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Register a new userComment on Isotope effect in multi-band and multi-channel attractive systems and inverse isotope effect in iron-based superconductors by T. Yanagisawa, et al
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Annette Bussmann-Holder
Publication date
2010
fields
Physics
and research's language is
English
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In a recent paper Yanagisawa et al. [1] claim from a theoretical analysis of a multi-channel multi-band superconductor model that an inverse isotope exponent on the superconducting transition temperature Tc can be realized in iron-based superconductors. Simultaneously, a subgroup of the authors of Ref. 1 performed the corresponding isotope effect experiment on (Ba, K)Fe2As2 by investigating the iron isotope exchange effect on Tc [2]. In accordance with their theoretical analysis they indeed report an unusually large sign reversed isotope exponent of {alpha} simeq -0.18(3) which is in strong contrast to previous experiments on the nominally same system with the same composition in Ba, K content, namely Ba0.6K0.4Fe2As2 [3], where the exponent was determined to be {alpha} simeq 0.37(3). This conflict remains unsolved until now with the exception of Ref. 4 where the iron isotope exponent has been determined for FeSe. In accordance with the results of Ref. 3 a large positive isotope exponent has been seen thus questioning the outcome of Ref. 1 and implicitly the findings of Ref. 2. Here, we do not comment on the controversial experimental situation but address the theoretical analysis of Ref. 1, where a variety of misleading assumptions have led to the conclusion that a sign reversed isotope exponent can be realized in a multi-band and multi-channel attractive model for iron based superconductors.
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The role of electron-phonon interactions in iron-based superconductor is currently under debate with conflicting experimental reports on the isotope effect. To address this important issue, we employ the renormalization-group method to investigate the competition between electron-electron and electron-phonon interactions in these materials. The renormalization-group analysis shows that the ground state is a phonon-dressed unconventional superconductor: the dominant electronic interactions account for pairing mechanism while electron-phonon interactions are subdominant. Because of the phonon dressing, the isotope effect of the critical temperature can be normal or reversed, depending on whether the retarded intra- or inter-band interactions are altered upon isotope substitutions. The connection between the anomalous isotope effect and the unconventional pairing symmetry is discussed at the end.
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 coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-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 TCc 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. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity.
We report the inelastic light scattering studies on SmFeAsO0.65 and SmFeAsO0.77H0.12 with iron isotopes namely 54Fe and 57Fe. In both of these systems under investigation we observed a significant shift in the frequency of the phonon modes associated with the displacement of Fe atoms around ~ 200 cm-1. The observed shift in the Fe mode (B1g) for SmFeAsO0.65 is ~ 1.4 % and lower in case of SmFeAsO0.77H0.12, which is ~ 0.65 %, attributed to the lower percentage of isotopic substitution in case of SmFeAsO0.77H0.12. Our study reveals the significant iron isotope effect in these systems hinting towards the crucial role of electron-phonon coupling in the pairing mechanism of iron based superconductors.
Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE on the transition temperature T_c in various HTS is briefly discussed. It is observed that different cuprate families show a similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR, and magnetization studies of the total and site-selective OIE on lambda_{ab} are described in some detail. A substantial OIE on lambda_{ab} was observed in various cuprate families at all doping levels, suggesting that cuprate HTS are non-adiabatic superconductors. The experiments clearly demonstrate that the total OIE on T_c and lambda_{ab} arise from the oxygen sites within the superconducting CuO_2 planes, demonstrating that the phonon modes involving the movement of planar oxygen are dominantly coupled to the supercarriers. Finally, it is shown that the OIE on T_c and lambda_{ab} exhibit a relation that appears to be generic for different families of cuprate HTS. The observation of these unusual isotope effects implies that lattice effects play an essential role in cuprate HTS and have to be considered in any realistic model of high-temperature superconductivity.
The isotope effect in the superconducting transition temperature is anomalous if the isotope coefficient $alpha<0$ or $alpha>1/2$. In this work, we show that such anomalous behaviors can naturally arise within the Bardeen-Cooper-Schrieffer framework if both phonon and non-phonon modes coexist. Different from the case of the standard Eliashberg theory (with only phonon) in which $alphale1/2$, the isotope coefficient can now take arbitrary values in the simultaneous presence of phonon and the other non-phonon mode. In particular, most strikingly, a pair-breaking phonon can give rise to large isotope coefficient $alpha>1/2$ if the unconventional superconductivity is mediated by the lower frequency non-phonon boson mode. Based on our studies, implications on several families of superconductors are discussed.
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