A deeper look into iron selenide: In the past 10 years, iron-based superconductors have created more puzzles than they have helped resolve. Some of the most fundamental outstanding questions are how strong the interactions are and what the electron pairing mechanism is. Now two groups have made contributions toward resolving these questions in the intriguing compound iron selenide (FeSe) (see the Perspective by Lee). Gerber et al. used photoemission spectroscopy coupled with x-ray diffraction to find that FeSe has a very sizable electron-phonon interaction. Quasiparticle interference imaging helped Sprau et al. determine the shape of the superconducting gap and find that the electron pairing in FeSe is orbital-selective. Science , this issue p. 71 , p. 75 ; see also p. 32

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$. We obtained a sheet carrier concentration of $N_{\mathrm{s}}\approx 5–9\times {10}^{13}{\mathrm{cm}}^{-2}$, an effective mass of $m^{*}=3.2\pm 0.4m_e$, and a strongly frequency dependent mobility. The latter are similar as in bulk ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ and therefore suggestive of polaronic correlations. We also determined the vertical concentration profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.

We investigated the electronic structures of the two-dimensional layered perovskite ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ ($M=4d$ Ru, $4d$ Rh, and $5d$ Ir) using optical spectroscopy and polarization-dependent O $1s$ x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $\sigma \left(\omega \right)$ exhibit similar interband transitions, indicative of the common electronic structures of the $4d$ and $5d$ layered oxides. The energy splittings between the two $e_g$ orbitals, i.e., $d_{3z^2-r^2}$ and $d_{x^2-y^2}$, are about $2\mathrm{eV}$, which is much larger than those in the pseudocubic and $3d$ layered perovskite oxides. The electronic properties of the ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ compounds are discussed in terms of the crystal structure and the extended character of the $4d$ and $5d$ orbitals.