Creation of High-Energy Phonons from Low-Energy Phonons in Liquid Helium

Abstract

We present a theory for the remarkable observation that high-energy phonons are created from a moving cloud of low-energy phonons in liquid ${}^{4}\mathrm{He}$. This phenomenon depends on the loss of high-energy $(h)$ phonons from a moving phonon cloud. The $h$ phonons are created from the low-energy $(l)$ phonons by four-phonon processes and escape from the back of the $l$-phonon cloud because they have a lower velocity than the $l$ phonons. This theory accounts for the high efficiency of the conversion process; a major part of the energy of the low-energy phonons can be converted to high-energy phonons in a time of order $10\ensuremath{\mu}\mathrm{s}$.