Dresden 2006 – wissenschaftliches Programm

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SYSF: Superfluidity

SYSF 1: Superfluidity 1

SYSF 1.1: Hauptvortrag

Montag, 27. März 2006, 09:45–10:15, HSZ 04

Rotons and superfluidity — •Sebastien Balibar — Laboratoire de Physique Statistique, Ecole Normale Superieure, 75231 Paris cedex 05, France

Fritz London understood that quantum mechanics shows up at the macroscopic level, and proposed in 1938 that superfluidity was a consequence of Bose-Einstein condensation. However, Lev Landau could not believe in London’s ideas; instead, he introduced quasiparticles to explain the thermodynamics of superfluid 4He and its critical velocity. One of these quasiparticles was his famous “roton” which he considered as an elementary vortex. In 1946, London criticized Landau’s “theory based on the shaky grounds of imaginary rotons”. Despite their rather strong disagreement, Landau was awarded the London prize in 1960. Today, we know that London and Landau had both found part of the truth: BEC takes place in superfluid 4He, and rotons exist.

In my early experiments on quantum evaporation, I found direct evidence for the existence of rotons which play the role of photons in the photoelectric effect. But rotons are now considered as particular phonons which are nearly soft, due to the local order of superfluid 4He. In our recent studies of nucleation, rotons show their importance again: by using acoustic techniques, we have extended the study of liquid 4He up to very high pressures where the liquid state is very dense and metastable. We now wish to demonstrate that, at high density, the vanishing of the roton gap destroys superfluidity and triggers an instability towards the crystalline state.