Study shows for the first time that phonons can be manipulated by means of a magnetic field

Results published in Physical Review Letters by scientists at the University of São Paulo and Rice University can be used to improve thermoelectric devices.

By José Tadeu Arantes  |  Agência FAPESP – A phonon is an excitation that propagates through the crystal lattice of a solid material. In classical physics, it could be described as an elastic wave, but quantum physics is needed here because the phenomenon occurs at the atomic scale, and in this case, a phonon must be thought of as a quantum of energy that travels through the lattice, or as a quasi-particle.

The manipulation of phonons is relevant to a wide array of technological applications, including heat transport in thermoelectric devices, modification of the properties of a material, and induction of quantum effects such as superconductivity.

“There are several ways of controlling phonons, but the use of magnetic fields for this purpose wasn’t expected until we completed our study,” said Felix Hernandez, a professor at the University of São Paulo’s Physics Institute (IF-USP) in Brazil.

An investigation conducted by Hernandez in collaboration with Junichiro Kono, a professor at Rice University in Houston, Texas (USA), achieved this kind of manipulation in lead telluride (PbTe), one of the most widely used materials for thermoelectric applications. An article on the study is published in Physical Review Letters.

Methodology

Thin PbTe films were produced by the National Space Research Institute (INPE) in Brazil and submitted to extremely intense magnetic fields (above 25 tesla) in the laboratory at Rice University. A magnetic field of this strength is 5,000 times that of a fridge magnet.

“When atoms vibrate in synchrony, the phonons are like sound waves that propagate through the lattice and transfer heat. But there’s another kind of phonon. If lead telluride is hit by a laser pulse, the tellurium and lead ions that make up the crystal lattice oscillate transversely in the same direction but in opposite senses. This type of phonon is classified as optical,” Hernandez explained.

When a strong magnetic field is applied, the tellurium and lead ions, which have different masses, acquire an angular momentum, and the transverse movement becomes circular. “Circularly polarized optical phonons absorb light differentially, in a phenomenon known as magnetic circular dichroism. In addition, we observed that the frequencies of these phonons separated as a function of field size. This was the result of magnetic moment in the Zeeman interaction and of diamagnetic displacement,” Hernandez said.

The Zeeman effect was discovered by Dutch physicist Pieter Zeeman (1865-1943), who was awarded the 1902 Nobel Prize in Physics. It describes the effect of the splitting of the spectral lines of an atom due to a strong magnetic field. Diamagnetism consists of the appearance of an angular momentum oriented in the opposite sense to that of an external magnetic field. “We showed that the observed data result from morphic changes induced by the magnetic field in the symmetries of the crystal,” he said.

The study resulted in a number of important findings. To summarize, it showed that phonon oscillation is not harmonic; that phonons’ optical mode softens in the sense that their energy decreases as temperature falls; that phonons display left- and right-handed circular polarization in a magnetic field; that left and right polarization absorb light differentially, so that the material displays magnetic circular dichroism; that energy separation of the types of polarization is due to the Zeeman interaction and diamagnetic displacement; and that the Zeeman interaction is 100 times stronger than predicted by theory.

“This is the first time phonon diamagnetism has been observed,” Hernandez stressed. “The study presents proof of concept of a novel mechanism for controlling phonons that can be used to improve thermoelectric devices.”

The next step, he said, is to investigate the magnetic properties of PbTe phonons doped with tin (Sn), which is a crystalline topological insulator. 

The study was supported by FAPESP via the Thematic Project “Research on new materials involving high magnetic fields and low temperatures”, led by Gennady Gusev, with Hernandez as one of the senior associated researchers; and the project “Kerr/Faraday rotation in quantum materials”, led by Hernandez.

The article “Magnetic control of soft chiral phonons in PbTe” is at: journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.075901. It can also be found in the arXiv preprint repository: arxiv.org/abs/2107.07616#:~:text=PbTe%20crystals%20have%20a%20soft,in%20anomalously%20low%20thermal%20conductivity.