Hot magnets: Homing in on phonons

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  • Published: Apr 1, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Control

A magnetic field can be used to control heat through the manipulation of acoustic phonons, the elementary particles that transmit thermal energy and sound, according to US researchers.

A magnetic field can be used to control heat through the manipulation of acoustic phonons, the elementary particles that transmit thermal energy and sound, according to US researchers.

Researchers at Ohio State University describe in the 23rd March issue of Nature Materials how they have used a magnetic field of the same approximate size as that used in a medical magnetic resonance imaging instrument and found that they could reduce the amount of heat energy flowing through a semiconductor by some 12 percent. This is the first work to show prove that acoustic phonons also possess magnetic properties.

“This adds a new dimension to our understanding of acoustic waves,” explains Joseph Heremans, Ohio Eminent Scholar in Nanotechnology and professor of mechanical engineering at OSU. “We’ve shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to control sound waves, too.”

Heat and sound

At the quantum level, heat and sound are simply the energetic vibrations of atoms, mechanically speaking - phonons are quasi-particles of heat and sound and thus kissing cousins of photons, particles of light. Of course, Einstein's focus on photons and discovery of the photoelectric effect have led to much research on photons, but their cousins have not seemed quite so interesting to "mainstream" quantum physics.

“We believe that these general properties are present in any solid,” adds post-doctoral researcher Hyungyu Jin. The implication for materials science and possible technological applications is that glass, minerals, polymers and other substances not normally considered to have conventional magnetic properties might be controlled using this effect with a sufficiently strong magnetic field. The effect would be of negligible consequence in metals because heat transmission by phonons is insignificant compared to their overall conductivity.

Presence

However, 7 Tesla magnets are uncommon outside laboratories and hospitals. Moreover, in order to observe the effect the semiconductor needs to be at a temperature of -268 degrees Celsius to slow atomic motion adequately that the phonon phenomenon can be detected. This is what made their experiments rather difficult to carry out, Jin says. Making thermal measurements at such a low temperature is tricky. As a workaround, Jin shaped the semiconductor, indium antimonide, into a lopsided tuning fork structure with one arm 4 millimetre thick and the other just 1 mm. A heating unit was placed at the base of each arm. This allowed the team to exploit conductivity differences that arise at such low temperatures because of sample size, 4 mm versus 1 mm. At -268 Celsius, the larger sample can transfer heat more quickly than the smaller one, so the thicker tuning fork arm transfers more heat than the thinner one.

Imagine that the tuning fork is an athletics track. "The phonons flowing up from the base are runners on the track," says Heremans, using a simple metaphor for a complex quantum phenomenon. "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?” The more collisions between "runners" the slower they go.

In their experiments, Jin observed the temperature change in both arms of the tuning fork with and without the magnetic field activated. 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. But in the presence of the magnetic field, heat flow through the larger arm was slowed by 12 percent. Heremans suggests that the magnetic field increases the frequency of collisions between phonons passing through the material. OSU materials scientists Nikolas Antolin, Oscar Restrepo and Wolfgang Windl identified and quantified this effect through computer simulations.

In the larger "tuning fork" 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 unaffected. The next step is to see whether the team can deflect sound waves laterally using a magnetic field.

Related Links

Nature Mater 2015, online: "Phonon-induced diamagnetic force and its effect on the lattice thermal conductivity"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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