Isotope effects on the pseudogap in high-temperature superconductors

Inelastic neutron scattering studies of the relaxation rate of crystal-field excitations have provided evidence for large oxygen and copper isotope effects on the pseudogap in high-temperature superconductors. Existing experiments in the slightly underdoped HoBa2Cu4O8 compound will be reviewed, as well as possible implications of such findings on the microscopic mechanism of high-temperature superconductivity. New relevant experiments performed in the La1-xSrxCuO4 family will also be presented.