First-ever images of heat ‘sloshing’ like sound waves captured by MIT in a superfluid

First-ever images of heat 'sloshing' like sound waves captured by MIT in a superfluid

First-ever images of heat ‘sloshing’ like sound waves captured by MIT in a superfluid

Heat is usually thought of as something that spreads and dissipates, but in some exotic states of matter, it can behave like a wave, bouncing back and forth like sound.

Visualizing heat waves in a superfluid

This phenomenon, known as “second sound,” has been observed in only a few materials. Now, for the first time, physicists at MIT have captured direct images of the second sound in action.

The images show how heat can move like a wave, independent of the material’s physical matter, which may move differently. The researchers used a superfluid, a state of matter where atoms flow without any friction, as a medium to visualize the second sound. They found that heat and matter can “slosh” against each other, creating oscillations similar to sound waves.

“It’s as if you had a tank of water and made one half nearly boiling,” says Richard Fletcher, an assistant professor of physics at MIT and a study co-author. “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 still.”

The study, published in Science, could help scientists understand how heat moves in superfluids and other related materials, such as superconductors and neutron stars. These materials exhibit unusual and fascinating properties, such as zero resistance and infinite conductivity, that could enable new technologies and applications.

First-ever images of heat 'sloshing' like sound waves captured by MIT in a superfluid

“There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars,” says Martin Zwierlein, the Thomas A Frank Professor of Physics at MIT and the leader of the research team. “Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach.”

First-ever images of heat 'sloshing' like sound waves captured by MIT in a superfluid

Sound can help us understand superfluids and other materials

The researchers used a cloud of lithium-6 atoms, which are fermions, a type of particle that normally repels each other. By cooling the atoms to near absolute zero and applying a magnetic field, they induced the atoms to pair up and form a superfluid. They then used a laser beam to create a hotspot in the superfluid and another laser beam to image the resulting heat waves.

They observed that the heat waves moved in a periodic fashion, similar to sound waves, and were out of phase with the matter waves, meaning that the heat and matter were oscillating in opposite directions. This is a signature of a second sound, distinct from ordinary sound, where heat and matter move together.

The second sound was first predicted by the physicist László Tisza in 1938, who proposed a two-fluid model for superfluidity — that a superfluid is a mixture of some normal, viscous fluid and a friction-free superfluid. This mixture of two fluids should allow for two types of sound, ordinary density waves and peculiar temperature waves, which physicist Lev Landau later named “second sound.”

Since fluid transitions into a superfluid at a certain critical, ultracold temperature, the MIT team reasoned that the two types of fluid should also transport heat differently: In normal fluids, heat should dissipate as usual, whereas in a superfluid, it could move as a wave, similarly to sound.

“Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it,” Zwierlein says. “The character of the heat wave could not be proven before.”

To prove it, Zwierlein and his team sought to isolate and observe the second sound, the wave-like movement of heat, independent of the physical motion of fermions in their superfluid. They did so by developing a new method of thermography — a heat-mapping technique. In conventional materials, one would use infrared sensors to image heat sources.

But at ultracold temperatures, gases do not give off infrared radiation. Instead, the team developed a method to use radio frequency to “see” how heat moves through the superfluid. They found that the lithium-6 fermions resonate at different radio frequencies depending on their temperature: When the cloud is at warmer temperatures and carries more normal liquid, it resonates at a higher frequency. Regions in the cloud that are colder resonate at a lower frequency.

The researchers applied the higher resonant radio frequency, which prompted any normal, “hot” fermions in the liquid to ring in response. The researchers then could zero in on the resonating fermions and track them over time to create “movies” that revealed heat’s pure motion — a sloshing back and forth, similar to sound waves.

“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,” Zwierlein says.

The experiments mark the first time scientists have been able to image second sound directly and the pure motion of heat in a superfluid quantum gas. The researchers plan to extend their work to map heat’s behavior more precisely in other ultracold gases. Then, they say their findings can be scaled up to predict how heat flows in other strongly interacting materials, such as high-temperature superconductors and neutron stars.

Source: Interesting Engineering

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First-ever images of heat ‘sloshing’ like sound waves captured by MIT in a superfluid/First-ever images of heat ‘sloshing’ like sound waves captured by MIT in a superfluid /First-ever images of heat ‘sloshing’ like sound waves captured by MIT in a superfluid

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