🔬 materials science

Coherent seeding and control of dynamical ferroelectricity by phonon anharmonicity

This paper demonstrates that intense terahertz excitation induces a coherent, light-driven ferroelectric state in centrosymmetric PbTe via anharmonic phonon coupling, enabling the deterministic amplification and suppression of polarization through double-pulse protocols.

Original authors: Junhan Huang, Yongkang Ju, Xinbo Wang, Li Yue, Hao Wang, Qiaomei Liu, Tianchen Hu, Yuchen Cui, Liyu Shi, Shangfei Wu, Sijie Zhang, Dong Wu, Peizhe Tang, Tao Dong, Nan-Lin Wang

Published 2026-06-23

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CC BY 4.0

Original authors: Junhan Huang, Yongkang Ju, Xinbo Wang, Li Yue, Hao Wang, Qiaomei Liu, Tianchen Hu, Yuchen Cui, Liyu Shi, Shangfei Wu, Sijie Zhang, Dong Wu, Peizhe Tang, Tao Dong, Nan-Lin Wang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a crystal made of lead and tellurium (PbTe) as a giant, perfectly symmetrical ballroom. In this ballroom, the atoms are like dancers holding hands in a perfect circle. Under normal conditions, for every dancer moving left, another moves right, keeping the whole room perfectly balanced. Because of this perfect balance, the room has no "direction" or "polarity"—it's like a magnet that has been turned off.

Scientists have long wanted to use light to force these atoms to break their symmetry and start moving in a specific direction, creating a temporary "ferroelectric" state (a state with a strong electric direction). However, usually, this only works in materials that are already on the verge of breaking, like a wobbly tower of blocks.

This paper reports a breakthrough: the team successfully forced this perfectly balanced PbTe ballroom to break its symmetry and create a directed state using only a powerful pulse of light, and they did it without the material being "wobbly" to begin with.

Here is how they did it, using simple analogies:

1. The "Shake" That Creates Order

The researchers used a very intense pulse of Terahertz (THz) light. Think of this not as a gentle breeze, but as a massive, rhythmic drumbeat hitting the crystal.

The Target: Inside the crystal, there are two types of atomic vibrations (phonons) that are identical twins (degenerate). They are like two identical springs attached to the same point.
The Trick: The light hits these springs so hard that they don't just vibrate back and forth; they start interacting with each other in a chaotic, "non-linear" way.
The Result: Because the springs are so "anharmonic" (meaning they get stiffer or looser depending on how hard you pull them, rather than staying perfectly springy), this intense shaking forces the atoms to settle into a new, slightly off-center position. It's like shaking a box of marbles so violently that they all suddenly roll to one side and stay there for a moment. This creates a macroscopic electric polarization—a "ferroelectric" state.

2. The Temperature Limit

Usually, heat acts like a noisy crowd that ruins the dance. In other materials, this light-induced state only works at extremely cold temperatures (near absolute zero).

The Discovery: The team found that in PbTe, this "off-center" state could survive up to about 100 Kelvin (roughly -173°C).
Why? The material has a special property called "giant anharmonicity." Think of it as the dancers having very sticky shoes. Even when the room gets a bit warmer, the "stickiness" of their movement helps them stay in that off-center position longer than expected, before the heat finally scrambles them.

3. The "Double-Clap" Control

The most exciting part of the paper is how they controlled this state. They didn't just turn it on and leave it; they used a double-pulse protocol.

The Analogy: Imagine pushing a child on a swing.
- Amplification: If you push the swing exactly when it's coming back toward you (in sync), the swing goes higher. The researchers did this with a second light pulse, and the electric state became stronger.
- Suppression: If you push the swing exactly when it's moving away from you (out of sync), you stop the swing dead in its tracks. The researchers did this too, and the electric state vanished instantly.
The Significance: They could turn the "electric direction" of the crystal on, off, amplify, or cancel it out in trillionths of a second, simply by timing the second light pulse perfectly.

4. The Direction Matters

They also discovered that the angle of the light matters.

The Analogy: Imagine trying to tip a table. If you push it straight down the middle, it might just wobble. But if you push it at a specific angle, it tips over easily.
The Finding: The light worked best when it was aimed at a specific angle (about 13 degrees) relative to the crystal's natural grid. If they aimed it straight on, the effect was weaker. This proves that the "tipping" relies on the complex interaction between the two twin vibrations.

