Comment on “Cleavage surface of the BaFe2−xCoxAs2and FeySe1−xTexsuperconductors: A combined STM plus LEED study”

Abstract

Massee et al. [Phys. Rev. B 80, 140507(R) (2009)] found on the cleavage planes of BaFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$ two different long-range ordered structures, i.e., a ($2\ifmmode\times\else\texttimes\fi{}1$) phase present only after cleavage at low temperature and a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ phase observed after cleavage at room temperature. These results apply generally to 122 Fe-based superconductors, but have been discussed controversially [for a summary of the conflicting views, see Hoffman, Rep. Prog. Phys. 74, 124513 (2011)]. Here we support the interpretation of Massee et al. In addition, we argue that the existence of different long-range ordered structures corresponding to the same coverage in different temperature regimes is associated with the melting of a charge density wave and removal of an associated periodic lattice distortion (CDW/PLD) in the substrate as $T$ is increased. At sufficiently low temperature the fluctuating CDW/PLD order parameter is stabilized by the adsorbate in a lock-in type mechanism. Accordingly, we interpret the surface structures observed on the 122 Fe pnictide surfaces as evidence for the presence of CDW fluctuations at low temperature, but with a wave vector differing from that of the antiferromagnetic spin-density fluctuations.