Liu R H, Wu T, Wu G, Chen H, Wang X F, Xie Y L, Ying J J, Yan Y J, Li Q J, Shi B C, Chu W S, Wu Z Y, Chen X H
Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
Nature. 2009 May 7;459(7243):64-7. doi: 10.1038/nature07981.
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 alpha(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.
近期在氧族铁基化合物中发现了超导现象，其临界转变温度（T(C)）高于麦克米兰极限温度39 K（巴丁-库珀-施里弗理论预测的理论最大值），这引起了极大的轰动。理论计算表明，电子-声子相互作用不够强，不足以产生如此高的转变温度，但有人提出强铁磁/反铁磁涨落是其原因。然而，铁基超导材料中的超导性和磁性对晶格表现出很强的敏感性，这表明存在非常规电子-声子耦合的可能性。在此，我们报告了氧和铁同位素取代对SmFeAsO(1 - x)F(x)和Ba(1 - x)K(x)Fe(2)As(2)体系中T(C)和自旋密度波（SDW）转变温度（T(SDW)）的影响。氧同位素对T(C)和T(SDW)的影响非常小，而铁同位素指数alpha(C) = -dlnT(C)/dlnM约为0.35（0.5对应完全同位素效应）。令人惊讶的是，铁同位素交换对T(SDW)的影响与对T(C)的影响相同。这表明电子-声子相互作用在超导机制中起到了一定作用，但简单的电子-声子耦合机制似乎不太可能，因为其中包含了强磁振子-声子耦合。
