Normal state spin dynamics in the iron-pnictide superconductors BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 probed with NMR measurements

TL;DR

This paper uses NMR measurements and SCR spin fluctuation theory to analyze the spin dynamics in iron-pnictide superconductors, revealing the role of AF spin fluctuations and their connection to superconductivity.

Contribution

It demonstrates that a SCR model with 2D AF spin fluctuations accurately describes NMR and resistivity data, highlighting the itinerant AF nature and quantum criticality in these materials.

Findings

Spin fluctuations are well described by SCR theory.

Crossover from non-Fermi to Fermi liquid behavior is linked to AF fluctuations.

Magnetism, lattice instability, and orbital degrees are interconnected.

Abstract

The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are analyzed based on the self-consistent renormalization (SCR) spin fluctuation theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi liquid to Fermi liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom.