Quantum state of muon and proton in crystalline silicon

Quantum state and dynamics of muon and proton in crystalline silicon have been studied by solving one-particle Schrodinger equations for the impurities. The ground state wavefunctions and energies are determined as a function of local distortion of the host Si lattice, where the Si-H interaction we used is that parameterized by Ramirez and Herrero following the results of earlier pseudopotential-density-functional calculation. It is shown that quantum zero-point motion of muon induces an effective potential barrier between the tetrahedral-symmetry (T) site and the bond-center (BC) site, which ensures bistability of muonium states observed in experiments. It is also shown that, if we fully consider the quantum effect with the present model potential, the BC site becomes less stable than the T site on the contrary to the experiments and former theoretical calculations.