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A large iron isotope effect in SmFeAsO1 - xF x and Ba1 - xK x Fe2As2


The recent discovery of superconductivity in oxypnictides with a critical transition temperature (TC) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen–Cooper–Schrieffer theory) has generated great excitement1,2,3,4,5. 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 responsible7,8,9. 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 TC and the spin-density wave (SDW) transition temperature (TSDW) in the SmFeAsO1 - xF x and Ba1 - xK x Fe2As2 systems. The oxygen isotope effect on TC and TSDW is very small, while the iron isotope exponent αC = -dlnTC/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on TSDW as TC. 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.

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Figure 1: Raman spectra at room temperature for the SmFeAsO1 -  xFx (x = 0 and 0.15) and Ba1 -  xKxFe2As2 (x = 0 and 0.4) samples.
Figure 2: Temperature dependence of resistivity ρ and its derivative d ρ /d T for the SmFeAsO 1 -  x F x samples with n O and 18 O.
Figure 3: Temperature dependence of ρ and d ρ /d T for the SmFeAsO 1 -  x F x and Ba 1 -  x K x Fe 2 As 2 samples isotopically substituted with 56 Fe and 54 Fe.


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This work is supported by the Nature Science Foundation of China, and by the Ministry of Science and Technology of China and Chinese Academy of Sciences. We acknowledge Z. X. Shen for discussion and encouragement, and D. L. Feng and S. Y. Li for discussions.

Author Contributions X.H.C. designed and coordinated the whole experiment, and analysed the data and wrote the paper. R.H.L. and T.W. performed the main experiments, including sample preparation and analysed the data. G.W., X.F.W. and B.C.S. synthesized the samples. H.C. and Y.L.X. partially measured the resistivity. J.J.Y. measured the susceptibility. Y.J.Y. and Q.J.L. did X-ray powder diffraction measurements. W.S.C. and Z.Y.W. provided the iron isotope 54Fe.

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Liu, R., Wu, T., Wu, G. et al. A large iron isotope effect in SmFeAsO1 - xF x and Ba1 - xK x Fe2As2. Nature 459, 64–67 (2009).

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