Scientists at MIT have directly captured signs of “second sound” in a superfluid for the first time. This bizarre phenomenon occurs when heat moves like sound waves through an unusual state of matter.
In our everyday experience of heat, the energy dissipates to its surroundings. A hotter object will cool to the temperature of other materials nearby, warming them up at the same time, until an equilibrium is reached.
But in unconventional materials, physics can operate in counter-intuitive ways. Superfluids are a rare state of matter with zero viscosity, meaning the substance can flow without any resistance or friction at all. It’s long been predicted that heat should be able to move through a superfluid kind of like sound waves – hence the name “second sound” – but it hadn’t been directly observed until now.
“It’s as if you had a tank of water and made one half nearly boiling,” said Assistant Professor Richard Fletcher, an author of the study. “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.”
To image the phenomenon, the researchers had to create an entirely new way of detecting heat. Normally, infrared sensors would be used, but creating a superfluid involves cooling a quantum gas almost to absolute zero, and infrared radiation isn’t emitted at such low temperatures. So, the team turned to radio instead.
The quantum gas the researchers used was made up of lithium-6 fermions, and it was found that the warmer these fermions were, the higher the frequency they resonated at. The team applied the higher radio frequency to the gas, which would cause the hotter fermions in it to resonate in response. By tracking which ones resonated at different times, the scientists were able to image the “second sound” as the heat waves oscillated back and forth.
“For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth,” said Martin Zwierlein, lead author of the study.
The team says that observing this weird phenomenon could help scientists better understand the thermal conductivity of rarer states of matter, including superconductors and neutron stars, which in turn could let them design better systems.
The research was published in the journal Science.
Source: MIT
