![]() The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA). Zwierlein and Fletcher’s co-authors on the study are first author and former physics graduate student Zhenjie Yan and former physics graduate students Parth Patel and Biswaroop Mukherjee, along with Chris Vale at Swinburne University of Technology in Melbourne, Australia. “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.” “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,” Zwierlein says. The new results, reported today in the journal Science, will help physicists get a more complete picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars. Second sound is the movement of heat, in which superfluid and normal fluid “slosh” against each other, while leaving the density constant. First sound, depicted in a simple animation, is ordinary sound in the form of density waves, in which normal fluid and superfluid oscillate together. ![]() ![]() In this superfluid state, theorists have predicted that heat should also flow like a wave, though scientists had not been able to directly observe the phenomenon until now. Led by Martin Zwierlein, the Thomas A Frank Professor of Physics, the team visualized 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 like a completely friction-free fluid. “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 totally still.” “It’s as if you had a tank of water and made one half nearly boiling,” Assistant Professor Richard Fletcher offers as analogy. The images capture the pure movement of heat, independent of a material’s particles. The new images reveal how heat can move like a wave, and “slosh” back and forth, even as a material’s physical matter may move in an entirely different way. Now MIT physicists have captured direct images of second sound for the first time. Signs of second sound have been observed in only a handful of materials. In fact, this wave-like heat is what physicists call “second sound.” But in rare states of matter, heat can behave as a wave, moving back and forth somewhat like a sound wave that bounces from one end of a room to the other. If left alone, a hotspot will gradually fade as it warms its surroundings. In most materials, heat prefers to scatter.
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