Strong magnets put new twist on phonons
Rice lab's RAMBO reveals unexpected influence on compound's crystal
lattice
Date:
February 15, 2022
Source:
Rice University
Summary:
Phonons, quasiparticles in a crystal lattice that are usually hard
to control by external fields, can be manipulated by a magnetic
field -- but it takes a very strong magnet.
FULL STORY ========================================================================== Phonons are collective atomic vibrations, or quasiparticles, that act as
the main heat carriers in a crystal lattice. Under certain circumstances,
their properties can be modified by electric fields or light. But until
now, nobody noticed they can respond to magnetic fields as well.
==========================================================================
That may be because it takes a powerful magnet.
Rice University scientists led by physicist Junichiro Kono and
postdoctoral researcher Andrey Baydin triggered the unexpected effect
in a totally nonmagnetic semiconducting crystal of lead and tellurium
(PbTe). They exposed the small sample to a strong magnetic field and
found they could manipulate the material's "soft" optical phonon mode.
Unlike acoustic phonons that can be understood as atoms moving in sync,
produce sound waves and influence a material's thermal conductivity,
optical phonons are represented by neighboring atoms oscillating in
opposite directions and can be excited by light. Hence, the "optical" tag.
Experiments revealed the material's phononic magnetic circular
dichroism, a phenomenon by which left-handed magnetic fields excite right-handed phonons and vice versa, under relatively low (9 Tesla)
magnetic fields. (By comparison, a refrigerator magnet is 5 milliTesla,
or 45,000 times weaker.) Pumping the field to 25 Tesla prompted the
sample to 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 exhibited an overall shift
with the magnetic field. They reported these effects were much stronger
than expected by theory.
========================================================================== "This work reveals a new way of controlling phonons," Kono said of the
study, which appears in Physical Review Letters. "Nobody expected that
phonons can be controlled by a magnetic field, because phonons usually
don't 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 tabletop spectrometer in Kono's lab that allows
materials to be cooled and exposed to high magnetic fields. Hitting the
sample with lasers allows researchers to track the motion and behavior
of electrons and atoms inside the material.
In this case, the alternating atoms react differently under the set of conditions -- low temperature, magnetized and triggered by terahertz waves
- - imposed by RAMBO. The spectrometer senses the phonons' absorption
of polarized light.
"The magnetic field forces these ions to oscillate in a circular orbit,"
said co-lead author Baydin, a postdoctoral researcher 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's impossible that electrons caused the magnetic
response of phonons," he said. "What's surprising is that the phonons themselves seem to be directly responding to the magnetic field, which
people hadn't seen before and didn't think was possible." Kono said the discovery's applications remain to be seen, but he suspects it will be
of interest to quantum technologists. "I think this surprising discovery
has long-term implications in quantum phononics because now there's a
way to control phonons using a magnetic field," he said.
Felix Hernandez of the University of Sa~o Paulo, Brazil, and Martin
Rodriguez- Vega of Los Alamos National Laboratory are co-lead authors
of the paper. Co- authors are Anderson Okazaki, Paulo Rappl and Eduardo
Abramof of the National Institute for Space Research, Sa~o Paulo, Brazil; applied physics graduate student Fuyang Tay and alumnus Timothy Noe of
Rice; Ikufumi Katayama and Jun Takeda of Yokohama National University,
Japan; Hiroyuki Nojiri of Tohoku University, Japan; and Gregory Fiete
of Northeastern University and the Massachusetts Institute of Technology.
Kono is the Karl F. Hasselmann Professor in Engineering and a professor
of electrical and computer engineering, of physics and astronomy and of materials science and nanoengineering.
The research was funded by the National Science Foundation (1720595),
a Brasil@Rice Collaborative Grant, the Sa~oPaulo Research Foundation (2015/16191- 5, 2018/06142-5) and the National Council for Scientific
and Technological Development (307737/2020-9), the Los Alamos Laboratory Directed Research and Development Program, the U.S. Department of Energy
and the Japan Society for the Promotion of Science (20H05662).
========================================================================== Story Source: Materials provided by Rice_University. Original written
by Mike Williams. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Andrey Baydin, Felix G. G. Hernandez, Martin Rodriguez-Vega,
Anderson K. Okazaki, Fuyang Tay, G. Timothy Noe, Ikufumi Katayama,
Jun Takeda, Hiroyuki Nojiri, Paulo H. O. Rappl, Eduardo
Abramof, Gregory A. Fiete, Junichiro Kono. Magnetic Control of
Soft Chiral Phonons in PbTe. Physical Review Letters, 2022; 128
(7) DOI: 10.1103/ PhysRevLett.128.075901 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/02/220215163423.htm
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