Phonons are collective atomic vibrations or quasiparticles that serve as the main heat medium for crystal lattices. Under certain circumstances, their properties can be altered by electric or light. But until now, no one was aware that it could react to magnetic fields.
Maybe it’s because you need a strong magnet.
Scientists at Rice University, led by physicist Junichiro Kono and postdoc Andrey Badin, have caused unexpected effects on completely non-magnetic semiconductor crystals of lead and tellurium (PbTe).They exposed a small sample to a strong magnetic field and discovered that they could manipulate the “soft” optics of the material. Phonon mode.
Unlike acoustic phonons, which can be understood to move atoms synchronously, generate sound waves, and affect the thermal conductivity of a material, optical phonons are represented by adjacent atoms that oscillate in opposite directions and are excited by light. Will be done. Therefore, the “optical” tag.
Experiments have revealed the phonon magnetic circular dichroism of the material. This is the opposite of the left-handed magnetic field exciting the right-handed phonons under a relatively low (9 tesla) magnetic field. (By comparison, refrigerator magnets are 5 millitesla, or 45,000 times weaker.)
Pumping the magnetic field to 25 Tesla results in a Zeeman split of the sample. In the Zeeman division, spectral lines are separated like light passing through a prism, but in a magnetic field, they are an important function of nuclear magnetic resonance devices. The line also showed an overall shift with the magnetic field. They reported that these effects were much stronger than expected by theory.
“This study reveals a new way to control phonons,” Kono said of the study. Physical review letter.. “No one expected phonons to be controllable by magnetic fields, as phonons usually do not react to magnetic fields at all unless the crystals are magnetic.”
This discovery was made possible by RAMBO (Rice Advanced Magnet with Broadband Optics), a tabletop spectrometer in Kono’s laboratory that can cool materials and expose them to high magnetic fields. By hitting a sample with a laser, researchers can track the movement and behavior of electrons and atoms in a material.
In this case, the alternating atoms react differently under a set of conditions imposed by RAMBO (low temperature, magnetization, triggered by terahertz waves). The spectrometer senses the absorption of polarization by phonons.
“The magnetic field causes these ions to oscillate in a circular orbit,” said Badin, co-lead author of Researcher Kono. “As a result, the effective magnetic moments of these phonons are very large.
“There is no resonance Interaction It is impossible because it is between phonons and electrons in a high magnetic field. Electronic “It caused the magnetic response of phonons,” he said. “Surprisingly, phonons themselves appear to respond directly to magnetic fields that they have never seen before and did not think they were possible.” ..
Kono said the discovery hasn’t been applied yet, but he thinks it will be interesting to quantum engineers. “I think this amazing discovery will have long-term implications for quantum phonons, because there are ways to control phonons using phonons. magnetic field,” He said.
Felix Hernandez of the University of Sao Paulo, Brazil, and Martin Rodriguez-Vega of the Los Alamos National Laboratory are co-lead authors of this paper. The co-authors are Anderson Okazaki, Paulo Lapple and Eduard Bramov of the National Institute for Space Research. Fuyang Tay, a graduate student in applied physics, and Timothy Noe of Rice, a graduate; Ikufumi Katayama and Atsushi Takeda of Yokohama National University. Hiroyuki Nojiri of Tohoku University; Gregory Fiete of Northeastern University and Massachusetts Institute of Technology.
Andrey Baydin et al, Magnetic Control of Soft Chiral Phonons in PbTe, Physical review letter (2022). DOI: 10.1103 / PhysRevLett.128.075901
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