In high-transition-temperature superconducting cuprates and iron arsenides, chemical doping plays an important role in inducing superconductivity. Whereas in the cuprate case, the dominant role of doping is to inject charge carriers, the role for the iron arsenides is complex owing to carrier multiplicity and the diversity of doping. Here, we present a comparative study of the in-plane resistivity and the optical spectrum of doped BaFe2As2, which allows for separation of coherent (itinerant) and incoherent (highly dissipative) charge dynamics. The coherence of the system is controlled by doping, and the doping evolution of the charge dynamics exhibits a distinct difference between electron and hole doping. It is found in common with any type of doping that superconductivity with high transition temperature emerges when the normal-state charge dynamics maintains incoherence and when the resistivity associated with the coherent channel exhibits dominant temperature-linear dependence.

Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transitiontemperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe 2 As 2 , is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe 2 P 2 , is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe 2 (As 1−x P x ) 2 make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal.

We carried out combined transport and optical measurements for BaFe2As2 and five isostructural transition-metal (TM ) pnictides. The low-energy optical conductivity spectra of these compounds are, to a good approximation, decomposed into a narrow Drude (coherent) component and an incoherent component. The iron arsenides, BaFe2As2 and KFe2As2, are distinct from other pnictides in their highly incoherent charge dynamics or bad metallic behavior with the coherent Drude component occupying a tiny fraction of the low-energy spectral weight. The fraction of the coherent spectral weight or the degree of coherence is shown to be well correlated with the TM -pnictogen bond angle and the electron filling of TM 3d orbitals, which are measures of the strength of electronic correlations. The iron arsenides are thus strongly correlated systems, and the doping into BaFe2As2 controls the strength of electronic correlations. This naturally explains a remarkable asymmetry in the charge dynamics of electron-and hole-doped systems, and the unconventional superconductivity appears to emerge when the correlations are fairly strong.The superconducting (SC) phase with unconventional pairing symmetry and with fairly high transition temperature (T c ) in iron-based superconductors emerges by doping (chemical substitution) into a parent compound showing an antiferromagnetic-orthorhombic (AFO) order [1]. This is similar to the high-T c cuprates, in which the parent compounds are Mott insulators due to strong electronic correlations. In contrast, the parent compounds of the iron-based superconductors are metals, albeit not good metals, and are not close to a Mott insulating state. From this, the iron pnictides/chalcogenides are usually thought to be in the intermediate correlation regime. Nevertheless, they possess features of strong electronic correlations, such as frozen spin magnetic moments [2,3] and temperature (T )-linear resistivity, characteristic of a non-Fermi liquid [4,5]. These "bad metallic" behaviors are usually observed for metals in close proximity to a Mott insulator and raise a puzzling question on the role of electronic correlations in the iron-based compounds, specifically iron arsenides.There exist theoretical attempts to solve this puzzle by incorporating the effect of the Hund's rule coupling (J H ) in addition to the onsite Coulomb interaction (U ), suggesting that, even when both J H and U are not large enough, the former coupling may be responsible for the aspects of strong correlations in multi-orbital metals [6,7]. However, so far there have been few experimental studies that are relevant to this issue [8], and it is far from clear why the iron arsenides are so special.Among various families of iron-based superconductors, the family with a ThCr 2 Si 2 crystal structure (so-called 122 system), representatively BaFe 2 As 2 and its doped compounds, has been most extensively studied. An advantage of this family is that isostructural compounds with various transition-metal (TM ) and pnictogen (Pn) atoms can be synthes...

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