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. 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. "The more collisions they undergo, the slower they go." "All of them end up passing through the material - the question is how fast," he continued. The runners who take the wider track can run faster, because they have lots of room. 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. "Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track. That means that the larger arm of the tuning fork could transfer more heat than the smaller arm. Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. 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. Normally, a material's ability to transfer heat would depend solely on the kind of atoms of which it is made. The design worked because of a quirk in the behavior of the semiconductor at low temperatures. He planted heaters at the base of the arms.
One arm of the fork was 4 mm wide and the other 1 mm wide. His solution was to take a piece of the semiconductor indium antimonide and shape it into a lopsided tuning fork. Taking a thermal measurement at such a low temperature was tricky.
That's why the experiment was so difficult, Jin said. There won't be any practical applications of this discovery any time soon: 7-tesla magnets like the one used in the study don't exist outside of hospitals and laboratories, and the semiconductor had to be chilled to -450 degrees Fahrenheit (-268 degrees Celsius) - very close to absolute zero - to make the atoms in the material slow down enough for the phonons' movements to be detectible. The effect would go unnoticed in metals, which transmit so much heat via electrons that any heat carried by phonons is negligible by comparison. The implication: In materials such as glass, stone, plastic - materials that are not conventionally magnetic - heat can be controlled magnetically, if you have a powerful enough magnet. "We believe that these general properties are present in any solid," said Hyungyu Jin, Ohio State postdoctoral researcher and lead author of the study. This study shows that phonons have magnetic properties, too. Phonons haven't received as much attention, and so not as much is known about them beyond their properties of heat and sound. But researchers have studied photons intensely for a hundred years - ever since Einstein discovered the photoelectric effect. The name "phonon" sounds a lot like "photon." That's because researchers consider them to be cousins: Photons are particles of light, and phonons are particles of heat and sound. "It's through vibrations that I talk to you, because my vocal chords compress the air and create vibrations that travel to you, and you pick them up in your ears as sound." "Sound is the vibration of atoms, too," he continued. The hotter a material is, the faster the atoms vibrate. "Heat is conducted through materials by vibrations. "Essentially, heat is the vibration of atoms," he explained. So any force that controls one should control the other. But both are expressions of the same form of energy, quantum mechanically speaking. People might be surprised enough to learn that heat and sound have anything to do with each other, much less that either can be controlled by magnets, Heremans acknowledged. With a strong enough magnetic field, we should be able to steer sound waves, too." "We've shown that we can steer heat magnetically. "This adds a new dimension to our understanding of acoustic waves," said Joseph Heremans, Ohio Eminent Scholar in Nanotechnology and professor of mechanical engineering at Ohio State. The study is the first ever to prove that acoustic phonons - the elemental particles that transmit both heat and sound - have magnetic properties. In the March 23 issue of the journal Nature Materials, they describe how a magnetic field roughly the size of a medical MRI reduced the amount of heat flowing through a semiconductor by 12 percent.
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