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 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.

The doping dependence of electronic states in an electron-doped high-temperature superconductor ͑HTSC͒ Nd 2−x Ce x CuO 4 was studied by high-resolution angle-resolved photoemission spectroscopy. We observed that the high-energy pseudogap around the hot spot shows an abrupt filling-in at the magnet-superconductor phase boundary, resulting in the unusual reconstruction of the Fermi surface. The magnitude ͑⌬ PG ͒ and the temperature ͑T * ͒ at which the pseudogap is filled-in show a close relation to the effective hyperfine coupling energy ͑J eff ͒ and the spin-correlation length ͑ AF ͒, respectively. These results suggest the magnetic origin of the pseudogap and the unconventional nature of the magnet-superconductor phase transition in electron-doped HTSC.

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We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O0.93F0.07)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s-or axial p-wave case, while it is 4.1 meV in the polar p-or d-wave case. We also found a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing temperature and closes at temperature far above Tc similarly to copper-oxide high-temperature superconductors.KEYWORDS: superconducting gap, pseudogap, iron-based superconductor, photoemission spectroscopy Superconductivity observed at as high as 26 K (onset temperature = 32 K) in La(O 1−x F x )FeAs 1 provided a deep impact in condensed-matter physics, since this new superconductor does not belong to any known categories of "high-temperature superconductors" such as copper-oxides (cuprates), 2 fullerenes, 3 and MgB 2 . 4La(O 1−x F x )FeAs consists of alternatingly stacked insulating lanthanum oxide (LaO) and conductive iron arsenide (FeAs) layers. While undoped LaOFeAs is a metal or a degenerate semiconductor at room temperature with no sign of superconductivity, substitution of oxygen with fluorine (F) atoms gives rise to the superconductivity at the F doping of x = 0.03. Further doping causes a gradual increase of superconducting transition temperature (T c ) up to the maximum T c of 26 K with the onset temperature over 30 K at x = 0.11. 1 Although the crystal structure is substantially different from that of cuprate superconductors -it is not a perovskite structure and does not contain CuO 2 planes -, both compounds share intriguing similarities such as the two-dimensional crystal/electronic structure, 1, 5-8 presence of a superconducting dome in the electronic phase diagram where the T c is controlled by a systematic aliovalent ion doping into the insulating block layers, and a characteristic anomaly in the transport property in the under-doped region.1 It has been suggested that La(O 1−x F x )FeAs is situated at the boundary to the ferromagnetic phase, since the F doping increases the number of Fe 3d electrons from six to that (seven) of Co 3d electrons in LaOCoAs which is known to undergo a ferromagnetic transition at 66 K.1, 9 Such a situation may invoke an exotic pairing mechanism related to ferromagnetic spin fluctuations. It is thus particularly important to elucidate the mechanism and origin of superconductivity in this new category of high-T c superconductor in relation to cuprates and other layered novel superconductors like ruthenium-10 and cobalt-oxides.11 * E-mail address: t-sato@arpes.phys.tohoku.ac.jpIn this letter, we report ultrahigh-resolution (∆E = 1.7 meV) photoemission spectroscopy (PES) on La(O 0.93 F 0.07 )FeAs to study the electronic structure and the superconducting gap. We found that the PES spectrum at 5 K shows a suppression of spectral intensity within 4 meV indicative of opening of a superconducting gap. We also observed a pseudogap ...