Theory of the negative isotope effect on copper in underdoped YBCO

An explanation is proposed of the negative isotope effect observed on coppe r in underdoped YBa2Cu3O7-delta. Calculations show that such an effect can be due to scattering of electrons from low-frequency phonons, which are not associated with oscillations of the total ionic charge density. The negati ve isotope effect is specific for superconducting layered curates with a d- wave order parameter and is absent in s-type superconductors of the same ki nd. Although the theory does not reproduce all observed features of the dep endence of the isotope effect on oxygen concentration, it agrees qualitativ ely with the general trend of enhancement with decreasing T-c. A discussion is presented, and several possible experiments are proposed. (C) 2000 Else vier Science B.V. All rights reserved.