The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermisurface (FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba0.6K0.4Fe2As2, the pairing strength is enhanced simultaneously (2⌬/TcϷ7) on the nearly nested FS pockets, i.e., the inner hole-like (␣) FS and the 2 hybridized electron-like FSs, whereas the pairing remains weak (2⌬/ TcϷ3.6) in the poorly nested outer hole-like (␤) FS. Here, we report that in the electron-doped BaFe1.85Co0.15As2, the FS nesting condition switches from the ␣ to the ␤ FS due to the opposite size changes for hole-and electron-like FSs upon electron doping. The strong pairing strength (2⌬/TcϷ6) is also found to switch to the nested ␤ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.angle-resolved photoemission ͉ band structure ͉ iron pnictide ͉ superconductivity I n charge-doped superconductors, such as copper oxides (cuprates), electron or hole doping may influence the superconducting (SC) properties differently (1, 2). As an example, angle-resolved photoemission spectroscopy (3) (ARPES) and Raman scattering (4) revealed a nonmonotonic behavior in the SC gap function of the electron-doped cuprates that is different from the simple dx 2 -y 2 -wave function observed in the hole-doped cuprates (5). On the other hand, in the new Fe-based superconductors (6-9), no direct comparison of the SC order parameter has been made between hole-and electron-doped systems. ARPES studies on hole-doped Ba 1-x K x Fe 2 As 2 have observed isotropic gaps that have different values on different Fermi surfaces (FSs) with strong pairing occurring on the nearly nested FS pockets (10-13). Thus, it is particularly important to conduct a comparison of the SC gaps and their FS dependence of an electron-doped pnictide. We have chosen BaFe 1.85 Co 0.15 As 2 , which is optimally electron doped (14) with the same crystal structure as the Ba 1-x K x Fe 2 As 2 system (9). ResultsFig . 1A and B show ARPES intensity plots of BaFe 1.85 Co 0.15 As 2 (T c ϭ 25.5 K) as a function of binding energy and momentum (k) along 2 high-symmetry lines in the Brillouin zone (BZ). We observe a hole-like dispersion centered at the ⌫ point and 2 electron-like FSs near the M point. Even though a reasonable agreement is found between experiment and renormalized band calculations (15), some experimental features such as the energy position of the 0.2 eV band at the ⌫ point and the bottom of the electron band at the M point, are not well reproduced by band calculations. This suggests a possible orbital and k dependence of the mass-renormalization factor. Fig. 1C shows the ARPES intensity at the Fermi level (E F ) plotted as a function of the in-plane wave vector. A circular and an...

We have performed angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 to investigate the electronic structure relevant to superconductivity. We observed a holelike Fermi surface (FS) and an electronlike FS at the Brillouin zone center and corner, respectively, which are nearly nested by the Q∼(π,π) wave vector. We do not find evidence for the nesting instability with Q∼(π+δ,0) reminiscent of the antiferromagnetic order in the parent compound Fe1+yTe. We have observed an isotropic superconducting (SC) gap along the holelike FS with the gap size Δ of ∼4 meV (2Δ/kBTc ∼ 7), demonstrating the strong-coupling superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenides and iron-arsenides strongly suggests that common interactions which involve Q∼(π,π) scattering are responsible for the SC pairing.

We have performed angle-resolved photoemission spectroscopy on the overdoped Ba 0.3 K 0.7 Fe 2 As 2 superconductor (T c = 22 K). We demonstrate that the superconducting (SC) gap on each Fermi surface (FS) is nearly isotropic whereas the gap value varies from 4.4 to 7.9 meV on different FSs. By comparing with under-and optimally doped Ba 1−x K x Fe 2 As 2 , we find that the gap value on each FS nearly scales with T c over a wide doping range (0.25 x 0.7). Although the FS volume and the SC gap magnitude are strongly doping dependent, the multiple nodeless gaps can be commonly fitted by a single gap function assuming pairing up to the second nearest neighbor, suggesting the universality of the short-range pairing states with the s ± -wave symmetry.

High-temperature superconductivity in iron-arsenic materials (pnictides) near an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the superconducting gap develops on underdoping. This pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic wavevector, where the superconducting pairing is stronger as well, suggesting that antiferromagnetic fluctuations drive both the pseudogap and superconductivity. Interestingly, we found that the pseudogap and the spectral lineshape vary with the Fermi surface quasi-nesting conditions in a fashion that shares similarities with the nodal-antinodal dichotomous behaviour observed in underdoped copper oxide superconductors.

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