Strongly correlated electron system without free carrier Yb₄(As_1-xP_x)₃

Mixed valence compound Yb₄As₃ which shows the charge ordering of Yb³⁺ and Yb²⁺ around 300K has been considered to be a heavy fermion system with extremely low carrier concentration. So the main interest in this system is to know whether the heavy fermion behavior surely comes from such low carrier concentration. To make this situation clear, single crystals of Yb₄(As_1-xP_x)₃ were grown and their transport properties, magnetic susceptibility and specific heat were measured. With increasing concentration of phosphor the system becomes semiconductive, however, the specific heat and magnetic susceptibility are almost the same as those of pure Yb₄As₃. This result shows that the large α-value of Yb₄As₃ is not due to the mass enhancement as seen in usual heavy fermion systems but arises from another magnetic correlation.