Electric field switching of chiral phonons

Abstract

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.

Figures (4)

• Figure 1: Chiral phonons in a ferroelectric. a) The bistable Ti displacements relative to the crystal centre (oxygen octahedra) are the origin of displacive ferroelectricity, quantified by the spontaneous electric polarisation, $\mathbf{P}_{s}$. By applying an electric field anti-parallel to the induced polarisation, the vector direction of $\mathbf{P}_{s}$ is inverted. b) Depending on the direction of the Ti displacement (along the c-axis of the crystal), the flipped polarity changes the handedness of the chiral phonons in the material. Double-well taken from Cohen Cohen1992. c) Chiral phonons are found in general non-symmetric points ($\mathbf{q}_{\alpha}$) in the reciprocal space, where the handedness follows the C4V symmetry of tetragonal BTO. Right and left handed phonons are non-degenerate in such instances. Reflection about a high symmetry point (to $\mathbf{q}_{\beta}$) inverts the phonon angular momentum.
• Figure 2: Free standing films. a) Freestanding films (FS) of ferroelectric BTO are prepared and placed on Au$||$Cr bottom electrodes. Yellow lines indicate the a and b axes (equivalent) where the c axis is out of plane. The top electrodes are deposited above the FS film where the area corresponds to roughly to the x-ray spot size (50 microns long axis). b) The ferroelectric nature and switching of the polarisation is confirmed using piezo force microscopy. The opening between increasing (blue) and decreasing (red) field yields a measure of coercivity of approx 3 V. c) Measurement scheme used during the experiment. The polarisation is switched by applying +(-) 3.5 V before a +(-) 0.2 V holding potential is maintained. In each ferroelectric state, RIXS is measured with circularly right- and left-handed x-rays in a '+ - - +' pattern.
• Figure 3: Electric-field switching of chiral phonons measured with RIXS. The RIXS spectra is recorded for two X-ray polarisations under two different directions of applied electric field. a) Low energy phonons are observed near the elastic line for x-rays energy corresponding to the near oxygen K-edge (531.25 eV) when the sample is measured under positive electric field (0.2 V). The exact position of the elastic line is found by fitting the energy gain (negative energy loss) region with a Lorentzian curve, while Savitzky-Golay (SG) smoothing is applied to guide the eye. b) We confirm the switching of PAM by measuring the CD in RIXS for a focused x-ray beam (50 microns). Upon reversing the applied electric field, CD is inverted and remains stable for at least 15 hours. Likewise, the field-switching is inverted by changing the left/right circular polarisation. Error bars are estimated from Poisson statistics on re-binned data (by a factor 2), see Supplementary.
• Figure 4: Chiral phonons q-dependence. a) Comparison of the field switching contrast (SG smoothed) seen in C- light for different values of $\mathbf{q}$. The shaded regions correspond to the assumed chiral phonons. Reversal of contrast is observed for inversions of $\mathbf{q}$ ($\vec{q}_2$ vs $\vec{q}_4$). b) Example of the circular motion (AM projected in-plane) of the oxygen atoms at $\vec{q}_2$ computed from the DFT simulations of the tetragonal BTO (videos are available in the Supplementary materials). c-e) Computed values of $\mathbf{J} \cdot \mathbf{q}$ where $q_z$ is fixed at 0.42 Å -1 for 3 phonon modes which have significant PAM, where the energies are 11, 38 and 100 meV (associated with regions I, II, and III above). The contrast follows the g-type C4v symmetry yang2025 of the BTO structure. f-h) Integrated CD contrast for the shaded regions (I, II, and III) in a) as a function of $\phi$. These correspond to points shown along the yellow line in e). The variation in the measured contrast matches the calculated values well for the 3 phonon modes across the directions measured.