Fast and slow sound in liquid Li_30 Bi_70

Taking advantage of the improved performances of hot neutron three-axis spectrometers, we were able to simultaneously resolve two inelastic doublets in the dynamic structure factor of liquid Li_30 Bi_70 Measuring two distinct branches in the dispersion curve of the alloy, we provide a striking experimental observation of the existence of the fast and slow sound phenomenon in Li_30 Bi_70. The comparison with an analytical mode, describing the frequency response of the equivalent crystal with mass disorder, establishes that the two modes arise from the decoupled dynamics of the light and heavy components at high frequency.