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Jeudi 6 Decembre, 16h
Amphi Howleck, Esc C, 1ème etage
How supersolids melt : vortices near a high-symmetry phase coexistence point
Chris Hooley
Scottish Universities Physics Alliance (SUPA), University of St Andrews, UK
According to the Mermin-Wagner theorem, superfluidity in two dimensions (e.g. a thin film) ought to be impossible. However, superfluid response in such films is routinely seen in experiments. The solution of this apparent contradiction is that the film is not showing true long-range order (which would indeed be impossible), but instead quasi-long-range order : superfluid phase coherence decays with distance, but only in a power-law fashion, which is enough to preserve macroscopic superfluid behaviour.
The way in which this quasi-long-range order eventually goes away is even more fascinating : it happens by a proliferation of free vortices, as explained by Berezinskii, and Kosterlitz and Thouless - collectively known as BKT - in the 1970s. But what becomes of this BKT transition when the system is near a point of competition with another (non-superfluid) phase ? This question is relevant to supersolidity (or the lack thereof) in helium, as well as to the high-temperature superconductors and to various experiments involving cold atomic gases. I shall present some of our recent results on this topic, as well as outlining some of the puzzles that remain unresolved.