  The recent discovery of superconductivity in oxypnictides with the critical
temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum
predicted by Bardeen-Cooper-Schrieffer (BCS) 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, while strong ferromagnetic/antiferromagnetic fluctuations have
been proposed to be responsible. However, superconductivity and magnetism in
pnictide superconductors show a strong sensitivity to the lattice, suggesting a
possibility of unconventional electron-phonon coupling. Here we report the
effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW)
transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The
results show that oxygen isotope effect on TC and TSDW is very little, while
the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to
0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows
the same effect on TSDW as TCc These results indicate that electron-phonon
interaction plays some role in the superconducting mechanism, but simple
electron-phonon coupling mechanism seems to be rather unlikely because a strong
magnon-phonon coupling is included. Sorting out the interplay between the
lattice and magnetic degrees of freedom is a key challenge for understanding
the mechanism of high-TC superconductivity.
