FAST SOUND IN LIQUID WATER

Both inelastic-neutron-scattering experiments and computer simulations have established that liquid water can support a longitudinal mode pr opagating with a velocity much larger than the ordinary speed of sound . The physics behind this peculiar behavior is studied by a comprehens ive microscopic approach. The effect is found to be a rather extreme c ase of a well-known phenomenon occurring even in monatomic liquids. To make the analogies and the differences more transparent, simulation d ata in water and in a simple alkali metal (liquid Cs near melting) are also reported. Finally, we discuss the ultimate microscopic origin of fast-sound effect in liquid water.