Enhancement of superconducting transition temperature due to antiferromagnetic spin fluctuations in iron pnictides LaFe(As1−xPx)(

Abstract

Systematic31 P-NMR studies on LaFe(As1−xPx)(O1−yFy) with y=0.05 and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a result, Tc exhibits respective peaks at 24 and 27 K against the P substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in Tc for LaFe(As1−xPx)(O1−yFy) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (La1−zYz)FeAsO δ compounds in which Tc reaches 50 K for z=0.95, we remark that a moderate development of AFMSFs causes Tc to increase up to 50 K under the condition that the local lattice parameters of the FeAs tetrahedron approach those of the regular tetrahedron. We propose that Tc of Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the AFMSFs and other factors originating from the optimization of the local structure.

Theoretical study of correlation between spin fluctuations andTcin isovalent-doped 1111 iron-based superconductors

“…In ref. [12], some of the present authors have shown that the variation of λ against the bond angle α exhibits a double dome feature as in Fig.2, which is consistent with the experimental observations 7,8,10 . There, the origin of the double dome structure has been explained as follows: superconductivity is suppressed when the bond angle is increased from α ∼ 110 because the hole Fermi surface around (π, π) originating from the d xy orbital vanishes.…”

“…As described in the Introduction, it was found in the NMR experiments 8,16 that the correlation between T c and the development of 1/T 1 T at low temperatures is material dependent. In ref.…”

“…[8] as shown in Fig.1. It was found that T c at finite phosphorous content is correlated with the development of 1/T 1 T upon lowering the temperature, indicating that the low energy spin fluctuation is responsible for the non- monotonic T c dependence with phosphorous doping.…”

“…Therefore, it was expected, from the empirical T c trend found by Lee et al 4 , that T c monotonically decreases by increasing the phosphorous content [4][5][6] . However, it has recently been revealed that another local maximum of T c (we will call this the second T c dome) exists in the high phosphorous content regime 7,8,10 . Moreover, for the non-fluorine-doped system, the antiferromagnetic phase sandwiched by the two superconducting phases were found in the finite P doping region [9][10][11] .…”

Titanium Pnictide Oxide Superconductors

“…However, the disappearance of superconductivity and a steep rise of Tc in the vicinity of the second superconducting phase implies the second superconducting phase may have a different mechanism behind superconductivity from the first superconducting phase. A two-dome structure in Tc has also seen in the iron pnictide superconductors such as LaFeAs(O1-xHx) [49][50][51]. In the phase diagram of LaFeAs(O1-xHx) (0 ≤ x ≤ 0.6), the first superconducting dome (0.05 ≤ x ≤ 0.20) with a maximum Tc of 29 K and the second dome (0.20 ≤ x ≤ 0.42) with a maximum Tc of 36 K appear with two antiferromagnetic phases (x < 0.05 and 0.4 < x) [49].…”