Properties of Layered Crystals and High Temperature Superconductors.
Studies of inelastic nuclear scattering events in a layered crystal led in 1995 to the prediction of a new kind of mobile, highly localised, lattice excitation called a quodon. Quodons resemble self-focussing optical-mode breathers and can have energies in the 0.01 to 10eV range. Fourier transform analysis of their internal optical-mode oscillation show that they have vibration frequencies above the highest phonon frequency and so do not couple to phonons. In 1996 we showed that certain atomic chains in typical high Tc superconductors have sufficient axial symmetry to allow the creation of quodons as a result of transient charge fluctuations. In 1997 the basic properties of quodons on quasi-one-dimensional (QOD) chains were explored, including their creation by impulses, their robust propagation along a chain and their inelastic scattering at large discontinuities in a chain. These results allowed a better understanding of the sputtering process. This work was extended in 1998 to two-dimensional arrays of QOD chains, specifically to explore the stability of quodons in 2-D sheets, and showed that they did not spread laterally during propagation. Lately we have shown that specific QOD chains in flat CuO2 sheets in typical YBCO compounds can support robust mobile quodons. Most recently, it has been shown that a pair of electrons can have a lower energy state in the presence of a quodon. This suggests a phenomenological model of HTSC in which pairs of charges are bound by the exchange of virtual quodons to form bosons.