Effects of carbon in MgB_2 thin films: Intrinsic or extrinsic

摘要

First-principles calculations are performed on MgB_2, carbon doped MgB_2, and MgB_2C_2. The trend of calculated lattice parameters of MgB_2 with increasing carbon content agrees with bulk experiments but not with thin films produced by hybrid physical-chemical vapor deposition. In this work, the authors propose a model to explain this behavior based on the coefficients of thermal expansion of MgB_2 and MgB_2C_2 as predicted from first principles and of graphite from literature. It is concluded that the effect of carbon on the lattice parameters of MgB_2 thin films is extrinsic and due to differences of the coefficients of thermal expansion of different phases.