Exoticd-Wave Superconducting State of Strongly Hole-DopedKxBa1−xFe2As

Abstract

We investigate the superconducting phase in the KxBa1−xFe2As2 122 compounds from moderate to strong hole-doping regimes. Using functional renormalization group, we show that while the system develops a nodeless anisotropic s ± order parameter in the moderately doped regime, gapping out the electron pockets at strong hole doping drives the system into a nodal (cos kx+cos ky)(cos kx− cos ky) d-wave superconducting state. This is in accordance with recent experimental evidence from measurements on KFe2As2 which observe a nodal order parameter in the extreme doping regime. The magnetic instability is strongly suppressed.PACS numbers: 74.20.Mn, 74.20.Rp, 74.25.Jb, 74.72.Jb The most elementary questions in the field of ironbased superconductors, such as the symmetry of the order parameter in the superconducting (SC) state, are still under vivid debate. The complexities involve an intricate band structure, a diversity of different material compounds which exhibit sometimes contradictory behavior, and the proximity of various symmetry-broken phases. Due to best single-crystal quality, the most studied pnictide compounds belong to the 122 family such as BaFe 2 As 2 . Their crystal structure is tetragonal I4/mmm, where the Fe and As atoms arrange into layers; the intralayer hybridization is dominant but, unlike other pnictide compounds such as the 1111 family, the inter-layer hybridization is also important. Soon after their discovery [1], the 122 compounds have been synthesized not only with Ba as a substituent between the FeAs layers, but also with K, Cs, and Sr. The SC transition temperatures achieved were up to 37 K [2].The current theoretical opinion on the SC order parameter has converged on a nodeless s ± order parameter that changes sign between the electron (e) pockets and hole (h) pockets. This order parameter comes out of both the strong and the weak-coupling pictures of the iron-based superconductors [3][4][5][6][7], and owes its origin to the pnictide Fermi surface (FS) topology of h pockets at the Γ and e pockets at the X (π, 0)/(0, π) point of the unfolded Brillouin zone. The dominant scattering contributions originate from h pocket scattering at Γ to e pockets at X, yielding the s ± SC order parameter for the doped case and the collinear antiferromagnetic phase in the undoped case. Detailed nesting properties of the pockets, the multi-orbital character of the FS, and the presence or absence of a third h pocket at M (π, π) in the unfolded Brillouin zone complicate this picture. For the 1111 compounds, it was shown that the absence of the M h pocket (whose Fermi level can be significantly tuned by the pnictogen height through replacing As by P) can modify the SC order parameter anisotropy from a nodeless to a nodal s ± phase, which gives the correct material trend for As-P substitution in other pnictide families [7][8][9]. With small exceptions, the anisotropic extended s-wave scenario (and its extension to the nodal s ± ) was consistent with experimental findings for most of the pnictide compounds [10][1...

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…As shown in 6(b), the larger (smaller) of the 3 energy gaps is found on the α (β) band while the ζ gap exhibits accidental nodes which arise from the angle-dependent interband interaction with the pocket. These results are at odds with all theoretical calculations [55][56][57][58] which predict the largest gap on the β band. Moreover, at the X point, we recover the large gap ∆ =1.9 k B T c inferred from our magnetization and field-dependent heatcapacity data.…”

s+isstate with broken time-reversal symmetry in Fe-based superconductors

“…These modes are the solutions of DetK(q, Ω) = 0 at small Ω, and to get these modes one can safely expand in both v F q/∆ and in Ω/∆ Evaluating the integrals and converting from Matsubara to real frequency axis we obtain the expressions for Π jk ii and K(q, Ω) at small Ω and q, which we presented in Eqs. (6) and (10) in the main text.…”

“…(iii) For the x = 1 material KFe 2 As 2 , ARPES measurements 33,34 show that only hole pockets are present. According to theory, in this situation, both d−wave and s−wave pairing amplitudes are attractive 5,[9][10][11]35 , and which state wins depends on delicate interplay between system parameters. d−wave gap is the largest on the hole pocket, which in the unfolded Brillouin zone is centered at (π, π) (Refs.10,11), and s−wave gap is the largest on the two Γ−centered hole pockets (GCP's), and changes sign between them 35 .…”

Magnetic order in orbital models of the iron pnictides

“…81 Although the existence of superconductivity in these models has been extensively studied, 40,[44][45][46]48,51,53,56,60,63,64,66 the magnetic behaviour remains rather poorly understood. The most popular approach to the appearance of AFM order in these models is to examine the ground state using a standard mean-field ansatz that allows for at most twosite magnetic unit cells; 47,50,52,54,58,59,61,64 very recently, this has been generalized to a Gutzwiller mean-field theory.…”