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 - xFx and Ba1 - xKxFe2As2 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.
Recent inelastic neutron scattering measurements on Ba1 - xKxFe2As2 (x = 0 and 0.4) provide evidence for the presence of magnetic excitations10,11, suggesting that spin fluctuation may be important in the mechanism of superconductivity. However, phonons couple selectively to the spin system12. The structural transition from tetragonal to orthorhombic is driven by the antiferromagnetic SDW order13, and the antiferromagnetic SDW exists only in the orthorhombic structure14,15. The pressure coefficient of TC, dTC/dP, changes from positive to negative with a crossover from orthorhombic to tetragonal symmetry for the superconducting phase14. The superconductivity and spin-density wave coexist in the orthorhombic structure14,16,17. These results indicate remarkable sensitivity of superconductivity and magnetism to the lattice.
We synthesized isotopically substituted polycrystalline samples with nominal compositions SmFeAsO1 - xFx (x = 0, 0.15) and Ba1 - xKxFe2As2 (x = 0, 0.4) by conventional solid-state reaction as described in refs 5 and 17, respectively. Figure 1 shows the Raman spectra for the samples SmFeAsO1 - xFx by replacing 16O with 18O, and for the samples SmFeAsO1 - xFx and Ba1 - xKxFe2As2 by replacing 56Fe with the isotope 54Fe. The frequency shift of 4.2% and 4.5% for the Eg mode of oxygen18 around 420 cm-1 suggests about 71% substitution of 18O for 16O in the x = 0 sample, and 77% for the x = 0.15 sample. The Raman shift of about 1.7% we observed in the four samples for the B1g mode of iron18 indicates almost 100% 54Fe substitution for 56Fe for the two systems. These data are listed in Table 1.
The temperature dependence of resistivity ρ and its derivative dρ/dT for typical samples SmFeAsO1 - xFx on replacing 16O with the isotope 18O are shown in Fig. 2. TC and TSDW are listed in Table 2 for all samples from different batches. We calculated the isotope exponent αC for the superconducting transition using αC = -dlnTC/dlnM to be -0.06(1). To compare quantitatively the isotope effect on TSDW with that on TC, we also define an isotope exponent αSDW = -dlnTSDW/dlnM for the SDW transition although no theory is yet established for the isotope effect on the magnetic phase transition. We obtained αSDW = -0.05(1). These results indicate that the oxygen isotope effect on TC and TSDW is very small.
The temperature dependence of resistivity and its derivative for typical samples SmFeAsO1 - xFx and Ba1 - xKxFe2As2 when 54Fe is substituted for 56Fe is shown in Fig. 3. An increase in TC is observed in the resistivity measurements, and dρ/dT shows an increase of the SDW transition when 54Fe is substituted for 56Fe. The average results for several different samples are listed in Table 2. The average αSDW values for several samples of SmFeAsO and BaFe2As2 are 0.39(2) and 0.36(2), and the average αC values for several samples of SmFeAsO0.85F0.15 and Ba0.6K0.4Fe2As2 are 0.34(3) and 0.37(3). These values are comparable to 0.5 for the full isotope effect in the framework of BCS theory and indicate a strong iron isotope effect on TC and TSDW. This implies that the electron–phonon interaction should play an important role for the superconducting mechanism. We note that αC and αSDW for TC and TSDW are almost the same for the two systems, and much larger than the oxygen isotope exponents.
Isotope effect studies require well-characterized samples with reproducible crystal chemistry properties. We found that TSDW is insensitive to the sample processing for the parent compounds. However, because the F content is not easy to control, TC for the SmFeAsO0.85F0.15 sample is sensitive to the sample processing. A detailed description of the synthesis procedure we used to ensure the same F content is given in the Supplementary Information. No difference in the lattice constants (see Supplementary Fig. S1) provides strong evidence for the same F content for isotope exchange. To confirm that the observed results are intrinsic instead of an impurity effect, we checked whether TC and TSDW differ for the Ba1 - xKxFe2As2 samples using natural abundance iron (nFe) with 99.9% purity and 56Fe with 99.78% purity. TSDW and TC are nearly the same for the samples with nFe and 56Fe (see Supplementary Fig. S3). We synthesized the Ba0.6K0.4Fe2As2 samples using nFe with 98% and 99.9% purity to determine the effect of impurity on TC. The difference of TC for the two samples is 0.07 K (see Supplementary Fig. S4), indicating that the effect of impurity on TC is very small, and does not affect the intrinsic isotope effect observed in Table 2.
We emphasize that iron isotope exchange has a strong effect on the SDW state. Substitution of 54Fe for 56Fe leads to a remarkable decrease in resistivity below the SDW ordering temperatures with a large αSDW for the two systems, suggesting a strong magnon–phonon coupling. A giant oxygen isotope effect has been observed in magnetoresistive La1 - xCaxMnO3 + y, and the isotope exponent αFM for ferromagnetic transition is as high as 0.85 (ref. 19). This large isotope shift is believed to arise from coupling of the charge carriers to Jahn–Teller lattice distortions19. In pnictide superconductors, the strong sensitivity of superconductivity and magnetism to the lattice may be responsible for the large isotope effect. These results indicate that the electron–phonon interaction is important in the superconducting mechanism, but the strong magnon–phonon coupling must also be considered.
The iron isotope effect on TSDW and TC is much larger than the oxygen isotope effect in pnictide superconductors. The reason could be that the iron–arsenide plane is the conducting layer and thus responsible for the superconductivity, and the SDW ordering originates from the Fe moment. For the MgB2 superconductor, no magnetic correlation is included and the superconductivity can be understood within BCS theory with αC = 0.32 (ref. 20). In the copper oxides, the isotope effect on TC is sensitive to doping level. The effect is vanishing at optimum doping, but increases systematically with decreasing doping level to a maximum at the border to the antiferromagnetic state21,22. It thus seems that the isotope effect is related to magnetic fluctuation. Such unconventional isotope effects demonstrate that the electron–phonon interaction is also important in the physics of copper oxides. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for our understanding the mechanism of high-TC superconductivity.
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.
This fie contains Supplementary Data and Supplementary Figures S1-S4 with Legends.
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Brazilian Journal of Physics (2015)