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...

Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as EuFe 2 As 2 , is responsible for superconductivity in the doped compounds, such as Ba͑Fe 0.92 Co 0.08 ͒ 2 As 2 and Ba͑Fe 0.95 Ni 0.05 ͒ 2 As 2 . Analyzing the dc resistivity and scattering rate of this contribution, a hidden T 2 dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.

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 magnetic, transport and optical properties of EuFe2As2 single crystals have been investigated parallel and perpendicular to the ab-plane. The anisotropy ρc/ρ ab ≈ 8 depends only slightly on temperature. In both orientations, the spin-density wave transition at TSDW = 189 K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu 2+ moments at TN = 19 K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only 4.2 × 10 21 cm −3 and a bandmass of m b = 2m0. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below TSDW, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at TSDW. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below TSDW, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.PACS numbers: 75.30.

The transport and complex optical properties of the electron-doped iron-arsenic superconductor BaFe 1.85 Co 0.15 As 2 with T c = 25 K have been examined in the Fe-As planes above and below T c . A BlochGrüneisen analysis of the resistivity yields a weak electron-phonon coupling constant ph Ӎ 0.2. The lowfrequency optical response in the normal state appears to be dominated by the electron pocket and may be described by a weakly interacting Fermi liquid with a Drude plasma frequency of p,D Ӎ 7840 cm −1 ͑Ӎ0.972eV͒ and scattering rate 1 / D Ӎ 126 cm −1 ͑Ӎ15 meV͒ just above T c . The frequency-dependent scattering rate 1 / ͑͒ has kinks at Ӎ12 and 55 meV that appear to be related to bosonic excitations. Below T c the majority of the superconducting plasma frequency originates from the electron pocket and is estimated to be p,S Ӎ 5200 cm −1 ͑ 0 Ӎ 3000 Å͒ for T Ӷ T c , indicating that less than half the free carriers in the normal state have collapsed into the condensate, suggesting that this material is not in the clean limit. Supporting this finding is the observation that this material falls close to the universal scaling line for a Bardeen, Cooper, and Schrieffer dirty-limit superconductor in the weak-coupling limit. There are two energy scales for the superconductivity in the optical conductivity and photoinduced reflectivity at ⌬ 1 ͑0͒Ӎ3.1Ϯ 0.2 meV and ⌬ 2 ͑0͒ Ӎ 7.4Ϯ 0.3 meV. This corresponds to either the gapping of the electron and hole pockets, respectively, or an anisotropic s-wave gap on the electron pocket; both views are consistent with the s Ϯ model.