April 17, 2020
Keio University
Osaka City University
Satoshi Yui, a visiting researcher (JSPS Research Fellow) at the Research and Education Center for Natural Sciences, Keio University; Professor Hiromitsu Kobayashi of the Department of Physics at the Hiyoshi Campus, Faculty of Law; Professor Makoto Tsubota of the Graduate School of Science, Osaka City University; and Associate Professor Wei Guo of Florida State University have revealed that the anomalous velocity fluctuations appearing in the gentle laminar flow of the normal fluid component in superfluid liquid helium-4 at cryogenic temperatures are caused by interactions with quantum vortices in the turbulent superfluid component. This is a groundbreaking achievement that successfully separates the motion of the two components—the superfluid and the normal fluid—of the "two-fluid model," the standard model of low-temperature physics proposed by Landau in 1941.
Liquid helium-4 ( 4 He) in its superfluid state at cryogenic temperatures below 2.17 K is described as a mixture of an inviscid "superfluid" and a viscous "normal fluid." This two-fluid model, theoretically proposed by Landau in 1941, is a standard model used not only for liquid helium but also for superconductivity. However, the motion of the superfluid and normal fluid components has never been demonstrated separately.
While velocity fluctuations in laminar flow are small in ordinary viscous fluids, anomalous velocity fluctuations dependent on the flow direction have been observed in flow experiments with superfluid helium, even though the normal fluid component is in a laminar state. In this paper, the researchers performed numerical simulations of quantum turbulence (a state where the superfluid becomes turbulent) by introducing a coupled numerical calculation method for the two fluids, involving quantum vortices that represent the rotation of the superfluid and the normal fluid. The analysis revealed that the anomalous, flow-direction-dependent velocity fluctuations of the laminar normal fluid are caused by the quantum turbulence created by quantum vortices.
This research result clarifies the picture of the two-fluid model theoretically proposed by renowned physicists such as Landau and Feynman, and separates the motion of the two fluids. This coupled numerical calculation of the two fluids is expected to have a significant impact on coherent matter-wave systems (such as superfluid helium, atomic gas Bose-Einstein condensates (BECs), and dark matter BECs) and multi-component fluid systems (such as liquid crystals, plasma and magnetohydrodynamics, and multiphase flows).
The results of this research were published in the online edition of "Physical Review Letters" on April 17, 2020 (JST).
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