1. L. Landau: J. Phys. USSR 11, 91 (1947). Even earlier [J. Phys. USSR 5, 71 (1941)], Landau had postulated the existence of rotons but at that time he envisaged them as forming a separate b5anch of excitations with energy-momentum dispersion relation e = A + p /21; where µ is an effective m5ss. In 1947 he changed the roton dispersion relation to e = A + (p - pa) /211 and connected the phonons (e = cp, where c is the velocity of sound) and rotons to form the continuous dispersion relation shown as an inset in Fig. 1
2. H. Palevsky, K. Otnes, K.E. Larsson, R. Pauli, R. Stedman: Phys. Rev. 108, 1346 (1957)
3. J.L. Yarnell, G.P. Arnold, P.J. Bendt, E.C. Kerr: Phys. Rev. Lett. 1, 9 (1958) and Phys. Rev. 113, 1379 (1959)
4. H. Palevsky, K. Otnes, K.E. Larsson: Phys. Rev. 112, 11 (1958)
5. D.G. Henshaw: Phys. Rev. Lett. 1, 127 (1958)