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.

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We present magnetic measurements in a single crystal of the newly discovered superconducting ironpnictide Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2. The magnetization loops exhibit a second magnetization peak ͑SMP͒ similar to that observed in most high-temperature superconductors ͑HTSs͒. Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, S = d ln M / d ln t, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 the onset and peak fields, as well as the minimum point in S, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of S. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.

The detailed optical properties of BaFe2As2 have been determined over a wide frequency range above and below the structural and magnetic transition at TN ≃ 138 K. A prominent in-plane infrared-active mode is observed at 253 cm −1 (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at TN ; for T < TN the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity. This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.

Improved resolution in both, energy and momentum in ARPES-data has lead to the establishment of a definite energy scale in the dressed quasiparticle dispersion relations. The observed structure around 80 meV has been taken as evidence for coupling to phonons and has re-focused the debate about the mechanism of superconductivity in the cuprates. Here we address the relative merits of phonon as opposed to spin fluctuation mechanisms. Both possibilities are consistent with ARPES. On the other hand, when the considerations are extended to infrared optical data, a spin fluctuation mechanism provides a more natural interpretation of the combined sets of data in Bi2Sr2Ca Cu2O 8+δ (Bi2212).