Phase order in superfluid helium films

¹ London Centre for Nanotechnology and Department of Physics and Astronomy, University College London 17-19 Gordon Street, London WC1H 0AJ, UK
² Laboratoire de Physique, Université de Lyon, École Normale Supérieure de Lyon - 46 allée d'Italie, 69364 Lyon Cedex 07, France
³ Department of Physics and Astronomy, Uppsala University - Box 516, 751 20 Uppsala, Sweden

^(a) s.t.bramwell@ucl.ac.uk

Received: 1 September 2015
Accepted: 29 November 2015

Abstract

Classic experimental data on helium films are transformed to estimate a finite-size phase order parameter that measures the thermal degradation of the condensate fraction in the two-dimensional superfluid. The order parameter is found to evolve thermally with the exponent , a characteristic, in analogous magnetic systems, of the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Universal scaling near the BKT fixed point generates a collapse of experimental data on helium and ferromagnetic films, and implies new experiments and theoretical protocols to explore the phase order. These results give a striking example of experimental finite-size scaling in a critical system that is broadly relevant to two-dimensional Bose fluids.

This paper is dedicated to the memory of our friend and colleague Maxime Clusel, with whom we enjoyed many stimulating discussions on related topics.