Imaging the second sound at MIT opens new avenues for understanding the wave-like behavior of heat in superfluids and its effects on different states of matter, expanding scientists’ understanding of heat flow in superconductors and neutron stars.

In most materials, heat prefers to dissipate. If the hot spot is left alone, it will gradually fade away and its surroundings will warm up. But in rare states of matter, heat can behave like a wave that moves back and forth, similar to a sound wave jumping from one end of a room to the other. In fact, this wave-like heat is called “second sound” by physicists.

Signs of the second sound were seen in only a few materials. Now MIT physicists have taken direct images of the second sound for the first time.

The new images show how heat can travel like a wave and “slap” back and forth, even though the physical substance of the material is moving in a completely different direction. The images capture the net movement of heat regardless of the particles of the material.

Associate Professor Richard Fletcher makes the analogy: “It’s like you have a tank of water and half of it is almost boiling.” “If you observe it later, the water itself may appear completely still, but suddenly one side gets hot, then the other side gets hot, and the heat goes back and forth while the water appears completely still.”

The team, led by Thomas A. Frank Professor of Physics Martin Zwirlein, visualized second sound in superfluid matter, a special state of matter that causes atoms to flow when a cloud of atoms cools to extremely low temperatures. like a frictionless fluid. In this superfluid state, theorists predicted that heat should also flow as a wave, although scientists have so far been unable to directly observe this phenomenon.

New results recently published in the journal ScienceIt will help physicists get a more comprehensive picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars.

“There is a strong connection between our 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 Zwirlein. “We can now test our system’s response to temperature in a rudimentary way, which teaches us things that are very difficult to understand or even achieve.”

Zwirlein and Fletcher are co-authors of the study, including first author and former physics graduate student Zhenjie Yan and former physics graduate students Parth Patel and Biswarup Mukherjee, as well as Chris Weil of Swinburne University of Technology in Melbourne, Australia. The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).

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When clouds of atoms are cooled to temperatures near absolute zero, they can collapse into rarer states of matter. Zwirlein’s group at MIT investigates exotic events that occur between ultracold atoms, particularly fermions (particles such as electrons that normally avoid each other).

But under certain conditions fermions can interact and couple strongly. In this bound state, fermions can flow in unusual ways. In their latest experiments, the team uses lithium-6 fermionic atoms trapped and cooled to nanokelvin temperatures.

In 1938, physicist László Tisa proposed the two-fluid model of superfluidity; The superfluid was actually a mixture of a normal viscous liquid and a frictionless superfluid. This mixing of the two fluids should allow for two types of sound, ordinary density waves and special temperature waves, which physicist Leo Landau later called “second sound”.

Because the liquid becomes superfluid at a certain critical ultra-cold temperature, the MIT team concluded that the two types of liquids must transport heat differently: In ordinary liquids, heat must dissipate normally, while in superfluids it can travel in a sound-like wave. .

“The second sound is a sign of superfluidity, but so far in extremely cold gases you have only been able to see it in this faint reflection of the density fluctuations that accompany it,” says Zwirlein. “The nature of the heat wave has not been proven before.”

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Zwirlein and his team tried to isolate and observe the second sound, the wave-like movement of heat, independent of the physical motion of fermions in superfluids. They did this by developing a new method of thermography, heat mapping. In traditional materials, infrared sensors can be used to image heat sources.

However, at very low temperatures, gases do not emit infrared radiation. Instead, the team developed a method that uses radio frequencies to “see” how heat moves through a superfluid. They found that lithium-6 fermions resonate at different radio frequencies depending on their temperature: When a cloud is at higher temperatures and carries a more ordinary liquid, it resonates at a higher frequency. Colder regions in the cloud resonate at a lower frequency.

The researchers applied a higher resonant radio frequency, which triggered the ringback of any normal, “hot” fermions in the liquid. The researchers were then able to focus on the resonating fermions and track them over time to create “movies” that revealed pure movement of heat (a back-and-forth flapping similar to sound waves).

“For the first time, we can photograph this material as it cools beyond its critical superfluid temperature and see directly how it goes from a normal liquid, where heat is opaquely balanced, to a superfluid where heat flows back and forth.” says Zwirlein.

The experiments mark the first time scientists have been able to directly image secondary sound and the pure motion of heat in a superfluid quantum gas. The researchers plan to expand their work to more accurately map thermal behavior in other ultracold gases. They say their discovery can then be extended to predict how heat flows in other strongly interacting materials, such as high-temperature superconductors and neutron stars.

“We will now be able to accurately measure thermal conductivity in these systems and hope to understand and design better systems,” Zwirlein concludes.