  A systematic investigation has been performed by synthesis and comprehensive
characterization of a series of SmFe1-xCuxAsO0.8F0.2 bulks (x = 0 to 0.2).
These samples are well characterized by structural, Raman spectroscopy,
microstructural, transport, magnetic measurements, and supplementary
calculations within density functional theory (DFT). The parent compound,
SmFeAsO0.8F0.2 (Sm1111), exhibits a superconducting transition temperature (Tc)
of approximately 54 K. The lattice volume (V) is increased with Cu substitution
(x) without observing any impurity phase related to copper, which confirms the
successful incorporation of Cu at Fe sites in the superconducting FeAs layers.
These analyses are also well in agreement with Raman spectroscopy measurements
and relevant DFT results. The superconducting transition is decreased
systematically with copper doping and completely suppressed for 7% Cu-doped
Sm1111 (x = 0.07). A large amount of Cu substitution (x greater than 0.07) has
demonstrated the metal to insulate transition in the low-temperature range, and
no impurity phase was observed even at high Cu doping levels (x = 0.2). The
calculated critical current density of the parent sample is suppressed with
copper substitution, suggesting the reduced pinning centers, sample density,
and grain connections, as confirmed by the microstructural analysis. Our
studies suggest that the substitution of Cu in the superconducting FeAs layer,
resulting the enlargement of the lattice volume, is a source of strong disorder
scattering, leading to the suppression of Tc and the emergence of
metal-to-insulator, unlike the more successful carrier doping by nickel (Ni) or
cobalt (Co), as previously reported.
