  In Ca1-xRExFeAs2 (RE= rare earth), an antiferromagnetic (AFM) phase as well
as a structural transition has been reported, even in the electron-overdoped
regime. Here we investigated the temperature-dependent in-plane optical
spectroscopy of overdoped Ca0.77Nd0.23FeAs2. Upon entering the AFM state, we
found an abrupt reduction of low-frequency (500-2 000 cm-1) spectral weight in
the optical conductivity. In sharp contrast to the parent compounds of 122
system, where spin-density-wave gaps have been clearly observed in the AFM
state, a gap signature is absent in Ca0.77Nd0.23FeAs2. This may be a
consequence of the poor nesting condition between hole and electron pockets.
However, a spectral weight analysis shows that the reduced spectral weight at
low frequency is transferred to the high frequency range (> 4 000 cm-1),
pointing to a localization effect. These observations suggest that the AFM
order in Ca0.77Nd0.23FeAs2 is most likely to originate from a localized nature
rather than Fermi surface nesting.
