Block antiferromagnetism and possible ferroelectricity in KFe2Se2

Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe₂Se₃, was predicted to be multiferroic Phys. Rev. Lett. 113 , 187204 (2014)] due to its unique one‐dimensional block‐type antiferromagnetism. Here, another iron‐selenide KFe₂Se₂, a parent state of iron‐based superconductor, is predicted to be multiferroic. Its two‐dimensional block‐type antiferromagnetism can generate a moderate electric dipole for each Fe–Se layer via the Fe–Se–Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe₂Se₂ may be switchable between antiferroelectric and ferroelectric phases.

Crystal structure of KFe₂Se₂. (a) Purple: K; green: Se; brown: Fe. Two Fe sheets in a minimum unit cell are indicated as A and B. (b) One Fe–Se layer with magnetism. Brown: spin up; blue: spin down. (c) A side view of Fe–Se bonds. The ionic displacements driven by exchange striction are indicated by arrows.