Normal and superconducting properties of LiFeAs explained in the framework of four-band Eliashberg theory

Abstract

In this paper we propose a model to reproduce superconductive and normal properties of the iron pnictide LiFeAs in the framework of the four-band s± wave Eliashberg theory. A confirmation of the multiband nature of the system rises from the experimental measurements of the superconductive gaps and resistivity as function of temperature. We found that the most plausible mechanism is the antiferromagnetic spin fluctuation and the estimated values of the total antiferromagnetic spin fluctuation coupling constant in the superconductive and normal state are λtot = 2.00 and λtot,tr = 0.77. PACS numbers: 74.70.Xa, 74.25.F, 74.20.Mn, Recent ARPES measurements of iron superconductor LiFeAs report four slightly anisotropic gaps [1]. Their isotropic values at 8 K are given by ∆ 1 = 5.0 meV, ∆ 2 = 2.6 meV, ∆ 3 = 3.6 meV, ∆ 4 = 2.9 meV and the critical temperature for this compound isIn an other work [3] we disregarded the anisotropic part of the gap values and we tried to reproduce the experimental data in the framework of s± wave multiband Eliashberg theory. At first, we calculated Z i (iω n ) have to be solved. If i is the band index (that ranges between 1 and 4) and ω n are the Matsubara frequencies, the imaginary-axis equations are:where Γ ij and Γ

Superconductivity from repulsion in LiFeAs: Novels-wave symmetry and potential time-reversal symmetry breaking

“…For small J/U the α − β interaction is repulsive, and its enhancement is expected if antiferromagnetic fluctuations are strong. Not surprisingly, our result for stronger α − β interaction coincides with spin-fluctuation studies of Wang et al 36 and Ummarino et al 39 .…”

“…However, the gap on the α pocket turns out to be smaller than the other gaps, in disagreement with the ARPES results, which show that the α gap is the largest. Ummarino et al argued 39 that one can match the ratios of all gaps in an s +− state if one combines magnetic fluctuations at large q ≈ Q and small q fluctuations, for which, they argued, the best candidate is electron-phonon interaction. Another group also found 37 that interactions at small momentum transfer are strong but attributed it to strong small q magnetic fluctuations.…”

Resistivity in Ba(FeCo)As: Comparison of thin films and single crystals

“…Incidentally, it is interesting to note, as shown in Table , that the representative energy of the electron–boson spectral function in the normal state decreases with increasing the cobalt content in the Ba(FeCo)As thin films. The same table also highlights that in this compound (no matter if in the form of crystal of thin film) as well as in other iron‐based superconductors the normal and superconducting states are characterized by very different values of the typical energy of the electron–boson spectral function and of the total electron–boson coupling constant. In particular, the typical energy of the electron–boson spectral function systematically increases going from the superconducting to the normal state (in agreement with inelastic neutron scattering experimental data ()) while the total electron–boson coupling constant significantly decreases.…”

“…Three‐ and four‐band Eliashberg models have been proposed to describe the superconducting phenomenology. However, a four‐band model contains a huge number of free parameters, and in the present case (in contrast with the case of LiFeAs ()) it is too hard to fix them in a unique way even if the four‐band model is reduced to a simpler effective two‐band one ().…”

“…Considering the fact that the electron–phonon coupling in all the compounds belonging to the class of iron‐based superconductors is weak (), it is logical to consider (as done for LiFeAs ()) that another mechanism contributes to the transport properties; taking into account the superconducting properties of the iron pnictides, the antiferromagnetic (AFM) SF are the best candidate to play the role of the principal actor also in the normal state. The electron‐SF transport spectral functions are similar to the standard Eliashberg (), but for they behave like and not like .…”