Physicists capture first sounds of heat in superfluid, revealing how heat can move like a wave

In most materials, heat prefers to disperse. If left alone, a hot spot will gradually disappear as it warms its surroundings. But in rare states of matter, heat can behave like a wave, moving back and forth much like a sound wave bouncing from one end of a room to the other. In fact, this wave heat is what physicists call the “second sound.”

Signs of second sound have only been observed in a handful of materials. Now, MIT physicists have captured direct images of a second sound for the first time.

The new images reveal how heat can move like a wave and “lap” back and forth, even though the physical matter of a material may move in an entirely different way. The images capture the pure movement of heat, independent of particles in a material.

“It’s like you have a tank of water and you almost boil half of it,” Assistant Professor Richard Fletcher offers by analogy. “If you then look, the water itself may appear totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat comes and goes, while the water seems totally motionless.”

Led by Martin Zwierlein, the Thomas A Frank Professor of Physics, the team visualized a second sound in a superfluid, a special state of matter created when a cloud of atoms is cooled to extremely low temperatures, at which point the atoms start to circulate. as a completely frictionless fluid. In this superfluid state, theorists predicted that heat should also flow like a wave, although scientists had not been able to directly observe the phenomenon until now.

The new results, reported in the journal Sciencewill help physicists get a more complete picture of how heat moves through superfluids and other associated materials, including superconductors and neutron stars.

“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,” explains Zwierlein. “We can now perfectly probe the thermal response of our system, which tells us things that are very difficult to understand or even achieve.”

Co-authors of Zwierlein and Fletcher’s study are first author and former physics graduate student Zhenjie Yan, and former physics graduate students Parth Patel and Biswaroop Mikherjee, as well as Chris Vale of the University of Technology. Swinburne in Melbourne, Australia. The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).

Great sound

When clouds of atoms are brought to temperatures near absolute zero, they can transition to rare states of matter. Zwierlein’s group at MIT explores exotic phenomena that emerge among ultracold atoms, particularly fermions, particles like electrons that normally avoid each other.

Under certain conditions, however, fermions can interact strongly and pair. In this coupled state, fermions can flow in an unconventional way. For their latest experiments, the team uses fermionic lithium-6 atoms, which are trapped and cooled to nanokelvin temperatures.

In 1938, physicist László Tisza proposed a two-fluid model for superfluidity: a superfluid is actually a mixture of a normal, viscous fluid and a frictionless superfluid. This mixture of two fluids should allow the production of two types of sounds, ordinary density waves and special temperature waves, which the physicist Lev Landau would later call “second sound”.

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

“The second sound is the hallmark of superfluidity, but until now in ultracold gases you could only see it in this faint reflection of the accompanying density ripples,” says Zwierlein. “The character of the heatwave could not be proven before.”

Tuning

Zwierlein and his team sought to isolate and observe a second sound, the wave motion of heat, independent of the physical motion of fermions in their superfluid. To do this, they developed a new thermography method: a thermal mapping technique. In conventional materials, infrared sensors would be used to image heat sources.

But at ultracold temperatures, gases do not emit infrared radiation. Instead, the team developed a method to use radio frequency to “see” how heat moves through the superfluid. They found that 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. Colder regions of the cloud resonate at a lower frequency.

The researchers applied a higher resonance radio frequency, which caused all the normal, “hot” fermions in the liquid to ring in response. The researchers were then able to focus on the resonant fermions and track them over time to create “movies” revealing the pure movement of heat – a back-and-forth movement similar to sound waves.

“For the first time, we can take pictures of this substance as we cool it to the critical temperature of superfluidity, and see directly how it goes from being a normal fluid, where the heat balances in a boring way, to a superfluid where heat flows back and forth,” says Zwierlein.

These experiments mark the first time that scientists have managed to directly image a second sound and the pure movement of heat in a superfluid quantum gas.

The researchers plan to expand their work to more precisely map the behavior of heat in other ultracold gases. Next, they say their findings can be extended to predict how heat flows in other strongly interacting materials, such as in high-temperature superconductors and neutron stars.

“We will now be able to precisely measure the thermal conductivity of these systems and hope to understand and design better systems,” concludes Zwierlein.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.