Flip a Voltage and the Atoms Spin the Other Way: NC State Reversed a Crystal's Handedness on Command

Heat a crystal and its atoms jiggle. In some materials they do something more specific: groups of atoms move in circles, carrying angular momentum through the material as they propagate. These are chiral phonons, and the direction of that circular motion — the handedness — determines the direction of the electron spin they generate. Chirality here means what it means for hands: the structure cannot be superimposed on its mirror image, and a left-handed glove will not fit a right hand.

Physicists have been able to observe chiral phonons for some time. What they have not been able to do is control them. A team publishing in Nature Communications has now switched the handedness on demand, using nothing more exotic than an applied electric field.

"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 North Carolina State University and co-corresponding author. "Before this work, chiral phonons have been observed with different techniques, but the switching and active control of chiral phonons has not been demonstrated."

The material choice did most of the work. The team used triglycine sulfate, or TGS, a ferroelectric molecular crystal whose structural chirality and ferroelectric polarization are coupled — change one and the other has to change too. Cooling TGS drives it from the paraelectric phase, where its electric dipoles point in random directions, into the ferroelectric phase, where they align and produce a spontaneous polarization. Applying an electric field during that phase aligns the dipoles one way; reversing the field flips both the polar direction and the chiral direction with it.

To confirm the spins followed, the researchers used the time-resolved magneto-optical Kerr effect, a technique that reads out spin polarization at a material's surface. They saw the full chain: field applied, polarization switched, chirality switched, spin direction reversed. "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 said.

Density functional theory simulations backed the measurement with an atomic-scale picture. "When TGS switches between its ferroelectric states, key glycine phonon modes reverse their circular motion, providing a microscopic picture of switchable phonon chirality," said Yi Xia, assistant professor of mechanical and materials engineering at Portland State University and co-corresponding author.

The device argument is straightforward. Electric fields are already the primary control parameter in solid-state spintronics; magnetic control requires extra hardware. "If you switch the handedness, or chirality, you can control the spin direction — essentially providing an electrical control knob for spin," said Xiang-Bin Han, co-first author and a postdoctoral researcher in Li's group. "If an electric field can regulate phonon chirality via electron–phonon coupling, it may eliminate the need for additional control mechanisms that would otherwise increase device complexity."

The researchers say the method adapts readily to techniques already in use in the field, which is the difference between a laboratory curiosity and something that shows up in a chip. Faster, more energy-efficient spintronic and photonic devices are the stated target.