Researchers have, for the first time, directly observed and reversibly switched the chirality of atomic vibrations, specifically, phonon angular momentum, within the ferroelectric material barium titanate (BaTiO3). Employing circularly dichroic resonant inelastic X-ray scattering, the team directly resolved phonon chirality through angular momentum transfer between X-rays and the crystal lattice. This work establishes a “robust and non-volatile gyroelectric effect,” offering a pathway to manipulate vibrational states even after power is removed and paving the way for potential advances in phonon-based technologies. The results demonstrate phonon angular momentum as an electrically controllable degree of freedom.
Electric-Field Control of BaTiO3 Phonon Chirality
The technologically relevant ferroelectric barium titanate, or BaTiO3, now exhibits reversible switching of phonon angular momentum when exposed to an electric field; this control stems from observations made using circularly dichroic resonant inelastic X-ray scattering. This technique directly resolves phonon chirality by transferring angular momentum between circularly polarized X-rays and the crystal lattice, offering a level of precision previously unavailable for studying these atomic vibrations.
Researchers observed a momentum-dependent dichroic response that changes in tandem with the ferroelectric polarization of the material, a phenomenon they describe as a “robust and non-volatile gyroelectric effect.” The observed switching isn’t merely a demonstration of control, but a confirmation of a specific physical mechanism at play within the BaTiO3 structure. The team’s measurements align quantitatively with first-principles calculations, validating their interpretation of how crystal symmetry gives rise to phonon angular momentum and chirality in these regions.
These chiral phonons, lattice vibrations carrying angular momentum, are understood to offer a route for coupling lattice, electronic, and magnetic properties, but achieving deterministic control over them has been a longstanding challenge. The ability to manipulate these vibrations with an external electric field opens possibilities for novel device concepts. The implications of this work extend beyond fundamental materials science; the researchers suggest a pathway toward phonon-based information and energy technologies.
The non-volatile nature of the observed effect is particularly noteworthy, meaning the vibrational state, and thus the potential for encoding information, is retained even when the electric field is removed. This contrasts with many other methods of controlling material properties that require continuous energy input to maintain a specific state. “We observe a momentum-dependent dichroic response that switches with ferroelectric polarization,” the team reports, highlighting the direct link between the material’s electric properties and the behavior of its atomic vibrations.
BaTiO3 as a Platform for Reversible PAM Switching
BaTiO3 thin films now demonstrate reversible switching of vibrational angular momentum with applied voltage, a capability enabled by direct observation of phonon chirality. The selection of barium titanate as a platform was deliberate; its broken inversion symmetry and switchable polar order provide an ideal environment for controlling these vibrations.
Beyond its fundamental properties, BaTiO3 is already established in low-power electronics, with existing applications in ferroelectric field-effect transistors, non-volatile memories, and emerging neuromorphic architectures. The ability to electrically control phonon angular momentum within this technologically relevant material expands its potential for advanced device designs.
To achieve reversible switching, the team applied a temporal potential of +3.5V or -3.5V, followed by a holding potential of +0.2V or -0.2V, ensuring stable ferroelectric domains during measurement. Quantitative agreement between the measured dichroism and first-principles calculations validates the experimental findings and confirms the underlying physics. This alignment supports the establishment of a “robust and non-volatile gyroelectric effect,” where the vibrational state remains even after the electric field is removed. The researchers addressed long-term signal drifts during polarization switching, allowing for reliable measurements of this subtle effect.
Circularly Dichroic RIXS Resolves Phonon Chirality
The ability to reversibly switch the angular momentum of atomic vibrations within barium titanate represents a step toward new information and energy technologies. This direct resolution of phonon chirality, the twisting direction of these vibrations, was achieved through careful selection of the outgoing angle of the X-ray beam, minimizing the component along the crystal’s ‘c’ axis where angular momentum is expected to be perpendicular. Quantitative agreement between experimental measurements and first-principles calculations validates the observation of electrically controlled phonon angular momentum.
Specifically, an applied voltage of 3 V is sufficient to fully switch and nearly saturate the BTO polarization, suggesting angular momentum exchange with these vibrational states. The sensitivity of the technique stems from its focus on the titanium-oxygen bonds within barium titanate; non-degenerate chiral phonons, essential for observing this effect, arise from broken inversion symmetry.
The measurements confirm that the contrast observed in the spectra emerges from the transfer of angular momentum to the phonon system, a key component of studies examining phonon spectra and electron-phonon coupling. The observed phonon energies are limited to approximately 100 meV, with features at higher energies presumed to be multiphonon processes.
