Creation of high-energy phonons from low-energy phonons in liquid helium

We present a theory for the remarkable observation that high-energy phonons are created from a moving cloud of low-energy phonons in liquid He-4. 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 mu s. [S0031-9007(99)08478-1].