Effects of Impurity Pinning on Commensurate Charge-Density-Wave State and Incommensurate-Commensurate Transition

The one-dimensional charge-density-wave (CDW) state containing impurities is studied theoretically. The localized large-amplitude excitation is taken into account to determine the ground state of the commensurate ( C )-CDW state at absolute zero. It is shown that the long-range order of the C -CDW is unstable against an arbitrarily weak impurity potential in spite of the existence of the commensurability energy. Investigations are also made on the impurity effect on the incommensurate ( I )- C transition as well as the temperature dependence of the wave number of the I -CDW. In this connection a mechanism is proposed for bringing the concave curvature in the temperature dependence of the wave number, which is observed in 2H-TaSe 2 .