Physics

A Second Second Sound?

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Science  22 Jan 2010:
Vol. 327, Issue 5964, pp. 394
DOI: 10.1126/science.327.5964.394-b

Superfluids, such as 4He below its transition temperature around 2 K, make for a great science demonstration: They flow without viscosity through tiny holes, creep up vertical walls, and refuse to boil. This last property arises because heat, which normally propagates by diffusion, travels through a superfluid following a wave equation (often referred to as the “second sound”), resulting in extremely high thermal conductivity. Meppelink et al. realize an analog of this phenomenon in a weakly interacting Bose-Einstein condensate of sodium atoms by physically separating the condensed (superfluid) from the thermal (normal) portion of the gas. They then cause out-of-phase oscillations of the two components during which the center of mass remains stationary, much as in the case of the second sound. The amplitude of these oscillations is damped as the condensate passes through the thermal cloud, leading to energy exchange between the two (Landau damping). The damping rate is measured as a function of the temperature and confinement of the gas. In the collisionless regime (gas cloud size smaller than the mean free path), the Landau damping mechanism explains the results, whereas in the hydrodynamic regime the experiments suggest another, collision-dependent mechanism, which lacks a thorough theoretical description.

Phys. Rev. Lett. 103, 265301 (2009).

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