Phonons are collective atomic vibrations or quasiparticles that act as the main heat carriers in a crystal lattice. Their properties can sometimes be changed by electric fields or light. But so far nobody has noticed that they can also react to magnetic fields.
This may be because it needs a strong magnet.
Rice University scientists led by physicist Junichiro Kono and postdoctoral researcher Andrey Baydin triggered the unexpected effect in a completely nonmagnetic semiconducting lead and tellurium (PbTe) crystal. They exposed the small sample to a strong magnetic field and found that they could manipulate the material̵
7;s “soft” optical phonon mode.
In contrast to acoustic phonons, which can be understood as atoms moving synchronously, generating sound waves and affecting the thermal conductivity of a material, optical phonons are represented by neighboring atoms vibrating in opposite directions and can be excited by light. Hence the “optical” label.
Experiments revealed the material’s phononic magnetic circular dichroism, a phenomenon whereby left-handed magnetic fields excite right-handed phonons and vice versa, at relatively low (9 Tesla) magnetic fields. (By comparison, a fridge magnet is 5 millitesla, or 45,000 times weaker.)
Pumping the field to 25 Tesla caused the probe to undergo Zeeman splitting, in which spectral lines separate like light through a prism, but in a magnetic field, a critical feature in nuclear magnetic resonance devices. The lines also showed an overall shift with the magnetic field. They reported that these effects were much stronger than expected from theory.
“This work shows a new way of controlling phonons”, Kono said about the study, which appears in Physical Verification Letters. “Nobody expected that phonons could be controlled by a magnetic field because phonons don’t usually respond to magnetic fields at all unless the crystal is magnetic.”
The discovery was made possible by RAMBO (the Rice Advanced Magnet with Broadband Optics), a benchtop spectrometer in Kono’s lab that can be used to cool materials and expose them to high magnetic fields. By hitting the sample with lasers, researchers can track the movement and behavior of electrons and atoms inside the material.
In this case, the alternating atoms react differently under the conditions imposed by RAMBO – low temperature, magnetized and triggered by terahertz waves. The spectrometer records the absorption of polarized light by the phonons.
“The magnetic field forces these ions to oscillate in a circular orbit,” said co-lead author Baydin, a postdoctoral fellow in Kono’s lab. “The result is that the effective magnetic moment of these phonons is very large.
“There are no resonant interactions between phonons and electrons in high magnetic fields, so it is impossible that electrons caused the magnetic response of phonons,” he said. “What is surprising is that the phonons themselves seem to respond directly to the magnetic field, which humans had never seen before and didn’t think was possible.”
Kono said the application of the discovery remains to be seen, but he suspects it will be of interest to quantum technologists. “I think this surprising discovery has long-term implications for quantum phononics because now there is a way to control phonons using a magnetic field.” he said.
Felix Hernandez from the University of São Paulo, Brazil, and Martin Rodriguez-Vega from Los Alamos National Laboratory are co-lead authors of the publication. Co-authors are Anderson Okazaki, Paulo Rappl and Eduardo Abramof from the National Institute for Space Research, São Paulo, Brazil; Fuyang Tay, PhD student in applied physics, and alumnus Timothy Noe von Rice; Ikufumi Katayama and Jun Takeda from Yokohama National University, Japan; Hiroyuki Nojiri from Tohoku University, Japan; and Gregory Fiete of Northeastern University and the Massachusetts Institute of Technology.
Kono is Karl F. Hasselmann Professor of Engineering and Professor of Electrical Engineering and Information Technology, Physics and Astronomy, and Materials Science and Nanotechnology.
Research was funded by the National Science Foundation (1720595), op [email protected] Collaborative Grant, São Paulo Research Foundation (2015/16191-5, 2018/06142-5) and National Council for Scientific and Technological Development (307737/2020-9), Los Alamos Laboratory Directed Research and Development Program, US Department of Energy and the Japan Society for the Promotion of Science (20H05662).
Source: https://www.rice.edu/