Isotope Effect in Iron-Based Superconductors
 
       
 
The iron-based high-temperature superconductors have attracted extensive researches since the discovery of the oxypnictide LaFeAsO1-xFx. The isotope effect gives a key issue in understanding the symmetry and mechanism of superconductivity. Recently, the inverse isotope effect

α ∼ -0.18

has been found for (Ba,K)Fe2As2 superconductor with a transition temperature Tc ∼ 38K [1], where the isotope exponent &alpha is defined as

Tc ∼ M-α

for the isotropic mass M. This surprising observation of the inverse isotope effect provides us important information for the pairing mechanism. The negative &alpha suggests that there is a competition between the electron-phonon interaction and the other interaction.

We have investigated the isotope effect by using the multi-band model of Fe pnictides. We have shown that the origin of superconductivity mainly lies in the interband attractive interaction on the basis of the multi-band model [2]. Let us assume that we have two channels for the Cooper-pair interaction, denoted as V1(k,k') and V2(k,k'), and at least one channel is attractive. In iron-based superconductors, two interactions act as the origin of electron pairings. As a result, the negative &alpha occurs, from the competition between two interactions.

We have investigated the isotope effect by using the multi-band model of Fe pnictides. We have shown that the origin of superconductivity mainly lies in the interband attractive interaction on the basis of the multi-band model [2]. Let us assume that we have two channels for the Cooper-pair interaction, denoted as V1(k,k') and V2(k,k'), and at least one channel is attractive. In iron-based superconductors, two interactions act as the origin of electron pairings. As a result, the negative &alpha occurs, from the competition between two interactions. &alpha is given as
,

where λAF* is the antiferromagnetic coupling constant that includes the retardation effect:
.



References

[1] P.M. Shirage et al.: Phys. Rev. Lett. 103 (2009) 257003.
[2] T. Yanagisawa et al.: J. Phys. Soc. Jpn. 78 (2009) 094718.
[3] H. Y. Choi et al.: Phys. Rev. B80 (2009) 052505.
 
 
 
  Condensed Matter Physic: Electronics Research Institute