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

Single-crystalline KFe2As2 and CaT2As2 (T = Fe, Co, Ni, 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-xRexFe2As2 (Re-rare earth element). The resistivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni, Cu) approximately follows the similar T^2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals 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, 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 the weaker electronic correlation than KFe2As2 and CaT2As2 (T = Fe, Co, 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 to seek the mechanism of magnetism and superconductivity in pnictides.