Electronic properties of 3d transitional metal pnictides: A comparative study by optical spectroscopy

Single-crystalline KFe2As2 and CaT2As2 (T = Fe, Co, Ni, and Cu) are synthesized and investigated by resistivity, susceptibility, and optical spectroscopy. It is found that CaCu2As2 exhibits a similar transition to the lattice abrupt collapse transitions discovered in CaFe2(As1-xPx)(2) and Ca1-xRx Fe2As2 (R = rare-earth element). The resistivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni, and Cu) approximately follows the similar T-2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals that the optical response of CaCu2As2 is not sensitive to the transition at 50 K, with no indication of development of a new energy gap below the transition temperature. Using Drude-Lorentz model, we find that two Drude terms, a coherent one and an incoherent one, can fit the low-energy optical conductivity of KFe2As2 and CaT2As2 (T = Fe, Co, and Ni) very well. However, in CaCu2As2, which is a sp-band metal, the low-energy optical conductivity can be well described by a coherent Drude term. Lack of the incoherent Drude term in CaCu2As2 may be attributed to a weaker electronic correlation than in KFe2As2 and CaT2As2 (T = Fe, Co, and Ni). Spectral weight analysis of these samples indicates that the unconventional spectral weight transfer, which is related to Hund's coupling energy J(H,) is only observed in iron pnictides, supporting the viewpoint that J(H) may be a key clue in the search for the mechanism of magnetism and superconductivity in pnictides.