Anharmonic contribution to mode softening inV3Si

In order to assess the magnitude and significance of anharmonic effects in ${\mathrm{V}}_3$Si the Landau theory of Bhatt and McMillan for the coupled-mode charge-density wave instability has been extended to higher order. The higher-order coupling parameters are determined empirically and are used to calculate nine second- through fourth-order elastic constants as a function of temperature. The nonlinear stress-strain curve calculated from these results is in good agreement with the experimental data of Patel and Batterman. The elastic-shear-mode softening has been investigated in the quasiharmonic anisotropic-elastic-continuum approximation, in which anharmonic effects enter through the third- and fourth-order elastic constants. A strong enhancement of the electronically driven instability by the anharmonic phonon-phonon interaction is found.