It's fascinating to know that heat behaves differently in certain states of matter. Just like a sound wave that echoes through a room, heat can also move in a wave-like manner. Physicists refer to this phenomenon as the "second sound." It's like a pot of hot water radiating heat and warming up the surrounding environment over time. Isn't science amazing? Let's explore more together and uncover the wonders of the universe!
The Second Sound Phenomenon
The traces of the second sound have been detected in a limited number of materials. Recently, MIT physicists have successfully captured direct images of the second sound, a groundbreaking achievement in the field of physics.
The images obtained shed light on how heat can move like a wave, and “slosh” back and forth, even when a material’s physical matter might be moving in an entirely different direction. These images document the independent movement of heat, detached from the movement of a material’s particles.
In the words of Assistant Professor Richard Fletcher, “It’s like having a tank of water with one half nearly boiling. Despite the water appearing calm, the heat moves back and forth, with one side becoming hot after another”.
The Role of Superfluid
The research team, led by Martin Zwierlein, the Thomas A. Frank Professor of Physics, visualized the second sound in a superfluid. A superfluid is a distinct state of matter that emerges when a cloud of atoms is cooled to extremely low temperatures, leading the atoms to flow like a friction-free fluid.
In this superfluid state, theorists have predicted that heat should also flow like a wave. However, this phenomenon had not been observed directly until this groundbreaking study. The results, published in the highly esteemed journal Science, contribute significantly to our understanding of how heat moves through superfluids and other similar materials, like superconductors and neutron stars.
The Connection with Superconductors and Neutron Stars
Zwierlein explained the broader implications of their study, stating, “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 ultra-dense neutron stars.”
The researchers' ability to probe the temperature response of their system provides insights into phenomena that are typically challenging to understand or reach. The study was a collaborative endeavor, with contributions from Zhenjie Yan, Parth Patel, Biswaroop Mikherjee, former physics graduate students at MIT, and Chris Vale at Swinburne University of Technology in Melbourne, Australia.
Super Sound in Rare States of Matter
When clouds of atoms are cooled to temperatures close to absolute zero, they can transition into rare states of matter. Zwierlein’s group at MIT has been investigating the exotic phenomena that emerge among ultra-cold atoms, and specifically fermions—particles that usually avoid each other.
Under certain conditions, however, fermions can interact strongly and pair up. In this coupled state, fermions can flow in unconventional ways. For their latest experiments, the team used fermionic lithium-6 atoms, which were trapped and cooled to nanokelvin temperatures.
Superfluidity – a Two-Fluid Model
In 1938, physicist László Tisza proposed a two-fluid model for superfluidity—suggesting 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 unique temperature waves, which physicist Lev Landau later named “second sound”.
Since a fluid transitions into a superfluid at a certain critical, ultra-cold 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.
Tuning in to the Movement of Heat
The research team developed a novel method of thermography—a heat-mapping technique—to isolate and observe the wave-like movement of heat, independent of the physical motion of fermions in their superfluid.
In typical materials, infrared sensors would be used to image heat sources. But at ultra-cold temperatures, gases do not emit infrared radiation. Instead, the team developed a method using 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: Warmer cloud regions resonate at a higher frequency, while colder regions resonate at a lower frequency.
Visualizing the Motion of Heat
The researchers applied the higher resonant radio frequency, which prompted any normal, “hot” fermions in the liquid to ring in response. This allowed the researchers to focus on the resonating fermions and track them over time, creating “movies” that revealed heat’s pure motion—a sloshing back and forth, similar to waves of sound.
“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 explained.
This experiment marks the first time that scientists have been able to directly image the second sound and the pure motion of heat in a superfluid quantum gas.
Future Directions
The researchers plan to extend their work to more precisely map heat’s behavior in other ultra-cold gases. Then, they believe their findings can be scaled up to predict how heat flows in other strongly interacting materials, such as in high-temperature superconductors, and in neutron stars.
“Now we will be able to measure precisely the thermal conductivity in these systems, and hope to understand and design better systems,” Zwierlein concluded.
Reference :
Zhenjie Yan et al, Thermography of the superfluid transition in a strongly interacting Fermi gas, Science (2024). DOI: 10.1126/science.adg3430.
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