MIT scientists have captured signs of a "second sound" directly in a superfluid for the first time. The bizarre phenomenon occurs when heat travels like sound waves through an unusual state of matter. In our daily experience, heat energy escapes to the surroundings. The hotter object cools down to the temperature of other nearby materials while heating them up until equilibrium is reached. However, in unconventional materials, physics can work in counterintuitive ways.

MIT scientists directly observe the strange effect of heat in superfluids Jose-Luis Olivares, MIT

A superfluid is a rare state of matter that has zero viscosity, meaning matter can flow without any resistance or friction. It has long been predicted that heat should be able to flow in superfluids like sound waves, hence the name "second sound", but it has not been directly observed until now.

"It's like you have a tank of water and make half of it almost boiling," said study author Richard Fletcher, an assistant professor. "If you then look, the water itself may look completely still, but then suddenly the other side gets hot, and then the other side gets hot again, and the heat flows back and forth, while the water looks completely still.

The first sound illustrates how sound waves oscillate together in ordinary fluids and superfluidsSecond sound illustrates how heat oscillates from one end to the other through a superfluid, as detected in new study

To image this phenomenon, researchers had to create an entirely new way of detecting heat. Typically infrared sensors would be used, but creating a superfluid requires cooling the quantum gas to almost absolute zero, and infrared radiation is not emitted at such low temperatures. So the team turned to radio.

The researchers used a quantum gas composed of lithium-6 fermions and found that the higher the temperature of these fermions, the higher the frequency at which they resonate. The team applied higher radio frequencies to the gas, which caused the hotter fermions within it to resonate in response. By tracking which fermions resonate at different times, scientists can catch the "second sound" as the heat waves swing back and forth.

Martin Zwierlein, first author of the study, said: "For the first time we can take pictures of this substance as it cools to the superfluid critical temperature and directly see how it switches from an ordinary fluid with a boring thermal equilibrium to a superfluid with heat sliding back and forth."

The team says observing this strange phenomenon could help scientists better understand the conductivity of heat in rarer states of matter, including superconductors and neutron stars, and in turn help them design better systems.

The research was published in the journal Science.