The transport and complex optical properties of the electron-doped iron-arsenic superconductor ${\text{BaFe}}_{1.85}{\text{Co}}_{0.15}{\text{As}}_2$ with $T_c=25\text{K}$ have been examined in the Fe-As planes above and below $T_c$. A Bloch-Grüneisen analysis of the resistivity yields a weak electron-phonon coupling constant ${\lambda }_{ph}\simeq 0.2$. The low-frequency 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 ${\omega }_{p,D}\simeq 7840{\text{cm}}^{-1}$ $\left(\simeq 0.972\text{eV}\right)$ and scattering rate $1/{\tau }_D\simeq 126{\text{cm}}^{-1}$ $\left(\simeq 15\text{meV}\right)$ just above $T_c$. The frequency-dependent scattering rate $1/\tau \left(\omega \right)$ has kinks at $\simeq 12$ 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 ${\omega }_{p,S}\simeq 5200{\text{cm}}^{-1}$ $\left({\lambda }_0\simeq 3000\text{Å}\right)$ for $T⪡T_c$, 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 ${\Delta }_1\left(0\right)\simeq 3.1\pm 0.2\text{meV}$ and ${\Delta }_2\left(0\right)\simeq 7.4\pm 0.3\text{meV}$. 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 $s^{\pm }$ 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 report tip-enhanced Raman imaging experiments in which information on sample topography and local electric fields is simultaneously obtained using an all-optical detection scheme. We demonstrate how a Raman-active 4,4′-dimercaptostilbene (DMS)-coated gold tip of an atomic force microscope can be used to simultaneously map the topography and image the electric fields localized at nanometric (20 and 5 nm wide) slits lithographically etched in silver, all using optical signals. Bimodal imaging is feasible by virtue of the frequency-resolved optical response of the functionalized metal probe. Namely, the probe position-dependent signals can be subdivided into two components. The first is a 500–2250 cm–1 Raman-shifted signal, characteristic of the tip-bound DMS molecules. The molecules report on topography through the intensity contrast observed as the tip scans across the nanoscale features. The variation in molecular Raman activity arises from the absence/formation of a plasmonic junction between the scanning probe and patterned silver surface, which translates into dimmed/enhanced Raman signatures of DMS. Using these molecular signals, we demonstrate that sub-15 nm spatial resolution is attainable using a 30 nm DMS-coated gold tip. The second response consists of two correlated sub-500 cm–1 signals arising from mirror-like reflections of (i) the incident laser field and (ii) the Raman scattered response of an underlying glass support (at 100–500 cm–1) off the gold tip. We show that both the reflected low-wavenumber signals trace the local electric fields in the vicinity of the nanometric slits.

We studied magnetic anisotropic properties, interlayer coupling, and spin wave relaxation in ten periods of CoFeB/Cr/CoFeB films grown on seed layers of Cu with a Co:Fe:B composition ratio of 2:2:1. The measurements were taken in samples with 50 Å layers of CoFeB using the ferromagnetic resonance technique. The thickness of the Cr interlayers was varied from 4 to 40 Å for understanding the mechanisms of interlayer coupling. We investigated the magnetic anisotropy parameters by rotating the sample with respect to the microwave magnetic field from in plane to perpendicular to the plane. We identify both the acoustic branch and the optical branch in the spin wave resonance spectra. The effective interlayer coupling constant and the out-of-plane anisotropy show an oscillatory change, while the uniaxial in-plane anisotropy increases monotonically with increasing the thickness of the spacing layers. Moreover, we show that the spin wave relaxation can be optimized by adjusting the interlayer exchange interactions.