Observation of Fermi-surface–dependent nodeless superconducting gaps in Ba _0.6 K _0.4 Fe ₂ As ₂

Abstract

We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba0.6K0.4Fe2As2 (Tc = 37 K). We have observed two superconducting gaps with different values: a large gap (∆ ∼ 12 meV) on the two small holelike and electron-like Fermi surface (FS) sheets, and a small gap (∼ 6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (Tc), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2∆/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

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

“…In the pnicitides, it is quite natural to attribute its origin to AF fluctuations, because it emerges near an AF phase, and its properties are shaped by the AF wave vector. Our observations of the SC coherence suppression and the reduction of the SC gap on underdoping, as well as the emergence of a PG that coexists with the SC gap below T c , support the scenario where the PG is the offspring of a competing state, although the SC pairing itself may be mediated by AF fluctuations, as suggested by various experiments 8,24,26,27 and theoretical works 12,[28][29][30][31] . In particular, a functional renormalization group approach has been applied to both cuprates and pnictides, suggesting that the pairing in both materials could be driven by AF correlations and enhanced by (π, π) FS quasi-nesting 32 .…”

“…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 .…”