Researchers at North Carolina State University, Portland State University and the Air Force Research Laboratory, have demonstrated electrical, reversible control over the "handedness" of chiral phonons in a ferroelectric crystal - a capability the team says could lead to faster, more energy-efficient spintronic devices.
Chiral phonons are collective vibrations that move through a material's atomic lattice in a circular motion, carrying angular momentum that can be transferred to electron spins, giving materials a spintronic function without requiring any magnetic ordering. The NC State group previously showed this angular momentum could generate spin current from a simple thermal gradient in a 2D hybrid perovskite. Until now, though, chiral phonons had only been observed passively - the ability to actively switch their handedness on demand had not been demonstrated. For the new work, the team turned to triglycine sulfate (TGS), a molecular ferroelectric crystal in which structural chirality and ferroelectric polarization are intrinsically coupled: flipping one necessarily flips the other.
Cooling TGS below its transition temperature takes it from a centrosymmetric, non-chiral paraelectric phase into a ferroelectric phase, and applying an electric field during that transition locks the crystal's dipoles - and its chirality - into one of two mirror-image states. Circularly polarized Raman spectroscopy and density functional theory calculations confirmed that glycine-related phonon modes rotate in opposite directions in the two ferroelectric states, while showing no rotational character at all in the non-chiral paraelectric phase.
To read out the effect electrically, the researchers used time-resolved magneto-optical Kerr effect (TR-MOKE) measurements on a silver-coated TGS device, relying on the chiral-phonon-activated spin Seebeck effect to convert a laser-induced thermal gradient into a measurable spin signal at the silver surface. The Kerr rotation signal flipped sign when the crystal's poling was reversed, tracked the crystal's chirality state directly, and vanished entirely in the non-chiral paraelectric phase - while control thermoreflectance measurements ruled out any purely thermal origin for the signal.
"What we have demonstrated here is the ability to switch the handedness within a specific ferroelectric material by applying an electric field to it," said Xiaotong Li, assistant professor of chemistry at NC State and co-corresponding author of the study. "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." Co-corresponding author Yi Xia, assistant professor of mechanical and materials engineering at Portland State University, added: "The experimental results were supported by density functional theory simulations, which reveal the atomic motions behind the signal. 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," said co-first author Xiang-Bin Han, a postdoctoral researcher in Li's 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 team said the approach opens a path toward chiral-phonon-driven spintronic and photonic devices, and that it can be adapted to work with electrode and device architectures already used in the field.