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.

In nature, specific biomolecules interacting with mineral precursors are responsible for the precise production of nanostructured inorganic materials that exhibit complex morphologies and superior performance. Despite advances in developing biomimetic approaches, the design rules for creating sequence-defined molecules that lead to the synthesis of inorganic nanomaterials with predictable complex morphologies are unknown. Herein we report the design of sequence-defined peptoids for controlled synthesis of highly branched plasmonic gold particles. By engineering peptoid sequences and investigating the resulting particle formation mechanisms, we develop a rule of thumb for designing peptoids that predictively enabled the morphological evolution from spherical to coral-shaped nanoparticles. Through a combination of hyperspectral UV-Vis extinction microscopy and three-photon photoemission electron microscopy, we demonstrate that the individual coral-shaped gold nanoparticles exhibit a plasmonic enhancement as high as 105-fold. This research significantly advances our ultimate vision of predictive bio-inspired materials synthesis using sequence-defined synthetic molecules that mimic proteins and peptides.

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically locked femtosecond laser pulses. Power-dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source launches a PSP through a linear interaction, and the second subsequently probes the PSP via two-photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (783 nm) and group velocity (0.95c). In addition, tr-PEEM images reveal that the launched PSP may be detected at least 250 μm away from the coupling trench structure.