Electric field switching of chiral phonons

The gist: Electric-field switching of phonon angular momentum in ferroelectric BaTiO3 demonstrates a robust mechanism for non-volatile control of chiral phonons, showing reversible switching stable for at least 15 hours.

Introduction to Chiral Phonons

Lattice vibrations carrying angular momentum, known as chiral phonons, have emerged as a promising route to control and understand complex material properties, yet their deterministic manipulation remains largely unexplored Lattice vibrations carrying angular momentum, known as chiral phonons, have emerged as a promising route to control and understand complex material properties, yet their deterministic manipulation remains largely unexplored. More than a century after chirality was defined as the distinction between left- and right-handed structures, this concept continues to shape condensed matter physics in the spirit of Curie’s symmetry principle: it is asymmetry that creates the phenomenon Its influence spans from the enantioselective mechanisms that regulate molecular chemistry [2] to crystalline solids, where the absence of inversion symmetry gives rise to chiral order that couples spin, orbital, and lattice degrees of freedom. Recently, attention has shifted from static structural chirality to lattice dynamics with the discovery of chiral phonons. These are defined as vibrational modes in which atoms break improper rotational symmetry [3] and such excitations carry quantised angular momentum and enable lattice-mediated angular-momentum transfer [4].

Mechanism of Control in BaTiO3

The researchers selected a polar ferroelectric titanate, BaTiO3 (BTO), as an ideal platform because its ferroelectric polarisation can be inverted in situ by a small out-of-plane electric field, offering a controllable parameter of inversion-breaking symmetry. The system is designed to test how the angular momentum (chirality) of certain phonon modes can be switched by an external electric field. In the room temperature tetragonal phase of BTO, the lack of inversion symmetry results in phonons being non-degenerate and chiral at generic points in reciprocal space. The projection of the phonon angular momenta in reciprocal space is reversed by inversion of the polarisation direction for structures described by the point group C4v. By applying an electric field, the Ti4+ ion can be displaced to the other stable site (double-well potential in Fig. 1 b) thereby inducing a polarisation of opposite sign.

Experimental Observation and Switching

The study uses circularly polarised RIXS to probe the phonon angular momentum, which is sensitive to symmetry breaking via the transfer of angular momenta. The circular dichroism (CD) contrast in RIXS originates from the excitation of O 2p electric quadrupoles, as CD is absent for isotropic charge scattering and BTO is non-magnetic. By symmetry arguments, switching between the opposite Ti4+ displacements shown in Fig. 1 a, the forces experienced by the oxygen ions would be inverted and angular momentum of the phonon modes should demonstrate reversal. The experimental procedure involves applying a potential of +(-) 3.5 V before measurements are performed with a holding potential of +(-) 0.2 V to flip the ferroelectric domains. The resulting spectra show switching under the application of external electric field, and the CD contrast is inverted.

Characterization and Confirmation

The strongest contrast in RIXS is seen around 20, 40 and 100 meV which are attributed to angular momentum exchange with the predicted phonon modes at 11, 38, and 100 meV. The contrast exhibits switching of phonon chirality with respect to the direction of the electric poling field as well as to the inversion of X-ray circular polarisation. The analysis confirms that CD-RIXS contrast can be inverted by measuring along an inverted q, verifying that the relationship follows crystallographic symmetry and PAM must arise from the broken inversion symmetry of the displacive ferroelectric phase. This demonstrates deterministic inversion of phonon angular momentum in a technologically relevant perovskite over at least 15 hours period.

Implications for Phononics

This work establishes a robust mechanism for non-volatile control of chiral phonons and points towards new opportunities for phonon-based information and energy technologies. The ability to control the symmetry of physical properties via direct control of the crystal symmetry, in this instance the electric field control of the inversion centre is manifested in the PAM of the chiral phonon. This approach allows for active manipulation of chiral phonon populations, thereby establishing an alternative route for phononics in which angular momentum, rather than frequency alone, becomes a tunable degree of freedom. This opens the prospect for integrating THz- or optically-driven, angular-momentum-carrying phonons (coupled to adjacent magnetic layers) to realise on-chip phonon–magnon reservoirs for neuromorphic computing.

Methods Summary

The BTO thin films were fabricated by pulsed laser deposition (PLD) using a KrF excimer laser with a wavelength of 248 nm. Freestanding BTO was obtained by chemical etching to remove the LSMO sacrificial layer. The polarization switching device architecture involved combining standard photolithography, thermal evaporation, and transfer of freestanding BTO membranes. RIXS measurements were performed at the RIXS end-station of Beamline ID32 at the ESRF. DFT calculations were performed using the Abinit software package with the PBE GGA exchange–correlation functional and vdwDFT-D3(BJ) dispersion correction. The experimental data acquired at the ESRF can be found in the ESRF data portal at DOI:10.15151/ESRF-ES-2166894199.

References

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[5] Coh, S. Classification of materials with phonon angular momentum and microscopic origin of angular momentum

[32] Zhang, X. et al. Catalogue of chiral phonon materials <ref:2603.

1. Deterministic manipulation of lattice–driven phenomena governed by chiral symmetries can be achieved by implementing an electric field-controlled switching mechanism that inverts phonon angular momentum, which is demonstrated to be stable for at least 15 hours.

2. The improved AI system could perform real-time, non-volatile control of phonon states in ferroelectric materials like BaTiO3, enabling applications such as non-volatile memories and neuromorphic architectures.

3. The system can utilize the relationship between electric field control and chirality described by the gyration tensor, where the sign of optical rotation could be reversed in chiral ferroelectrics such as Pb5Ge3O11 in a phenomenon known as gyroelectricity.

4. The AI could model and predict how CD contrast is determined by the projection parameter J·q, allowing for the prediction of phonon chirality based on specific momentum transfers, confirming that the contrast observed in this work is inaccessible to optical measurements which are at a negligible q.

Lattice vibrations carrying angular momentum, known as chiral phonons, have emerged as a promising route to control and understand complex material properties, yet their deterministic manipulation remains largely unexplored. Here we demonstrate electric-field switching of phonon angular momentum in the technologically relevant ferroelectric BaTiO3. Using circularly dichroic resonant inelastic X-ray scattering (CD-RIXS) at the oxygen K edge, we directly probe the phonon angular momentum and compare the measured dichroism with first-principles predictions of phonon-mode chirality. We find excellent agreement, revealing a momentum-dependent circular-dichroism contrast that exhibits a reversible gyroelectric effect, stable for at least 15 hours. Our results establish a robust mechanism for non-volatile control of chiral phonons and point towards new opportunities for phonon-based information and energy technologies.