In a new study, researchers used an electric field to switch the chirality of phonons within a ferroelectric crystal. The work could lead to the creation of faster and more energy-efficient spintronic devices.
Chiral phonons are groups of atoms that move in a circular direction when excited by an energy source, such as heat. As the phonons move through a material, they propagate that circular motion, or angular momentum, through the material. The angular momentum serves as the source of spin, and the chirality dictates the direction of the spin, enabling spin control in spintronics.
Chirality, in simple terms, means that a molecule or material cannot be superimposed on its mirror image – think of your left and right hands, for example. A left-handed glove does not fit on your right hand, and vice-versa.
“What we have demonstrated here is the ability to switch the ‘handedness’ within a specific ferroelectric material by applying an electric field to it,” says Xiaotong Li, assistant professor of chemistry at North Carolina State University and co-corresponding author of the research. “Previous to this work, chiral phonons have been observed with different techniques, but the switching and active control of chiral phonons has not been demonstrated.”
The researchers used a ferroelectric molecular crystal, triglycine sulfate (TGS), in their experiments. TGS was selected because its structural chirality and ferroelectric polarization are coupled, meaning that if one changes, the other must as well.
When TGS is cooled down, it changes from the paraelectric phase where all the dipoles are randomly oriented to the ferroelectric phase, in which the dipoles become aligned to generate a spontaneous polarization. The researchers applied an electric field during the ferroelectric phase, which caused the dipole moments to align in one direction. The polar and chiral directions can also be switched under the electric field, achieving active control of the phonon chirality and spin.
Using Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE), the researchers measured spins on the surface and their polarization direction, to determine whether the spin polarization direction reversed when chirality was switched.
They saw a chain reaction – as the electrical field was applied, the polarization and chirality of TGS was switched, which in turn affected the spin direction of the phonons.
“We found that ferroelectricity controls structural chirality, chirality activates chiral phonons under heat flow, and chiral phonons transfer angular momentum to electronic spins, thereby enabling electrical control of spin,” Li says.
“The experimental results were supported by density functional theory simulations, which reveal the atomic motions behind the signal,” says Yi Xia, assistant professor of mechanical and materials engineering at Portland State University and co-corresponding author of the work. “When TGS switches between its ferroelectric states, key glycine phonon modes reverse their circular motion, providing a microscopic picture of switchable phonon chirality.”
“If you switch the handedness, or chirality, you can control the spin direction – essentially providing an electrical control knob for spin,” says Xiang-Bin Han, co-first author of the work and a postdoctoral researcher in the Li group at NC State. “From a device standpoint, if an electric field – the primary control parameter in solid-state spintronic systems – can regulate phonon chirality via electron-phonon coupling, it may eliminate the need for additional control mechanisms that would otherwise increase device complexity.”
The work opens opportunities for chiral-phonon-driven spintronic and photonic devices, the researchers say, and this method can be easily adapted to work with technologies already in use in the field.
The research appears in Nature Communications and was supported by the Air Force Office of Scientific Research under grant numbers FA9550-23-1-0311 and RX23COR003; the National Science Foundation (award numbers DMR-2521953, DMR-2317008, and CBET-2445361). Co-corresponding authors of the work are: Xiaotong Li, assistant professor of chemistry and member of the Organic and Carbon Electronics Lab (ORaCEL) at NC State; Dali Sun, professor of physics and member of ORaCEL at NC State; Jun Liu, associate professor of mechanical and aerospace engineering at NC State and member of ORaCEL; Yi Xia, assistant professor of mechanical and materials engineering at Portland State University. Xiang-Bin Han from chemistry and Cong Yang from mechanical and aerospace engineering are the co-first authors of this work. Other NC State contributors include Rui Sun, Xiaotong Zhang, Zhengze Xu, Aryan Jouneghaninaseri, and Xiaoning Jiang. Thuc Mai and Rahul Rao from the Air Force Research Laboratory also contributed to the work.
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