Non-Volatile Gyroelectric Effect in Ferroelectric BTO
This non-volatility, demonstrated through resonant inelastic X-ray scattering, distinguishes this effect from prior work focused solely on controlling atomic vibrations and opens possibilities for novel data storage methods. This sensitivity arises from the material’s broken inversion symmetry, a key characteristic enabling the electric field control of phonon angular momentum.
The observed momentum-dependent dichroic response, which changes sign with the ferroelectric polarization, confirms that the effect is directly linked to the material’s electrical state and not an artifact of the measurement process. This contrasts with earlier studies of chiral phonons, which primarily focused on their coupling to secondary effects.
The ability to deterministically control phonon angular momentum within a technologically relevant perovskite, barium titanate, suggests a pathway toward manipulating chiral phonon ground states for applications in transport phenomena. The team’s analysis confirms that the observed contrast originates from the orbital moment of these chiral phonon modes, linking the electrical control to a fundamental property of the lattice vibrations.
By applying and maintaining an electric field, the researchers were able to switch and hold the ferroelectric domain within a defined region of the barium titanate film, allowing for controlled measurements of the dichroic response in different polarization states. This process involved alternating between two circular polarization states of the X-rays, designated C+ and C−, while maintaining the applied voltage, ensuring reliable and repeatable measurements.
PAM as a Dynamical Order Parameter for Chirality
This research demonstrates that phonon angular momentum (PAM) can function as a dynamical order parameter, similar to established properties like magnetization or ferroelectric polarization, providing a quantifiable framework for chiral states. Identifying PAM at specific points within a material’s structure was previously achieved in α-quartz using circularly polarized resonant inelastic X-ray scattering, but extending this control to materials with practical applications remained a challenge. Barium titanate (BaTiO3) emerges as a key material in this advancement, possessing both broken inversion symmetry and a switchable ferroelectric polarization, allowing for controllable manipulation of its asymmetry.
This work establishes a new pathway for phononics, where angular momentum, not just frequency, becomes a tunable characteristic of vibrational modes. The measurements reveal element-specific evidence of the orbital origin of these non-degenerate chiral phonons, demonstrating that the observed chirality isn’t simply a byproduct of atomic arrangement but a fundamental property linked to electron behavior.
The implications extend beyond simply controlling vibrations; the demonstrated effect suggests the potential for creating materials where angular momentum can be harnessed for novel applications. Classifying materials based on their phonon angular momentum and understanding its microscopic origins, as outlined in recent publications, could lead to the design of materials with tailored vibrational properties.
C4v Symmetry Dictates g-Wave Phonon Behavior
The four-fold symmetry inherent in the crystal structure of barium titanate dictates how angular momentum is expressed in its g-wave phonons, influencing the observed response to applied electric fields. This symmetry, designated C4v, manifests as a predictable pattern in the gyration tensor governing phonon behavior, effectively linking crystal orientation to vibrational characteristics. Researchers found that the observed contrast in resonant inelastic X-ray scattering experiments aligns with this four-fold symmetry, providing a direct link between the material’s structure and its vibrational response.
Specifically, the team demonstrated that the switching of circular dichroism contrast in the RIXS signal directly follows the sign of the applied electric polarization, a phenomenon attributable to the orbital moment of chiral phonon modes. This gyroelectric effect in reciprocal space arises because the C4v symmetry dictates how the forces on oxygen ions invert with titanium displacement, reversing the angular momentum of the phonon modes.
For a non-symmetric point within the Brillouin zone, a phonon will only exhibit zero angular momentum if the material possesses both time-reversal and space inversion symmetry; barium titanate breaks this symmetry, allowing for non-zero angular momentum and thus, controllable chirality. The ability to control phonon angular momentum is a consequence of directly influencing the crystal symmetry itself. Demonstrating the link between macroscopic polarization and microscopic vibrational states.
This control, rooted in the Curie principle, allows for the manipulation of physical properties through alterations in crystal symmetry, with the electric field acting as the mediating force. The signal from phonon contributions exhibits the expected four-fold symmetry, consistent with the C4v symmetry of the barium titanate unit cell.
This symmetry dictates that the contrast observed is governed by the chiral projection parameter, known as the phonon helicity, which is non-zero only for phonons that break improper rotational symmetry. The observed effect establishes a pathway to manipulate vibrations and potentially create new devices based on phonon angular momentum.
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