Superconducting gap symmetry of Ba _0.6 K _0.4 Fe ₂ As ₂ studied by angle-resolved photoemission spectroscopy

Abstract

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally doped Ba0.6K0.4Fe2As2 compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M-point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable (∆ ∼ 11 meV) to that of the inner electron and hole pockets (∼ 12 meV), although it is substantially larger than that of the outer hole pocket (∼ 6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with the ∆(k) = ∆0 cos kx cos ky formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.

Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides

“…Moreover, the evolution of the spectral lineshape on underdoping indicates a dichotomy behaviour between unnested and quasi-nested FSs that shows striking similarities with the nodal-antinodal dichotomy found in cuprates, and that is naturally explained in pnictides in terms of short-range AF scattering. Figure 1d, the FS of UD26K differs from that of the OPD material previously reported 8,9,11,17 only by the size of the holelike and electronlike FS pockets, which become, respectively, smaller and larger in UD26K and satisfy the Luttinger theorem (Supplementary Discussions for the electron counting). Similarly to the OPD sample, the quasi-nesting condition between the inner hole-FS (α) and the electron-FSs is maintained, albeit slightly weakened in the UD samples.…”

“…We clearly observe the opening of SC gaps on all the FS sheets at low T and their disappearance above T c . The SC gap value (∆) on each FS sheet of the UD26K samples is reduced by approximately 30% as compared with the OPD37K samples 8,11 , which is almost the same as the reduction of the transition temperature (from 37 to 26 K). Interestingly, the 2∆ values vary linearly with T c from moderate underdoping to optimal doping, as illustrated in Figure 1e Intensity (a.u.)…”

“…Below T c , we observe the opening of SC gaps at their corresponding Fermi vectors (k F ). As with the OPD compound 8,9,11 , these gaps are isotropic within experimental uncertainties (Supplementary Discussion for the superconducting gap symmetry). To characterize these SC gaps further, temperature (T)-dependent ARPES measurements have been performed on different FS sheets (α, β and γ,δ).…”

“…Its size evolves smoothly along the Fermi surface (FS) from zero at locations referred to as nodes to a large maximum ∆ at the 'antinodes' that puts the value of 2∆/k B T c in the strong coupling regime 6,7 . In contrast, angle-resolved photoemission spectroscopy (ARPES) studies on optimally hole-doped pnictide Ba 0.6 K 0.4 Fe 2 As 2 (T c = 37 K; labelled as OPD37K), have revealed isotropic gaps that have different values on different FSs [8][9][10][11] , with strong pairing occurring on the quasi-(or nearly) nested FSs. The properties associated with the quasi-nested FSs have been described in a previous theoretical work 12 .…”

A unified description of cuprate and iron arsenide superconductors

“…There is substantial evidence that the pairing gap is spin singlet [19,20,21], but the gap orbital symmetry remains unsettled. Many experiments suggest that there are no gap nodes on the Fermi surfaces but some find evidence for nodes [19,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37], suggesting that there may be more than one orbital gap symmetry playing a role in the FeAs compounds.Many theoretical proposals have been made for the gap symmetry (see, for example, Refs. [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]).…”