Possible nodal superconducting gap and Lifshitz transition in heavily hole-doped Ba0.1K0.9Fe2As

Abstract

We performed a high energy resolution ARPES investigation of over-doped Ba0.1K0.9Fe2As2 with Tc = 9 K. The Fermi surface topology of this material is similar to that of KFe2As2 and differs from that of slightly less doped Ba0.3K0.7Fe2As2, implying that a Lifshitz transition occurred between x = 0.7 and x = 0.9. Albeit for a vertical node found at the tip of the emerging off-M-centered Fermi surface pocket lobes, the superconducting gap structure is similar to that of Ba0.3K0.7Fe2As2, suggesting that the paring interaction is not driven by the Fermi surface topology.PACS numbers: 74.70. Xa, 74.25.Jb, The discovery of high-temperature superconductivity without hole Fermi surface (FS) pocket [1][2][3][4][5][6][7] in AFe 2 Se 2 (A = Tl, K, Cs, Rb) imposed severe constraints to the electron-hole quasi-nesting scenario as the main Cooper pairing force in the Fe-based superconducting (SC) materials [8] and raised fundamental questions related to the importance of their FS topology. To answer these questions, it is necessary to investigate heavily hole-doped compounds. Previous angle-resolved photoemission spectroscopy (ARPES) studies of fully hole-doped KFe 2 As 2 indicate that the FS near the M(π, 0) point is formed by small off-M-centered hole FS pocket lobes [9,10] rather than the M-centered ellipsoid-like electron FS pockets commonly observed in the other materials [8]. Interestingly, KFe 2 As 2 has a very low critical temperature (T c ) of only 3 K [11,12], which was early [9] interpreted as, and is still widely considered as, a consequence of the evolution of the FS topology. Despite their incapability to access the band structure at the M point and thus to reveal completely the SC gap structure, laser-ARPES measurements suggest a rather complicated nodal SC gap profile around the Γ point [13], which is consistent with the finite residual thermal conductivity (κ 0 /T ) of this material at low temperature [14,15].While their existence is widely accepted, the origin of the nodes in KFe 2 As 2 and their relationship with the pairing mechanism remain unclear and could possibly involve a fundamental change in the SC order parameter upon doping K from the optimal Ba 0.6 K 0.4 Fe 2 As 2 composition, for which both the ARPES [16][17][18] and thermal conductivity [19] techniques agree on a nodeless SC gap. Indeed, Tafti et al. [20] recently reported a sudden reversal in the pressure (P ) dependence of T c in KFe 2 As 2 without discontinuity in the Hall coefficient R H (P ) and in the electrical resistivity ρ(P ), which was interpreted as an evidence for a change in the order parameter incompatible with a Lifshitz transition (change in the FS topology [21]). Based on a rigid-band shift model, the Lifshitz transition corresponding to the apparition of the small off-M-centered hole FS pocket lobes was estimated to occur within the 0.8 ≤ x ≤ 0.9 doping range [22]. Determining the k-space structure of the SC gap in the vicinity of this transition is of critical importance.In this Letter, we investigate the electronic structure...

Critical fields and vortex pinning in overdopedBa0.2K0.8Fe2As2

“…5 and TABLE I. The results obtained here are in overall agreement with recent ARPES measurements [16,17] on heavily over-doped Ba 1-x K x Fe 2 As 2 . Those measurements indicate the simultaneous appearance of Fermi surface sheets with nodal gaps and with complete gaps.…”

“…Those measurements indicate the simultaneous appearance of Fermi surface sheets with nodal gaps and with complete gaps. Although there are discrepancies regarding the exact gap structure [16,17], they bear out the general feature of large complete gaps coexisting with small nodal (or highly anisotropic) gaps and are consistent with the results of the fits of our H c1 -data. Furthermore, from the H c1 -measurements and its fits we can directly deduce the corresponding temperature variation of the penetration depth, which is also included in the inset of Fig.…”

“…Furthermore, quantum oscillation measurements [13] show that in KFe 2 As 2 the electron Fermi surface sheets have been replaced by narrow cylindrical hole sheets that are slightly displaced from the X-point. Thus, with increasing hole-doping the Fermi surface of Ba 1-x K x Fe 2 As 2 undergoes significant reconstruction in a Lifshitz transition that occurs in the neighborhood of x ~ 0.8 [14][15][16][17][18][19]. Magnetic penetration depth [20,21], thermal transport [22][23][24][25], heat capacity [26][27][28][29], and ARPES measurements 3 [16,17] on heavily doped Ba 1-x K x Fe 2 As 2 indicate the presence of line nodes in the superconducting gap.…”

Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1−xKx)FeLiu1, Tanatar2, Straszheim3 et al. 2014Phys. Rev. B6215722View full textAdd to dashboardCite

“…The origin of the feature was discussed in terms of multiband character of conductivity in which one of the bands has strongly temperature dependent contribution, while the other has nearly temperature independent conductivity [38], as contribution from phonon-assisted scattering between two Fermi-surface sheets [45] and as a feature associated with pseudogap, as suggested by its correlation with the maximum of the interplane transport ρ c (T ) in underdoped compositions [24,25,30]. The position of the crossover does not change with doping, and since the Fermi surface topology reveals quite significant changes [61], the explanation of the maximum in term of special features of band structure [38,45] is very unlikely.…”

“…13, we plot 5 (depending on criterion) with increasing K doping levels. The increase starts in the heavily overdoped compositions x > 0.82, not far from the point where the Fermi surface topology change was found in angle-resolved photoelectron spectroscopy (ARPES) studies [61] and where the magnetism of the compounds changes according to neutron scattering [69,70] and NMR [71] studies. According to ARPES studies, the electron sheet of the Fermi surface transforms to four tiny cylinders.…”