Nodal versus Nodeless Behaviors of the Order Parameters of LiFeP and LiFeAs Superconductors from Magnetic Penetration-Depth Measurements

Abstract

High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting behaviors. In LiFeAs the low-temperature λ(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density infinity λ(-2), indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of ~1.33 Å.

Doping-dependent anisotropic superconducting gap in Na1−δ(Fe1−xCoCho1, Tanatar2, Spyrison3 et al. 2012Phys. Rev. B

“…Instead, ρ s (T ) at x = 0.05 is remarkably similar to that of optimally isovalent-doped BaFe 2 (As 1−x P x ) 2 , a known nodal pnictide superconductor even at the optimal doping [20]. Our observations suggest that, despite the universal tendency for highest anisotropy at the dome edges in LiFe(As,P) [17,18], BaCo122 [9], BaK122 [13] and a layered Ca10-3-8 system [19], the gap structure at the dome center can highly anisotropic even in the charge-doped systems.…”

“…The isovalent-doped LiFe(As,P) [17,18] ("Ca10-3-8") compounds [19] obey this trend as well. The only exception to this universal trend is found in isovalent -doped materials such as BaFe 2 (As 1−x P x ) 2 ("BaP122") [20,21] and Ba(Fe 1−x Ru x ) 2 As 2 [22] where nodal behavior was concluded even at the optimal doping.…”

“…These single-gap curves clearly deviate in the whole temperature range, confirming multi-gap nature of superconductivity in Na(Fe 1−x Co x )As (as well as other FeSCs). Surprisingly, ρ s (T /T c ) of NaFe 0.95 Co 0.05 As is virtually the same as that of optimally -doped BaP122 (x = 0.33, black solid dots), a known nodal isovalent substituted pnictide [20], and shows notable difference from the exponential saturation in a fully gapped LiFeAs [17,18].…”

“…The only exception to this universal trend is found in isovalent -doped materials such as BaFe 2 (As 1−x P x ) 2 ("BaP122") [20,21] and Ba(Fe 1−x Ru x ) 2 As 2 [22] where nodal behavior was concluded even at the optimal doping. It was suggested by Kuroki et al that nodal vs. nodeless behavior is controlled by the pnictogen height at least in a 1111 family [23] and Hashimoto et al extended this suggestion to 111 and 122 families [18]. It is also believed that the same parameter controls the transition temperature, T c [24,25].…”

“…To explain such sensitivity to the doping level and appearance of nodes in some materials, several proposals have been put forward. (i) Following a suggestion by Kuroki et al that nodal vs. nodeless behavior is controlled by pnictogen height in 1111 family [23], Hashimoto et al extended this suggestion to 111 and 122 families [18]. (ii) Nodal structure of the superconducting gap was linked with the evolution of the Fermi surface topology and orbital content in different bands [3,4,26,45,46].…”