In most materials, heat prefers to disperse. If left alone, a hot spot will gradually disappear as it warms the room. But in rare states of matter, heat can behave like a wave, moving back and forth like a sound wave bouncing from one end of a room to the other. In fact, this wavelike heat is what physicists call a “second sound.”
Second sound signals were observed in only a few materials. Now, MIT physicists have captured direct images of the second sound for the first time.
The new images reveal how heat can move like a wave and “sway” back and forth, even though the physical matter of a material can move in an entirely different way. The images capture the pure movement of heat, independent of the material’s particles.
“It’s like you have a tank of water and half of it is almost boiling,” offers assistant professor Richard Fletcher as an analogy. “If you notice, the water itself may appear completely calm, but suddenly the other side becomes hot, and then the other side becomes hot, and the heat comes and goes, while the water appears completely still.”
Led by Martin Zwierlein, Thomas A Frank professor of physics, the team visualized the second sound in a superfluid – a special state of matter that is created when a cloud of atoms is cooled to extremely low temperatures, at which point the atoms begin to Flow. as a completely friction-free fluid. In this superfluid state, theorists predicted that heat should also flow like a wave, although scientists have not been able to directly observe the phenomenon until now.
The new results, reported today in the journal Sciencewill help physicists get a more complete picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars.
“There are strong links between our gas cloud, 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 Zwierlein. “We can now precisely probe the temperature response of our system, which teaches us things that are very difficult to understand or even achieve.”
Zwierlein and Fletcher’s co-authors on the study are first author and former physics postgraduate student Zhenjie Yan and former physics postgraduate students Parth Patel and Biswaroop Mikherjee, along with Chris Vale at Swinburne University of Technology in Melbourne, Australia. MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).
When clouds of atoms are reduced to temperatures close to absolute zero, they can transition into rare states of matter. Zwierlein’s group at MIT is exploring the exotic phenomena that emerge between ultracold atoms and, specifically, fermions—particles, like electrons, that normally avoid each other.
Under certain conditions, however, fermions can interact strongly and form pairs. In this coupled state, fermions can flow in unconventional ways. For their latest experiments, the team employs fermionic lithium-6 atoms, which are trapped and cooled to nanokelvin temperatures.
In 1938, physicist László Tisza proposed a two-fluid model for superfluidity—that a superfluid is actually a mixture of some normal, viscous fluid and a frictionless superfluid. This mixture of two fluids was supposed to allow two types of sound, ordinary density waves and peculiar temperature waves, which physicist Lev Landau later called “second sound.”
Because a fluid transitions to a superfluid at a certain critical, ultracold temperature, the MIT team argued that the two types of fluid should also transport heat differently: in normal fluids, heat should dissipate normally, while in a Superfluid he could move like 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 heat wave could not be proven before.”
Zwierlein and his team sought to isolate and observe the second sound, the wavelike motion of heat, independent of the physical motion of fermions in their superfluid. They did this by developing a new thermography method – a heat mapping technique. In conventional materials, infrared sensors would be used to image heat sources.
But at ultra-low temperatures, gases do not emit infrared radiation. Instead, the team developed a method to use radiofrequency 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 higher temperatures and carries more normal liquid, it resonates at a higher frequency. Regions of the cloud that are colder resonate at a lower frequency.
The researchers applied the highest resonant radio frequency, which caused any normal, “hot” fermions in the liquid to chirp in response. The researchers were then able to focus on the resonant fermions and track them over time to create “movies” that revealed the pure motion 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 through the critical temperature of superfluidity and directly see how it transitions from a normal fluid, where the heat evens out boringly, to a superfluid where the heat oscillates forward. and back. ,” says Zwierlein.
The experiments mark the first time that scientists have been able to directly image the second sound and pure heat movement in a superfluid quantum gas. The researchers plan to expand their work to more accurately map the behavior of heat in other ultracold gases. Then, they say their findings can be expanded 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,” concludes Zwierlein.
This work was supported by the National Science Foundation (NSF), the Air Force Office of Scientific Research, and the Vannevar Bush Faculty Fellowship. The MIT team is part of the MIT-Harvard Center for Ultracold Atoms (an NSF Physics Frontier Center) and is affiliated with the MIT Department of Physics and the Research Laboratory of Electronics (RLE).