Electric field switching of chiral phonons

This paper demonstrates the reversible, non-volatile electric-field switching of chiral phonon angular momentum in ferroelectric BaTiO3, verified through circular dichroic resonant inelastic X-ray scattering and first-principles calculations, establishing a robust mechanism for phonon-based information and energy technologies.

The Gist

Imagine a crystal not as a rigid, frozen block of ice, but as a bustling dance floor where atoms are constantly wiggling, vibrating, and spinning. In the world of physics, these vibrations are called phonons.

Usually, we think of these vibrations just as sound or heat moving through a material. But in this new study, scientists discovered something magical: in certain materials, these atomic vibrations don't just wiggle back and forth; they spin. They have a "handedness," just like your left and right hands. This is called a chiral phonon.

Here is the simple story of what the researchers achieved, using some everyday analogies:

1. The Material: A Switchable Crystal

The scientists used a material called Barium Titanate (BaTiO3). Think of this crystal as a tiny, internal compass. Inside, the atoms are slightly off-center, creating a permanent electric "push" (called polarization).

• The Analogy: Imagine a ball sitting in a valley with two hills on either side (a "double-well" potential). The ball naturally rolls to one side. If you push it hard enough, it rolls over to the other side.
• The Switch: In this crystal, you can push the atoms from one side to the other using a tiny electric voltage. This flips the direction of the internal compass.

2. The Discovery: Spinning Atoms

When the atoms in this crystal vibrate, they don't just shake up and down. Because the crystal is "lopsided" (it lacks symmetry), the atoms trace out tiny circles as they vibrate.

• The Analogy: Imagine a group of dancers. If the room is perfectly symmetrical, they might just march in a line. But if the room is tilted, they might start spinning in a circle.
• The Twist: Depending on which side of the "valley" the atoms are sitting in (which side the compass is pointing), they spin clockwise or counter-clockwise. This spinning motion carries "angular momentum," which is basically a measure of how hard they are spinning.

3. The Experiment: Seeing the Spin

How do you see something as small as a spinning atom? You can't use a regular microscope. The scientists used a super-powerful X-ray machine (a synchrotron) that shoots "circularly polarized" light.

• The Analogy: Think of the X-rays as a corkscrew beam of light. Some corkscrews twist right-handed, others left-handed.
• The Test: When the scientists shot a "right-handed" corkscrew beam at the crystal, it interacted strongly with the atoms spinning clockwise. When they shot a "left-handed" beam, it interacted with the atoms spinning counter-clockwise.
• The Result: By measuring how much light was absorbed or scattered, they could tell exactly which way the atoms were spinning.

4. The Breakthrough: Flipping the Spin with Electricity

This is the "magic trick" of the paper.

• The Action: The scientists applied a small electric voltage to the crystal to flip the internal compass (the polarization).
• The Reaction: As soon as the compass flipped, the direction of the atomic spin flipped too. The clockwise dancers instantly became counter-clockwise dancers.
• The Stability: Even better, once they flipped it, it stayed that way for at least 15 hours without needing any more power. It's like a light switch that stays "on" or "off" even after you let go of the switch.

Why Does This Matter?

Think of this as a new way to store information.

• Current Tech: We store data on hard drives using magnetic fields (North vs. South poles).
• New Tech: This research suggests we could store data using vibrational spin (Clockwise vs. Counter-clockwise).

Because these vibrations are related to heat and sound, this opens the door to a new field called "Phononics." Imagine computer chips that don't just use electricity to process information, but use the spin of heat waves. This could lead to:

1. Super-fast memory: Switching states instantly.
2. Low-power devices: Since it's non-volatile (it remembers without power), it saves energy.
3. Neuromorphic Computing: Mimicking the human brain's synapses using these vibrational states.

In a Nutshell

The scientists found a way to use a tiny electric switch to control the direction of a microscopic "spin" inside a crystal. They proved that by flipping the crystal's electric polarity, they can reverse the direction of its atomic vibrations. This is a giant step toward building future computers that use the "spin" of heat and sound, not just electricity, to think and remember.

Technical Summary

Here is a detailed technical summary of the paper "Electric field switching of chiral phonons."

1. Problem Statement

Chiral phonons are lattice vibrations that carry quantized angular momentum (phonon angular momentum, or PAM) due to the breaking of improper rotational symmetry. While these excitations have been theoretically predicted and recently observed to couple with spin, orbital, and electronic degrees of freedom (enabling effects like the phonon Hall effect and ultrafast magnetic switching), their deterministic, reversible control remains a significant challenge.

Existing studies have confirmed the existence of chiral phonons in non-centrosymmetric crystals (e.g., $\alpha$-quartz) using circularly polarized light. However, there is no established method to actively switch the handedness (chirality) of these phonons in a stable, non-volatile manner. The authors aim to bridge this gap by demonstrating that the angular momentum of chiral phonons can be switched deterministically using an external electric field in a technologically relevant material.

