few Ba doping levels. As in the parent perovskite compounds, LSCO is made of O octahedra unstable against tilting, which give rise to low symmetry phases. The layered coordination of such octahedra and the possibility of achieving a low density of pinning defects (interstitial O atoms), makes it possible to observe solitonic tilt waves and fast local motion of the octahedra among the several minima of the local tilt potential far below the structural transition temperature. The fast local motion is driven by tunneling of the O atoms and is enormously enhanced and accelerated by even small doping, demonstrating direct coupling between the tilt modes of the octahedra and the hole excitations. In addition, it has been possible to probe the dynamics of the stripes into which the holes segregate, at liquid He temperature, when they act as walls between domains of antiferromagnetically correlated spins (cluster spin glass), and also at higher temperature, when they can overcome by thermal activation the pinning barriers provided by Sr2+ dopants. A picture is proposed in which the high temperature motion of the stripes involves the formation of kink pairs, while at lower temperature only the motion of the existing kinks can occur. In YBCO the anelastic spectra are dominated by the motion of the O atoms in the CuOx planes, which are responsible for the doping of the charge carriers (holes) into the supercon-