Evidence of Nematicity in K0.8Fe1.7Se2

It has been proposed that the superconducting state of K0.8Fe1.7Se2 is phase separated from a non-superconducting magnetic state. The results from a recent neutron diffraction study on a single crystal of K0.8Fe1.7Se2 provide evidence for a continuous transition between the I4/m m m high temperature phase in which the Fe vacancies are randomly distributed and the I4/m vacancy ordered phase in the temperature range between T (C) and T (S). Upon cooling, the I4/m phase becomes more populated, increasing the superlattice structure, resulting in an enhancement of the (101) superlattice peak. The same temperature dependence is observed for the magnetic peak as well. Moreover, due to the Fe site splitting with the transition, its z-coordinate fluctuates, and so must the d (x z) and d (y z) orbitals. The orbital fluctuations couple to the magnetic ordering as seen here and may lead to a realization of nematic order in this system.