Anelastic spectroscopy studies of high-Tc superconductors

Abstract

YBCO, LSCO and Ru-1212 have been extensively studied by measuring the complex dynamic Young's modulus at 0.5-20 kHz between 1 and 900 K. The elastic energy loss is peaked at the temperatures where the relaxation rate of any microscopic process coupled to strain equals the measuring frequency. The reliability of the anelastic experiments and of the assignments to various microscopic mechanisms is also discussed. In LSCO, the layers of octahedra present solitonic tilt waves and fast local motion. The latter is driven by atomic tunneling and is enormously enhanced and accelerated by doping, demonstrating direct coupling between tilt modes and hole excitations. The dynamics of the hole stripes has been probed when they act as walls between domains of antiferromagnetically correlated spins (cluster spin glass) at LHe temperature, and at higher temperature, when they overcome by thermal activation the pinning barriers provided by Sr dopants. In YBCO various diffusion and ordering processes of the O atoms in the CuOx planes are identified, involving: i) Cu-O chains in the orthorhombic O-I phase (x~1); ii) sparser chains fragments at lower O content; iii) isolated O atoms. The hopping barriers are ~1 eV for the first two types of jumps and 0.11 eV for the latter. It is discussed how such widely different barriers are possible and how the extraordinarily high mobility of isolated O atoms is compatible with slow O ordering. In addition, relaxation processes have been attributed to hopping of O between off-center positions within the Cu-O chains and to bipolarons on orbitals that do not contribute to superconductivity. The O mobility in the RuO2-d planes of Ru-1212 is explained in terms of hopping of O vacancies whose elastic quadrupoles are weakly interacting and start becoming parallel to each other below TC ~ 470 K.