Editorial: High-Tc Superconductivity in Electron-Doped Iron Selenide and Related Compounds

Iron-selenide superconductors comprise a particularly interesting group of materials inside the family of iron-based superconductors. The simplest member of the group is bulk FeSe, which has a modest critical temperature of T c = 9 K. Like iron-pnictide superconductors, bulk FeSe shows a structural transition at T s = 90 K from a tetragonal to an orthorhombic phase driven by nematic ordering of the electronic degrees of freedom. Angle-resolved photoemission spectroscopy (ARPES), for example, reveals a small hole Fermi surface pocket at the center of the Brillouin zone and two electron Fermi surface pockets at the corner of the Brillouin zone, each with unequal d xz /d yz -orbital character. Unlike in iron-pnictide superconductors, however, no magnetic order coexists with the nematic order at temperatures below the structural transition. Inelastic neutron scattering (INS) spectroscopy finds a spin resonance inside the energy gap of the superconducting phase in bulk FeSe, however, at wavevectors corresponding to a stripe spindensity wave (SDW) It strongly suggests s +-superconductivity across the hole and electron Fermi surface pockets driven by associated antiferromagnetic spin fluctuations. INS also finds spin fluctuations at the Nel wavevector (, ) above the superconducting energy gap This suggests that superconductivity, nematic order, stripe-SDW order, and some type of Nel antiferromagnetic order compete at low temperature in bulk FeSe. One of the editors of the research topic has proposed that the latter is hidden Nel order The superconducting critical temperature increases dramatically to 30-40 K and above upon doping iron selenide with electrons. The latter has been achieved in various ways; for example, by alkali-metal intercalation, by placing a monolayer of FeSe on a substrate, and by organic-molecule intercalation. ARPES finds that the hole bands at the center of the Brillouin zone lie buried below the Fermi level. INS finds a spin resonance inside the superconducting energy gap, but it lies midway between the SDW and Nel wavenumbers INS also finds peaks and rings of low-energy spin excitations above the energy gap around the Nel wavevector ARPES and scanning tunneling microscopy (STM) find a non-zero superconducting energy gap. The situation with electron-doped FeSe is rather puzzling then, with high-T c superconductivity existing over electron Fermi surface pockets alone! This is not expected in ironselenide superconductors, where electron-electron repulsion is strong The latter requires that the sign of the pair wave function oscillates over the Brillouin zone