Vacancy and copper-doping effect on superconductivity for clathrate materials

We present a joint experimental and theoretical study of the superconductivity and electronic structures in type-I Cu-doped silicon clathrates and germanium clathrates. The superconducting critical temperature in Ba8Si46-xCux is shown to decrease strongly with copper content increasing. These results are corroborated by CASTEP approach, first-principles simulations calculated from the density-functional theory with plane waves and pseudopotentials. The simulations show that Cu-doping results in a large decrease of electronic density of states in Fermi level, which can explain the superconducting critical temperature decrease with Cu-doping in the BCS theoretical frame. Further, comparison of Ba8Ge46 and Ba8Si46 within the CASTEP approach shows that the superconductivity is an intrinsic property of the sp3 silicon and germanium clathrates without vacancy in the cage framework. By analysis of the density of states (DOS) and reported experimental results of the Zintl-like Ba8Ge43, a new mechanism of vacancy defect is suggested to explain the absence of superconductivity in Ge clathrates, which is of benefit to eliminating the divarication between theoretical prediction and the experimental observation for superconductivity in Ge clathrates. Keeping an entire Si and Ge cage structure without vacancy is the prerequisite for occurrence of superconductivity in clathrates.