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 performed an angle-resolved photoemission spectroscopy study of BaFe2As2, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. The bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electrondoped non-superconducting (SC) BaFe1.7Co0.3As2. We find that the two hole Fermi surface pockets at the Brillouin zone center observed in the hole-doped superconducting Ba0.6K0.4Fe2As2 are absent or very small in this compound, while the two electron pockets at the zone corner significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.

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