Temperature-pressure anomalies of electrooptic coefficients in C-60-TTF derivatives

A sharplike modulation of the linear electrooptic (LEO) coefficient with pr essure (p) and temperature (T) has been observed for the first time. The mo dulation depth of the LEO (r(222)) tenser component in the C-60-TTF (for th e He-Ne laser, lambda = 633 nm) was equal to about 2.6%. By varying the app lied hydrostatic pressure to up to 15 GPa and the temperature from 4.2 to 2 5 K, we have unambiguously demonstrated that the observed effect is caused by the incorporation of the TTF moiety. This effect is absent from the unsu bstituted fullerenes when the applied hydrostatic pressure is increased to 19 GPa and the temperature varied from 4.2 to 150 K. Norm-conserving self-c onsistent pseudopotential and molecular dynamics simulations independently reproduce these modulations and attribute them to the electron-vibration in teractions associated with the LEO coefficients. The contribution of the in terconformational tunneling under the applied pressure and temperature is d emonstrated. The data obtained give a possibility for using the discovered effect for contactless measurements of low temperatures and high pressures.