$\alpha \text{-FeSe}$ with the PbO structure is a key member of the family of high-$T_c$ iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted ${\text{FeSe}}_4$ tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that the application of hydrostatic pressure first rapidly increases $T_c$ which attains a broad maximum of 37 K at $\sim 7\text{GPa}$ before decreasing to 6 K upon further compression to $\sim 14\text{GPa}$. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $\alpha \text{-FeSe}$, revealing an extremely soft solid with a bulk modulus, $K_0=30.7\left(1.1\right)\text{GPa}$ and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed $\beta$ polymorph above $\sim 9\text{GPa}$. The nonmonotonic $T_c\left(P\right)$ behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties.

The newly discovered superconductor FeSe_1−x (x≈ 0.08, T_c ^onset≈ 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure—the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO_1−xF_x (RE = rare earth) superconductors.

The body-centered cubic A15-structured cesium fulleride Cs ₃ C ₆₀ is not superconducting at ambient pressure and is free from disorder, unlike the well-studied face-centered cubic A ₃ C ₆₀ alkali metal fulleride superconductors. We found that in Cs ₃ C ₆₀ , where the molecular valences are precisely assigned, the superconducting state at 38 kelvin emerges directly from a localized electron antiferromagnetic insulating state with the application of pressure. This transition maintains the threefold degeneracy of the active orbitals in both competing electronic states; it is thus a purely electronic transition to a superconducting state, with a dependence of the transition temperature on pressure-induced changes of anion packing density that is not explicable by Bardeen-Cooper-Schrieffer (BCS) theory.

Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above T_c is a major challenge for all unconventional superconductors. The molecular A₃C₆₀ fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C₆₀^3– electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of T_c with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.