Experimental Evidence for a Two-Roton Bound State in Superfluid Helium

We have used Raman scattering to study roton pairs in liquid helium at 1.2\ifmmode^\circ\else\textdegree\fi{}K. We find that the energy required to create two rotons is less than twice the energy of a single roton. This result can be explained by the existence of a two-roton bound state. Comparison of our spectra with theoretical calculations gives a binding energy of (0.37 \ifmmode\pm\else\textpm\fi{} 0.10)\ifmmode^\circ\else\textdegree\fi{}K and shows that the pair is in a $D$ state ($L=2$) of angular momentum.