Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba_0.7K_0.3Fe_2As_2: NMR in a single crystal

Abstract

We report the 75 As-NMR study on a single crystal of the hole-doped iron-pnictide superconductor Ba0.72K0.28Fe2As2 (Tc = 31.5 K). We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T1 decreases below Tc with no coherence peak and shows a step wise variation at low temperatures, which is indicative of multiple superconducting gaps, as in the electron-doped Pr(La)FeAsO1−xFx. Furthermore, no evidence was obtained for a microscopic coexistence of a long-range magnetic order and superconductivity.

Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co- and K-doped iron pnictides

“…Indeed, a sharp drop of 1/T 1 is observed below T c (Fig. 3d,e), in agreement with other results for Ba 1−x K x Fe 2 As 2 where 1/T 1 was fit with one [14] or two [17,25] gap functions. We also observed a sharp drop of the magnetic hyperfine shift 75 K below T c , indicating spin-singlet pairing (not shown).…”

“…3d) reveals an important difference (besides the plateau of 1/T 1 between 60 and 40 K): There is a 30% increase of 1/T 1 on cooling below ∼110 K, much larger than for the Co doped sample. Interestingly, for a slightly smaller K doping value of x = 0.3, Matano et al report a hump in (T 1 T ) −1 vs. T around 90 K, which they attribute to a structural (tetragonal to orthorhombic) transition because a small change of the quadrupole coupling is observed at the same temperature [17]. For x = 0.4, on the other hand, Yashima et al do not report any hump in 1/T 1 [25] and Takeshita and Kadono report the absence of frozen magnetism [31].…”

Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As Nuclear Quadrupole Resonance and Specific Heat

“…This is quite different from the 1/T 1 below T c in Ba 1−x K x Fe 2 As 2 with T c ≃ 38 K in which 1/T 1 follows T 3 -T 5 without coherence peak. 3,20,30) The 1/T 1 of KFe 2 As 2 more steeply decreases below about 0.6 K. Such kind of gradual T dependence of 1/T 1 in the SC state was observed in La 0.87 Ca 0.13 FePO. 29) In order to understand the nature of this anomalous T dependence, we analyzed the data by assuming simple two-independent SC-gap model with gap with line (T c ≃ 38 K), [18][19][20]30) and are consider to have main contribution to total density of states (DOS) at the Fermi level in KFe 2 As 2 .…”

“…Furthermore, we assumed two SC gap symmetries because these symmetries are experimentally and theoretically favorable for iron-based superconductors. [13][14][15][16][17][18][19][20][21]30) The 1/T 1 in the SC state is proportional to,…”

Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba0.6K0.4Fe2As2Probed by57Fe-NMR

“…Furthermore, ARPES results for Ba 0.6 K 0.4 Fe 2 As 2 show that there are two SC gaps with different values: a large gap on the two, small, hole-like and electron-like FS sheets, and a small gap on the large, hole-like FS [10]. This two-SC-gap phenomenon was suggested by NMR studies as well [20,21]. In this letter, we report the results of microscopic 57 Fe-NMR measurements on hole-doped Ba 0.6 K 0.4 Fe 2 As 2 with T c = 38 K enriched with the isotope 57 Fe to address its normal-state and SC characteristics.…”