Abstract

It is intuitive to think that heating any material leads to an increase in random thermal motion and, thereby, usually counteracts any ordering efforts. For example, heating ice leads to the formation of a liquid and ultimately to a gas. However, experiments with dipolar Bose-Einstein condensates suggested that this general phenomenology does not always take place [1]. In order to describe the surprising superfluid-supersolid transition upon an increase in temperature at fixed condensed particle number we employ Bogoliubov theory with local density approximation [2,3] and, indeed, find that such behaviour can also be expected theoretically [Fig. 1(a)]. With this approach we find [3] that the scaling with respect to density of the shift in energy due to thermal fluctuations is opposite to that of quantum fluctuations and, therefore, promotes the formation of high density regions [Fig. 1(b)]. Consistent with that we find that an increase in temperature promotes roton-softening [Fig. 1(c)]. Furthermore, we find good agreement between the experimentally measured density and the theoretical prediction including thermal fluctuations, whereas not taking thermal corrections into account yields a different result [Fig. 1(d)]. These findings call for further investigation of the surprising thermodynamic behaviour of ultracold quantum ferrofluids.

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