Nucleation of crystals and superfluid droplets in ⁴ He by heat pulse

Abstract

Both nucleation of a4 He crystal in superfluid and that of a 4 He superfluid droplet in crystal were found to be induced by a heat pulse at the melting pressure. The time evolutions of nucleation on a heater were directly recorded by a high-speed camera. Duration of the heat pulse was usually 2 msec in this report. The nucleated crystal grew with facets for 4 msec and melted with an irregular shape in 40 msec at 500 mK. The nucleated superfluid droplet in crystal also grew with facets, which was a so-called negative crystal, and disappeared in 10 msec. Introduction Crystal-superfluid transition in4 He is an example of the first order phase transition at very low temperatures where the thermal fluctuations are much smaller than the energy barrier for nucleation of the stable phase. In such a temperature region, nucleation through macroscopic quantum tunneling was theoretically predicted by Kagan and Lifshitz [1]. Nucleation of the 4 He crystal has been studied extensively using several kinds of driving forces. Application of hydrostatic pressure and electric field can induce the nucleation of a 4 He crystal in metastable superfluid with a typical overpressure of 5 mbar above the melting pressure [2][3][4][5]. An overpressure of about 5 bar was achieved by focused sound at the focusing spot on a clean glass plate and nucleation was realized [6]. We recently demonstrated that an acoustic wave can induce both nucleation of a 4 He crystal in superfluid and that of a superfluid droplet in crystal at melting pressure [7,8]. We attributed this reversible nucleation to the acoustic radiation pressure.However, it is not known whether or not a heat pulse can nucleate a crystal in 4 He. In case of 3 He which has a large negative slope of the melting curve below 300 mK, heat pulses have been used to nucleate a crystal of the nuclear spin ordered phase from metastable superfluid B-phase at ultra low temperatures [9,10,11]. Warming by the heat pulse with constant pressure resulted in the overpressure of the system and the crystal was nucleated. The melting curve of 4 He has a much smaller negative slope; minimum of the melting curve exists at 775 mK and the melting pressure increases about 8 mbar with cooling [12]. Slight heating of the system in 4 He causes only a small overpressure and it has never been regarded as an effective driving force. However, we did observe not only nucleation of 4 He crystals in a superfluid but also nucleation of superfluid droplets in a crystal by heat pulse as in the case of acoustic waves. Heat pulse nucleation is nucleation under a steady heat current and it is very interesting to study the nucleation in such a non-equilibrium condition. We report high-speed images of the nucleation of 4 He crystals and superfluid droplets in this paper.