The study has shown that phonons, the elementary particles that transmit both heat and sound, have magnetic properties. It implies that a strong magnetic field could be used to steer and control sound waves in the future.
Heat and sound are essentially the same form of energy, focused around the vibration of atoms, so by controlling one you can usually gain control of the other too.
The researchers published their report in the March 23 issue of the Nature Materials journal. It was funded by donors including the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research and the National Science Foundation.
The report describes how a magnetic field of around the same size of a medical MRI scanner reduced the amount of heat that could flow through a semiconductor by 12%. The semiconductor had to be chilled to -450 degrees Fahrenheit (-268 degrees Celsius) to slow the movements of the atoms in the conducting material. Close to absolute zero, this temperature made the movements of the phonons detectable.
Led by Ohio State postdoctoral researcher Hyungyu Jin, a piece of indium antimonide semiconductor was shaped into a tuning fork. One arm was 4mm wide and the other arm was 1mm wide. Heaters attached to the end of both arms.
The low temperatures of the experiment meant that the size of the semiconductor sample being tested became important as well as what atoms it was made of. A larger sample could transfer heat more quickly than a smaller sample so the larger arm of the tuning fork could transfer more heat than the smaller arm.
Jin measured the temperature change in both arms of the tuning fork and subtracted one from the other, alternately with and without a 7-tesla magnetic field turned on, akin to those used in hospitals.
When the magnetic field was disabled, the larger arm of the tuning fork transferred more heat as expected. With the magnetic field, the heat flow through the larger arm slowed by 12% though.
The researchers found that the magnetic field had caused some of the phonons passing through the material to vibrate out of sync and collide with each other. In the larger arm, more collisions were experienced because of the increased area so more phonons were lost and less heat was transferred.
The study concludes that phonons must have magnetic properties and that heat and sound could be controlled magnetically in substances such as glass and plastic that are ordinarily magnetic, if a powerful enough magnet could be found. The effect isn’t possible in metals, though: so much heat is transferred by electrons anyway that the contribution of the phonons would be essentially undetectable.
