Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

Abstract

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using 75 As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole-and electron-doped BaFe2As2 families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of Tc and the shape of the superconducting dome in these and other iron-pnictide families.PACS numbers: 74.70. Xa, 75.40.Gb The role of magnetic fluctuations in iron pnictide superconductors (SCs) has been extensively studied since their discovery. As the parent materials have antiferromagnetic (AFM) ground states, attention has been understandably focused on stripe-type AFM fluctuations, which are widely believed to give rise to the Cooper pairing in these systems. In the standard picture, carrier doping or pressure application results in suppression of the AFM order and the emergence of a SC state, with T c ranging from a few K to 56 K [1]. However, as of yet, there is no accepted theory for T c in these materials with which to explain the large variability in maximum T c between different iron arsenide families and the different shapes of the SC dome with electron and hole doping.Recent nuclear magnetic resonance (NMR) measurements on non-SC, paramagnetic (PM) SrCo 2 As 2 , the x = 1 member of the electron-doped Sr(Fe 1−x Co x ) 2 As 2 family, revealed strong ferromagnetic (FM) spin fluctuations in the Co layer coexisting with stripe-type AFM fluctuations [2,3]. Since stripe-type AFM fluctuations are a key ingredient to SC in the iron pnictides, this result suggested that FM fluctuations might compete with the stripe-type AFM fluctuations, suppressing SC in SrCo 2 As 2 . FM correlations were also observed in isostructural BaCo 2 As 2 [2, 4]. Similarly, CaCo 1.86 As 2 has an A-type AFM ground state with inplane FM order [5]. These results also raise the question of whether similar FM correlations exist generally in the SC A(Fe 1−x Co x ) 2 As 2 compounds, not just at the x = 1 edges of their phase diagrams.According to density functional theory calculations [6-10], the generalized static magnetic susceptibility χ(q) is enhanced at both the FM and stripe-type AFM wavevectors in all the iron-based SCs and parent compounds. Experimentally, the uniform χ(q = 0) of the parent compounds is enhanced by a factor of order five over band structure values, which is consistent with FM correlations [1]. Nevertheless, FM fluctuations have not been investigated microscopically, perhaps because low-energy FM fluctuations are difficult to observe via inelastic neutron scattering (INS). The peak in the inelastic structure factor at q = 0 coincides with the elastic Bragg diffractio...

Nontrivial Role of Interlayer Cation States in Iron-Based Superconductors

“…Hence, one can go from As to Se/Te, i.e., right in the periodic table, and find further FeBS, but not to the left towards Ge. In agreement with recent NMR measurements [29], our study highlights the role of presence or absence of ferromagnetic fluctuations in determining the value of T c in FeBS.…”

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“…Nuclear magnetic resonance (NMR) has been pivotal in studies of iron-based superconductors [20][21][22][23][24][25][26][27][28][29] as well as in studies of systems close to the quantum critical point [30][31][32][33][34][35][36][37] [39], taken at T = 300 K, shows a single line with a characteristic powder pattern of an axially symmetric shift anisotropy (Fig. 1b).…”

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“…6(c), we compare the 1/T 1 T data for the x = 0.028 Co substituted CaFe 2 As 2 with those for the x = 0.047 in Ba(Fe 1−x Co x ) 2 As 2 (T N = 50 K, T C = 15 K). 70 Here we chose the 4.7% Co substituted BaFe 2 As 2 sample because T N = 50 K is close to T N = 53 K of the 2.8% Co substituted CaFe 2 As 2 . As seen in the figure, 1/T 1 T for the 4.7% Co substituted BaFe 2 As 2 shows clear divergent behavior around T N = 50 K and a gradual decrease below T N , contrast to the case of the Co substituted CaFe 2 As 2 .…”