A large iron isotope effect in SmFeAsO1 x Fx and Ba1 x Kx Fe 2 As2

Abstract

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T C) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T C and the spin-density wave (SDW) transition temperature (T SDW) in the SmFeAsO 1 - x F x and Ba 1 - x K x Fe 2 As 2 systems. The oxygen isotope effect on T C and T SDW is very small, while the iron isotope exponent α C = -dlnT C /dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T SDW as T C. This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included. © 2009 Macmillan Publishers Limited. All rights reserved.