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Atoms in contact (9): Breaking the superfluid speed limit.

P. V. E. McClintock

Published Mar 17, 1977• 0 citations
OpenAlex

Abstract

The Russian physicist Lev Landau predicted that an object moving through a superfluid would meet no resistance - until a certain critical velocity. In this last of two articles on the motion of charged particles through superfluid helium the authors describe how they discovered an unexpected phenomenon when the ions exceeded the critical speed

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Quantum Gases A superfluid can flow without viscosity but only if the speed of the flow is lower than the so-called critical velocity. Sobirey et al. measured the critical velocity of a system of ultracold fermionic atoms confined to two dimensions. The researchers trapped a gas of lithium-6 atoms in a box-shaped potential and then moved another, periodic potential through the trap. The response of the gas to this perturbation showed a sudden increase when the speed of the periodic potential reached the critical velocity. Science , abc8793, this issue p. [844][1] [1]: /lookup/doi/10.1126/science.abc8793

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The Russian physicist Lev Landau predicted that an object moving through a superfluid would meet no resistance - until a certain critical velocity. In this last of two articles on the motion of charged particles through superfluid helium the authors describe how they discovered an unexpected phenomenon when the ions exceeded the critical speed
An object can move through a superfluid without resistance provided that its speed is below a critical value; higher speeds result in the formation of vortices, which are small volumes of normal fluid and superfluid that rotate and dissipate energy. Although the dynamics of these vortices are considered to be crucial in understanding superfluidity, the strong and complex interactions of the superfluid, dissipative forces, and surface effects in most superfluids (such as liquid helium) all combine to obscure observations.
The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding.
Finding the breaking point A superfluid can flow without viscosity but only if the speed of the flow is lower than the so-called critical velocity. Sobirey et al. measured the critical velocity of a system of ultracold fermionic atoms confined to two dimensions. The researchers trapped a gas of lithium-6 atoms in a box-shaped potential and then moved another, periodic potential through the trap. The response of the gas to this perturbation showed a sudden increase when the speed of the periodic potential reached the critical velocity. Science , abc8793, this issue p. 844
In January 1938, the Russian physicist Pyotr Kapitza reported that, at temperatures below 2.2 degrees above absolute zero, liquid helium seemed to flow with no loss of energy due to friction. The term “superfluid” was suggested to describe this new behavior—which was eventually understood as a dramatic illustration of the quantum-mechanical wave nature of matter. The study of such macroscopic quantum effects now lies at the heart of modern physics. The complete analogue of the Landau two-fluid hydrodynamic equations, including various transport coefficients associated with the normal fluid, has recently been derived for the trapped atomic Bose gases. In the next few years, experiments should be able to probe this two-fluid domain in Bose gases, a region first explored with such spectacular success in superfluid helium by Kapitza and Allen and Misener. The complexity of analyzing any liquid at a microscopic level has resulted in many aspects of the theory of superfluid.
Quantum Gases A superfluid can flow without viscosity but only if the speed of the flow is lower than the so-called critical velocity. Sobirey et al. measured the critical velocity of a system of ultracold fermionic atoms confined to two dimensions. The researchers trapped a gas of lithium-6 atoms in a box-shaped potential and then moved another, periodic potential through the trap. The response of the gas to this perturbation showed a sudden increase when the speed of the periodic potential reached the critical velocity. Science , abc8793, this issue p. [844][1] [1]: /lookup/doi/10.1126/science.abc8793