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Experiment Reveals Mysterious Properties of Sound Waves

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The design worked because of a quirk in the behavior of the semiconductor at low temperatures. Normally, a material’s ability to transfer heat would depend solely on the kind of atoms of which it is made. But at very low temperatures, such as the ones used in this experiment, another factor comes into play: the size of the sample being tested. Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. That means that the larger arm of the tuning fork could transfer more heat than the smaller arm.

Heremans explained why: “Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track. The runners who take the narrow side of the fork barely have enough room to squeeze through, and they keep bumping into the walls of the track, which slows them down. The runners who take the wider track can run faster, because they have lots of room."

“All of them end up passing through the material - the question is how fast,” he continued. “The more collisions they undergo, the slower they go.”

In the experiment, Jin measured the temperature change in both arms of the tuning fork and subtracted one from the other, both with and without a 7-tesla magnetic field turned on.

In the absence of the magnetic field, the larger arm on the tuning fork transferred more heat than the smaller arm, just as the researchers expected. However, in the presence of the magnetic field, heat flow through the larger arm slowed down by 12 percent.

So what changed? Heremans said that the magnetic field caused some of the phonons passing through the material to vibrate out of sync so that they bumped into one another, an effect identified and quantified through computer simulations performed by Nikolas Antolin, Oscar Restrepo and Wolfgang Windl, all of Ohio State’s Department of Materials Science and Engineering.

In the larger arm, the freedom of movement worked against the phonons - they experienced more collisions. More phonons were knocked off course, and fewer - 12 percent fewer - passed through the material unscathed.

The phonons reacted to the magnetic field, so the particles must be sensitive to magnetism, the researchers concluded. Next, they plan to test whether they can deflect sound waves sideways with magnetic fields.

Co-authors on the study included Stephen Boona, a postdoctoral researcher in mechanical and aerospace engineering; and Roberto Myers, an associate professor of materials science and engineering, electrical and computer engineering and physics.

Funding for the study came from the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research and the National Science Foundation (NSF), including funds from the NSF Materials Research Science and Engineering Center at Ohio State. Computing resources were provided by the Ohio Supercomputer Center.

* Pam Frost Gorder, Assistant director of research and innovation communications of The Ohio State University

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