Dynamics and morphology of superfluid bubbles inHe4quantum crystals

Abstract

Superfluid bubbles in a 4He quantum crystal, which we refer to as quantum negative crystals, were investigated in bcc and hcp phases by visualizing their forms and motions at various temperatures. They were nearly spherical in bcc phase and faceted on the c-plane in their upper part in hcp phase. They steadily rose in the crystal due to gravity. Their direction was vertical in bcc phase and obliquely parallel to the c-facet in hcp phase. We also observed a slowing down of negative crystal in hcp phase caused by the appearance of the a-facet when temperature was lowered. Dynamics and morphology became successively anisotropic with cooling. The driving force by gravity for this motion was derived in the spherical case. Growth coefficient obtained by this model agreed well with the reported values.

Tunneling Defect Nanoclusters in hcp 4He Crystals: Alternative to Supersolidity

“…The same can be true for clusters of a different metastable phase in hcp-crystals [20]. Large growth-introduced vacancy clusters are probably the faceted liquid bubbles studied experimentally in 4 He crystals [21].…”

Falling4He crystals in superfluid

“…This reasonable agreement indicates that the falling was not greatly influenced by the phase transition although the drastic shape change was observed during the falling. This is in contrast to a 'falling' of 4 He crystals in superfluid attached to a vertical wall or a 'rising' of superfluid droplets included in a host 4 He crystal at higher temperatures [23,24]. In these cases, the actual movement of 4 He atoms in the crystals never took place and the 'falling' or 'rising' was just a pretense.…”

Ripening of splashed4He crystals by acoustic waves with and without gravity

“…It is also true that gravity sometimes has a dramatic effect on the relaxation dynamics in 4 He crystals. For example, relaxation of the facet around the roughening transition was investigated utilizing gravity to obtain the temperature dependence of the step energy [12]; negative crystals rise in a host crystal due to buoyancy [13][14][15]; sudden reduction of gravity induces the transformation of the crystal to relax to the gravity-free quasiequilibrium crystal shape which is determined by surface energy [16]; and so on. Here, we report the Ostwald ripening process of 4 He crystals in a superfluid without gravity and compare it with a ripening process with gravity.…”