Nodeless Versus Nodal Order Parameters in LiFeAs and LiFeP

Abstract

High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting low-energy quasiparticle excitations. 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 ∝ 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 ∼ 1.33Å.There is growing evidence that the superconducting gap structure is not universal in the iron-based superconductors [1]. In certain materials such as optimally doped (Ba,K)Fe 2 As 2 and Ba(Fe,Co) 2 As 2 , strong evidence for the fully gapped superconducting state has been observed from several low-energy quasiparticle excitation probes including magnetic penetration depth [2, 3] and thermal conductivity measurements [4,5]. In contrast, significant excitations at low temperatures due to nodes in the energy gap have been detected in several Fe-pnictide superconductors. These include LaFePO (T c = 6 K) [6-8], BaFe 2 (As,P) 2 (T c ≤ 31 K) [9-11], and KFe 2 As 2 (T c = 4 K) [12][13][14]. It is quite extraordinary that such distinct pairing states appear in closely related members of the same class of superconductors. To understand the mechanism of superconductivity in iron-based superconductors, it is essential to identify what determines nodal and nodeless states [15][16][17][18][19][20][21].Theories based on antiferromagnetic spin fluctuations suggest that the pnictogen height h P n above the iron plane (see Fig. 1(a)) is an important factor in determining the structure of the superconducting order parameter [16,[18][19][20]. Generally, h P n is much shorter for the P based iron-pnictides in comparison to their As counterparts, so a good test of the theory would be to systematically compare As and P based superconductors. Although this can be achieved in part in the BaFe 2 (As,P) 2 series, the fully As containing end member BaFe 2 As 2 is a nonsuperconducting antiferromagnet. The same is true for LaFeAsO which is the As analogue of the nodal superconductor LaFePO. Charge doping of the arsenides induces superconductivity, but also introduces disorder which complicates the identification of the pairing state.The 111 materials, LiFeAs [22,23] and LiFeP [24, 25] provide a unique route to study this problem as both materials are superconducting (T c ≈ 17 K and 4.5 K, respectively), nonmagnetic, and importantly very clean, with long electronic mean-free paths. In LiFeAs, antiferromagnetic fluctuations have been observed [26,27] and fully gapped superconductivity has been demonstrated by several experiments [28][29][30][31][32][33], but no information has been reported for the pairing state in LiFeP. Bandstructure calculations show t...

Nodal superconducting-gap structure in ferropnictide superconductor BaFe2(As0.7P0.3)2

“…This would induce significant amount of d 3z 2 −r 2 orbital character into α near E F . This scenario together with our findings provides an possible explanation for the recent intriguing observation that the nodal gap will appear in iron pnictides when the distance between the pnictogen and Fe plane (h Pn ) is smaller than 1.33Å [9]. In the case of BaFe 2 (As 1−x P x ) 2 , h Pn is reduced by phosphor doping, and consequently, it would cause larger k z -dispersion and strong mixing of the d 3z 2 −r 2 orbital for the α band as observed, which would create the line node as proposed [27].…”

“…It has been proposed that the pairing interactions between the electron and hole Fermi surfaces will induce nodeless s-wave order parameter with opposite signs on them [13][14][15]. While this nodeless s ± -wave pairing symmetry has gained increasing experimental support [16][17][18], nodal gap has been reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 , and FeSe by thermal conductivity, penetration depth, nuclear magnetic resonance, and scanning tunneling spectroscopy studies [5][6][7][8][9][10][11][12]. However, no direct measurement on any of these compounds has been reported regarding the gap structure so far, and especially the location of the nodes remains unknown.…”

Direct evidence for a pressure-induced nodal superconducting gap in the Ba0.65Rb0.35Fe2As2 superconductor

“…On the one hand, nodeless isotropic gap functions were observed in optimally doped Ba 1−x K x Fe 2 As 2 , Ba 1−x Rb x Fe 2 As 2 and BaFe 2−x Ni x As 2 as well as in BaFe 2−x Co x As 2 , K x Fe 2−y Se 2 , and FeTe 1−x Se x [1][2][3][4][5][6][7][8]. On the other hand, signatures of nodal SC gaps were reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 , FeSe as well as in over-doped Ba 1−x K x Fe 2 As 2 and BaFe 2−x Ni x As 2 [7,[9][10][11][12][13][14][15][16][17]. Understanding what parameters of the systems control the different SC gap structures observed experimentally is paramount to elucidate the microscopic pairing mechanism in the Fe-HTS's and, more generally, to provide a deeper understanding of the phenomenon of high-temperature superconductivity.…”