1. Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
3. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China

Correspondence to: X. H. Chen¹ Correspondence and requests for materials should be addressed to X.H.C. (Email: chenxh@ustc.edu.cn).

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^{1, 2, 3, 4, 5}. Theoretical calculations indicate that the electron–phonon interaction is not strong enough to give rise to such high transition temperatures⁶, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible^{7, 8, 9}. 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 - xF_x and Ba_1 - xK_xFe₂As₂ systems. The oxygen isotope effect on T_C and T_SDW is very small, while the iron isotope exponent _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.

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.