Electric field reverses phonon chirality and spin direction in ferroelectric crystal

An international team of researchers has demonstrated that an electric field can reverse both the chirality and spin direction of phonons in a ferroelectric crystal, marking a breakthrough in spintronics. The study, conducted using triglycine sulfate (TGS), a ferroelectric molecular crystal, shows that the application of an electric field during the ferroelectric phase allows for the active control of phonon chirality and associated spin direction. This finding could pave the way for the development of faster and more energy-efficient spintronic devices. The experiment involved cooling TGS into its ferroelectric phase, where dipoles align to produce spontaneous polarization. Researchers then applied an electric field, causing the dipole moments to align in a particular direction. This alignment enabled them to switch the polar and chiral directions of the crystal, effectively controlling the chirality of phonons. The process was monitored using the time-resolved magneto-optical Kerr effect (TR-MOKE), allowing the researchers to measure the spin polarization direction and observe how it changed in response to the reversal of chirality. As the electric field was applied, the polarization and chirality of TGS underwent a switch, which in turn influenced the spin direction of the phonons. According to Xiaotong Li, an assistant professor of chemistry at North Carolina State University and co-corresponding author of the study, the findings demonstrate that ferroelectricity controls structural chirality, which in turn activates chiral phonons under heat flow. These phonons then transfer angular momentum to electronic spins, enabling electrical control over spin. The researchers confirmed their observations using density functional theory simulations, which revealed the atomic motions underlying the phenomenon. When TGS transitions between its ferroelectric states, key glycine phonon modes reverse their circular motion, offering a detailed microscopic view of switchable phonon chirality. Yi Xia, an assistant professor of mechanical and materials engineering at Portland State University and co-corresponding author of the work, noted that these simulations support the experimental results and provide insight into the fundamental physics at play. Xiang-Bin Han, a postdoctoral researcher in the Li group at NC State and co-first author of the study, emphasized the practical implications of the discovery. He explained that the ability to control spin direction through an electric field, rather than relying on external magnetic fields or other complex mechanisms, could simplify the design of spintronic devices. From a device perspective, he added, the use of an electric field to regulate phonon chirality via electron, phonon coupling might reduce the need for additional components, making the technology more compact and efficient. This work represents a significant step forward in the field of spintronics, where the manipulation of electron spin rather than charge is being explored for advanced computing applications. By demonstrating the direct electrical control of phonon chirality and spin, the researchers have opened up new possibilities for developing next-generation electronic devices that operate with greater speed and lower power consumption. Future studies will likely focus on refining the technique and exploring its potential applications in a broader range of materials and technologies.