Polaron density matrix and effective mass at finite temperature

We calculate the density matrix elements of the polaron using Feynman's var iational method. The density matrix is diagonalized, and the eigenvalues an d eigenfunctions are derived. These results and the calculated density matr ix are used to evaluate the root-mean-square (rms) displacements Rp and the effective mass (EM) m* for various values of coupling strengths between el ectron and medium and of the temperature. We find that EM is related to the rms displacement. The temperature dependence of the EM is in qualitative a greement with experiment. [S0163-1829(99)08333-2].