Soft Transverse Phonons in Nonequilibrium fcc Al:Si Solid Solution Quenched under High Pressure

We have determined for the first time the phonon density of states of a nonequilibrium fcc ${\mathrm{Al}}_{0.94}$ ${\mathrm{Si}}_{0.06}$ solid solution quenched under high pressure by means of inelastic neutron scattering and low-temperature specific heat. When compared with aluminum, a large softening of the transverse acoustic modes is observed similar to that found in amorphous metals. This shows that a phonon softening can be found in nonequilibrium crystalline alloys. The nonequilibrium state is due to the metallic state of silicon atoms trapped in the fcc lattice during quenching.