1. Landau, L. D. & Lifshitz, E. M. (eds) Course of Theoretical Physics Vol. 9, Statistical Physics Part 2 (by Lifshitz, E. M. & Pitaevskii, L. P.) 125–139 (Pergamon, Oxford, 1981).
2. Abrikosov, A. A., Gorkov, L. P. & Dzyaloshinski, I. E. Methods of Quantum Field Theory in Statistical Physics 235–247 (Dover Publications, New York, 1975)
3. Pitaevskii, L. P. Properties of the spectrum of elementary excitations near the disintegration threshold of the excitations. Sov. Phys. JETP 9, 830–837 (1959)
4. Kapitza, P. Viscosity of liquid helium below the λ-point. Nature 141, 74 (1938)
5. Stone, M. B., Zaliznyak, I., Reich, D. H. & Broholm, C. Frustration-induced two dimensional quantum disordered phase in piperazinium hexachlorodicuprate. Phys. Rev. B 64, 144405 (2001)