Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5

The recent discovery of intercalated iron-selenide superconductors [1] has stirred up the condensed-matter community accustomed to the proximity of the superconducting and magnetic phases in various cuprate and pnictide superconductors. Never before has a superconducting state with a transition temperature as high as 30 K been found to coexist with such an exceptionally strong antiferromagnetism with Néel temperatures up to 550 K as in this new family of iron-selenide materials. The very large magnetic moment of 3.3 μB on the iron sites, however, renders a microscopically homogeneous coexistence of superconductivity and magnetism unlikely. In contrast to their 122 iron-arsenide counterparts, the structurally similar intercalated iron-selenide compounds (K,Rb,Cs)0.8Fe1.6Se2 have so far defied all efforts to synthesize a bulk single-phase material of this family. Absence of such electronically homogeneous superconducting single crystals and a strong correlation between the superconducting and antiferromagnetic phases necessitate a detailed research into the nature of their coexistence.