Incommensurate Spin Fluctuations in High-Transition Temperature Superconductors

Neutron scattering experiments have revealed a fascinating interplay between the hole doping, the spin fluctuations, and the superconductivity of the cuprate superconductors. Recently, electrochemical techniques have been used to produce large single crystals of La$_2$CuO$_{4+y}$, which has mobile oxygen dopants. Staging behavior of the excess oxygen has been demonstrated, and the low-energy spin fluctuations in stage 6 La$_2$CuO$_{4+y}$ have been measured. The spin fluctuations are incommensurate with the lattice and have spatial, energy, and temperature dependencies very much like those in La$_{2-x}$Sr$_x$CuO$_4$, with similar high transition temperature. This establishes the universality of the incommensurate spin fluctuations among the La$_2$CuO$_4$-based superconductors.