  We use angle-resolved photoemission spectroscopy to study twinned and
detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic
reconstruction are observed to occur at the structural (TS) and magnetic (TSDW)
transitions. At TS, C4 rotational symmetry is broken in the form of an
anisotropic shift of the orthogonal dxz and dyz bands. The magnitude of this
orbital anisotropy rapidly develops to near completion upon approaching TSDW,
at which temperature band folding occurs via the antiferromagnetic ordering
wave vector. Interestingly, the anisotropic band shift onsetting at TS develops
in such a way to enhance the nesting conditions in the C2 symmetric state,
hence is intimately correlated with the long range collinear AFM order.
Furthermore, the similar behaviors of the electronic reconstruction in NaFeAs
and Ba(Fe1-xCox)2As2 suggests that this rapid development of large orbital
anisotropy between TS and TSDW is likely a general feature of the electronic
nematic phase in the iron pnictides, and the associated orbital fluctuations
may play an important role in determining the ground state properties.
