Superconductivity and isotope effect in pnictide superconductors

Chen Xianhui

University of Science and Technology of China

In this talk, I shall briefly introduce the development of superconductors including the conventional superconductors and cuprate supercoductors, mainly on pnictide supercodncutors. I mainly talk about the isotope effect in pnictide superconductors. 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 (e-p) coupling. Here we report the effect of isotope on TC and spin-density wave (SDW) TSDW in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems by either oxygen isotope or iron isotope exchange. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent ? C=-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 TC. 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.