2. Methodology

Material System:

• Material: Barium Titanate ($BaTiO_3$ or BTO), a polar ferroelectric perovskite.
• Sample Architecture: The researchers utilized freestanding epitaxial films of BTO. This was achieved by growing BTO on a sacrificial $La_{0.7}Sr_{0.3}MnO_3$ (LSMO) layer on a $SrTiO_3$ substrate, followed by chemical etching to release the film.
• Rationale: Freestanding films minimize epitaxial strain and domain pinning, allowing for robust ferroelectric switching. The films possess out-of-plane polarization ($P_s$), which can be inverted by an external electric field, thereby reversing the crystal's inversion symmetry breaking direction.

Experimental Technique:

• Circular Dichroic Resonant Inelastic X-ray Scattering (CD-RIXS): Measurements were performed at the Oxygen K-edge ($\approx 531.25$ eV) at the ESRF synchrotron (Beamline ID32).
• Mechanism: The technique probes the transfer of angular momentum from circularly polarized X-rays ($C^+$ and $C^-$) to the lattice. The dichroic contrast ($A_m$) arises from the coupling between the X-ray polarization and the circular motion of Oxygen $2p$ orbitals (electric quadrupole transitions).
• Switching Protocol:
  1. A "write" voltage ($\pm 3.5$ V) is applied to switch the ferroelectric domains.
  2. A "hold" voltage ($\pm 0.2$ V) maintains the state during measurement.
  3. RIXS spectra are collected for both polarization states ($C^+$ and $C^-$) in a specific sequence to cancel drift and isolate the chiral signal.
• Control of Variables: To mitigate artifacts from birefringence, measurements were taken at normal incidence with the beam parallel to the optical axis ($c$-axis). Momentum transfer ($\vec{q}$) was varied by rotating the sample to probe different points in reciprocal space.

Theoretical Support:

• Density Functional Theory (DFT): Calculations were performed using the Abinit package to predict phonon frequencies, eigenvectors, and mode effective charges. These were used to identify specific phonon modes and calculate the expected phonon helicity ($J \cdot q$).

3. Key Contributions

1. First Demonstration of Electric-Field Switching of PAM: The paper provides the first experimental evidence that the angular momentum (handedness) of chiral phonons can be reversibly switched by inverting the ferroelectric polarization via an external electric field.
2. Non-Volatile Control: The switched state is stable for at least 15 hours, establishing a mechanism for non-volatile control of lattice dynamics, analogous to non-volatile memory in electronics.
3. Direct Correlation with Theory: The experimental CD-RIXS contrast shows excellent agreement with first-principles DFT predictions regarding the chirality and momentum dependence of specific phonon modes.
4. Gyroelectric Effect in Phononics: The authors identify a "gyroelectric" effect where the sign of the phonon circular dichroism is linearly proportional to the spontaneous polarization ($P_s$), extending the concept of optical activity to lattice dynamics.

4. Key Results

• Observation of CD Contrast: The RIXS spectra revealed distinct circular dichroism in the energy loss region (0–100 meV), specifically at energy losses of ~11 meV, ~38 meV, and ~100 meV.
• Reversible Switching:
  • When the electric field direction was reversed (flipping the Ti displacement and $P_s$), the CD contrast inverted (i.e., the signal for $C^+$ became the signal for $C^-$ and vice versa).
  • This inversion was also observed when the circular polarization of the X-ray beam was flipped, confirming the chiral nature of the interaction.
• Momentum Dependence ($q$-dependence):
  • The CD contrast was measured at four different momentum vectors ($\vec{q}$) in reciprocal space.
  • The results confirmed that the contrast follows the $C_{4v}$ symmetry of the tetragonal BTO phase.
  • Crucially, reversing the momentum vector (e.g., comparing $\vec{q}_2$ to $\vec{q}_4$) inverted the sign of the contrast, matching the theoretical prediction that $J \cdot q$ changes sign with $q$.
• Stability: The switched state remained stable for over 15 hours without degradation, proving the non-volatile nature of the control.
• Microscopic Origin: The contrast is attributed to the circular motion of Oxygen atoms. The inversion of the Ti displacement (ferroelectric switching) inverts the force constants acting on the Oxygen ions, thereby reversing the direction of their circular rotation and the associated angular momentum.

5. Significance and Implications

• New Degree of Freedom: This work establishes phonon angular momentum as a tunable, controllable order parameter, distinct from frequency or amplitude.
• Phononics and Information Technology: The ability to non-volatilely switch chiral phonons opens pathways for phonon-based information storage and processing. It suggests the possibility of "phononic memory" where information is encoded in the handedness of lattice vibrations.
• Neuromorphic Computing: The coupling of chiral phonons to magnetic layers could enable on-chip phonon-magnon reservoirs for neuromorphic architectures, bridging ultrafast lattice dynamics with synaptic states.
• Fundamental Physics: The study validates the theoretical framework of chiral phonons in ferroelectrics and demonstrates the "gyroelectric" control of lattice dynamics, offering a new route to manipulate multi-ferroic properties and spin-lattice interactions without magnetic fields.

In summary, the paper successfully demonstrates a robust, electrically controlled mechanism to switch the chirality of lattice vibrations in a technologically mature material, paving the way for a new class of chiral phononic devices.