Superfluids are an exotic state of matter with astonishing properties. When certain materials are cooled to extremely low temperatures near absolute zero, their atoms enter a quantum state where they can flow without friction or viscosity. This allows superfluids to exhibit behaviors unlike any other known substance.

One such remarkable property predicted over 75 years ago but never directly observed is second sound – the ability for heat to propagate through the superfluid as a temperature wave. A team of physicists at MIT have now captured the first images showing second sound in action using ultracold lithium gases.

Normal materials conduct heat diffusively, where hot regions spread out and dissipate into cooler areas. But in superfluids and some other exotic materials, heat can oscillate back and forth without resistance. This peculiar behavior was theorized in the 1940s to manifest as second sound, a wave-like transfer of heat energy.

“It’s as if you had a tank of water and made one half nearly boiling,” explained MIT Assistant Professor Richard Fletcher. “If you then watched, the water itself might look totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat goes back and forth while the water looks totally still.”

Directly observing second sound waves has proven extraordinarily challenging. Superfluids exhibit no infrared radiation that could track heat flow. The MIT team got around this problem by using lithium-6 fermions that resonate at different frequencies based on their temperature. This allowed them to visualize the oscillating fermions, revealing heat propagating in a wavelike pattern characteristic of second sound.

“For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity,” said Professor Martin Zwierlein, “and directly see how it transitions from being a normal fluid where heat equilibrates boringly to a superfluid where heat sloshes back and forth.”

The Strange World of Superfluids

Superfluidity was first discovered in 1937 by Pyotr Kapitsa and John Allen, who found that liquid helium-4 below 2.17 K (-270.98°C) would suddenly lose all viscosity, allowing it to flow without resistance ^[1]. This permits spectacular phenomena like helium creeping up walls and dripping out of containers.

The key to superfluidity lies in quantum effects taking over. At low enough temperatures, the helium atoms’ motions become correlated and they condense into a single quantum state. This allows them to flow with no friction since there are no individual particles scattering off each other.

In addition to lack of viscosity, superfluids display other exotic behaviors. They have zero entropy, infinite thermal conductivity, and quantized vortices. Superfluid helium can also “creep” up walls, defying gravity through effects like the Rollin film where the liquid climbs vertically along a surface ^[2].

Understanding superfluidity has proven vitally important for fields like condensed matter physics and even astronomy. The inside of neutron stars are expected to contain superfluid neutrons and superconducting protons, which could explain phenomena like glitches in pulsars ^[3].

Second Sound – A New Form of Heat Conduction

In ordinary materials, heat conduction occurs by diffusion. Hot regions pass kinetic energy to adjacent colder areas through particle collisions and vibrations until thermal equilibrium is reached. But in superfluids and some other exotic media, heat can propagate as a temperature wave called second sound.

Second sound was first predicted in 1944 by Lev Landau, who theorized that due to superfluids having zero viscosity, heat would not spread out diffusively but oscillate as a wave ^[4]. This would cause the temperature to fluctuate locally, akin to how sound creates pressure fluctuations as it moves through air.

Evidence for second sound emerged in the 1950s through experiments on superfluid helium. Researchers including Richard Packard observed that applying heat on one end of a superfluid produced a thermal wave traveling to the far end, indicative of second sound ^[5]. But despite decades of work, second sound proved difficult to directly visualize.

MIT Team Captures First Direct Images of Second Sound

The new MIT study succeeded in imaging second sound waves using an ultracold gas of lithium-6 atoms cooled to near absolute zero. At these temperatures, the lithium enters a superfluid state analogous to helium.

The team applied localized heating to the superfluid lithium gas and tracked the resulting thermal oscillations. They achieved this by exploiting the fact that lithium-6 fermions resonate at frequencies dependent on their temperature. By imaging the resonating fermions throughout the superfluid, they visualized second sound waves in action for the first time.

“For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity,” said Professor Zwierlein, “and directly see how it transitions from being a normal fluid where heat equilibrates boringly to a superfluid where heat sloshes back and forth.”

The researchers found the second sound waves propagated at speeds around 1.5 meters per second. The images also revealed a gradual onset of second sound when cooling through the superfluid transition temperature.

Going forward, the team plans to continue mapping out heat flow in ultracold superfluids. The experimental techniques they developed could also be applied to other exotic states of matter predicted to exhibit second sound, such as neutron star crusts and potentially other forms of quantum matter.

The new findings open many avenues for research into heat transport far from equilibrium. Besides deepening our understanding of superfluidity, second sound could potentially lead to applications such as thermal diodes or logic devices that exploit the unique properties of heat waves in these materials. The implications of this discovery are vast, offering a new perspective on thermal dynamics in low-temperature physics.

References

1. Kapitza, P. Viscosity of Liquid Helium below the λ-Point. Nature 141, 74 (1938).
2. Rollin, B. & Osheroff, D. D. On the (de)stabilization of a liquid helium film creeping on a flat surface. Journal of Low Temperature Physics 110, 191–206 (1998).
3. Chamel, N. & Haensel, P. Physics of Neutron Star Crusts. Living Reviews in Relativity 11, 10 (2008).
4. Landau, L. Theory of the Superfluidity of Helium II. Physical Review 60, 356–358 (1941).
5. Peshkov, V. P. Second sound in helium II. J. Phys. USSR 10, 389–394 (1946).