The

The discovery of Fe-based pnictide superconductors [

At present it is not completely clear what is the microscopic pairing mechanism responsible for superconductivity. The conventional phonon-mediated coupling mechanism is too week and cannot explain the observed high

By using this strong-coupling approach, I show here that the superconducting iron pnictides represent a case of dominant negative interband-channel superconductivity (

The electronic structure of pnictides can be approximately reproduced by using a three-band model (Figure

Schematic drawing of the multiband model used in this work. The two hole bands (1 and 2) are centered around the

The generalization of the Eliashberg theory [

In the real axis formulation the multiband

In principle the solution of the three-band Eliashberg equations shown in (

It is obvious that a practical solution of these equations requires a drastic reduction in the number of free parameters of the model. According to the work of Mazin et al. [

Within these approximations, the electron-boson coupling-constant matrix

I initially solved the Eliashberg equations on the imaginary axis to calculate the critical temperature and, by means of the technique of the Padè approximants [

I reproduced the critical temperature and the gap values in three representative cases: (i) the compound LaFeAsO

Inelastic neutron-scattering experiments suggest that the typical boson energy possibly responsible for superconductivity ranges roughly between 10 and 30 meV [

In the

By properly selecting the values of these parameters it is relatively easy to obtain the experimental values of the critical temperature and of the small gap, which is well known. It is more difficult to reproduce the values of the large gaps of bands 1 and 3 since, due to the high

Full symbols, left axis: calculated gap values at

Full symbols, left axis: calculated gap values at

Full symbols, left axis: calculated gap values at

I also tested the effect into the model of a small intraband coupling (possibly of phonon origin). In the case of Ba

The effect of the intraband terms and Coulomb interaction on the gap values.

Pure interband | |||

Free parameters | — | ||

Results | |||

Intraband | |||

Free parameters | — | ||

Results | |||

Intraband and Coulomb | |||

Free parameters | — | ||

Results |

The effect of Coulomb interaction was also investigated for the case shown in Table

I also calculated the superconductive density of states in all three cases. The parameters used for the

The value of coupling constant

Calculated energy dependence of the superconductive normalized tunneling conductance at

The penetration depth as a function of temperature has been calculated in the three cases and is reported in Figure

Calculated temperature dependence of the penetration depth (

In conclusion, I have shown that the newly discovered iron pnictides very likely represent a case of dominant negative interband-channel pairing superconductivity where an electron-boson coupling, such as the electron-spin fluctuactions one, can become a fundamental ingredient to increase

The author thanks I. I. Mazin for useful discussions, Mauro Tortello and Dario Daghero for manuscript revision.