Summary

In short, the scientists used a powerful, rhythmic light pulse to shake a perfectly symmetrical crystal into a temporary, directional state. They proved that this state is created by the chaotic, "bouncy" nature of the atoms themselves (anharmonicity), not by pre-existing flaws. Furthermore, they showed they could act like a conductor, using a second light pulse to either boost this state or silence it instantly, all within a fraction of a second.

This work shows that we can use light to create and control new electronic states in materials that were previously thought to be too stable to change, opening a door to ultrafast control of material properties.

Technical Summary: Coherent Seeding and Control of Dynamical Ferroelectricity by Phonon Anharmonicity

Problem Statement
Optical control of quantum materials generally follows two paths: creating non-equilibrium states inaccessible at equilibrium and coherently steering ultrafast dynamics via multi-pulse protocols. Light-induced ferroelectricity (FE) is particularly promising because the macroscopic polarization order parameter directly links broken inversion symmetry to functional response. However, previous experimental demonstrations have been largely restricted to quantum paraelectrics (e.g., SrTiO $_3$ ) near ferroelectric instabilities. In these systems, the induced state is often obscured by pre-existing mesoscopic dipolar fluctuations and critical fluctuations, making the microscopic origin of the state ambiguous and complicating deterministic coherent control. The central challenge is to realize an unambiguous, light-induced FE state in a material without pre-existing order parameter fluctuations and to harness the coherent lattice dynamics that create it for ultrafast control.

Methodology
The authors investigated lead telluride (PbTe), a centrosymmetric rock-salt semiconductor known for giant lattice anharmonicity but lacking a finite-temperature ferroelectric transition.

Experimental Setup: Intense, single-cycle terahertz (THz) pulses were used to resonantly drive the doubly degenerate transverse optical (TO) phonon modes. The transient breaking of inversion symmetry was probed using second-harmonic generation (SHG) of an 800 nm pulse (THz-field-induced second harmonic, TFISH).
Control Protocols:
- Field Strength: THz field strengths were varied from 0.25 to 1.00 MV/cm to identify threshold behaviors.
- Temperature: Measurements were conducted across a temperature range (10 K to 200 K) to study thermal damping and anharmonic effects.
- Polarization: The THz field polarization angle ( $\alpha$ ) was rotated relative to the crystal axes to probe directional dependence.
- Double-Pulse Excitation: A double-THz-pump protocol was employed, introducing a second identical THz pulse with a tunable time delay ( $\tau$ ) relative to the first, to test coherent amplification and suppression.
Theoretical Modeling: First-principles density functional theory (DFT) calculations were used to derive the anharmonic potential energy surface for the degenerate TO modes. Classical equations of motion were solved numerically to simulate the nonlinear phonon dynamics and the resulting transient polarization.

Key Results

Induction of a Ferroelectric State: Above a threshold THz field of approximately 0.5 MV/cm, PbTe exhibits a macroscopic symmetry-broken state. This is evidenced by:
- A persistent, non-oscillatory TFISH signal (steady state) lasting >15 ps.
- The emergence of a fundamental frequency peak ( $f_{TO}$ ) in the TFISH spectrum, indicating heterodyne detection of a THz-induced inversion-symmetry-broken state (as equilibrium PbTe is SHG-inactive).
- Asymmetric splitting of the $2f_{TO}$ peak, signaling strong anharmonic renormalization.
Temperature Dependence: The induced FE state is observable up to $\sim$ 100 K ( $T^*$ ). This upper limit aligns with a previously reported anomaly in PbTe where thermally activated anharmonicity enhances phonon scattering. Above $T^*$ , increased damping suppresses the large-amplitude coherent motion required for the state, distinguishing this mechanism from the local dipolar fluctuations seen in SrTiO $_3$ .
Mechanism of Anharmonic Coupling: The study identifies the microscopic mechanism as the nonlinear coupling between the two degenerate TO phonon modes.
- The THz field directly excites one mode ( $Q_{y'}$ ), which, through anharmonic terms in the potential, exerts a resonant nonlinear driving force on the orthogonal mode ( $Q_{x'}$ ).
- This coupling drives both modes into large-amplitude oscillations around finite quasi-static offsets, breaking inversion symmetry.
- The efficiency of this process is polarization-dependent, reaching a maximum when the THz field is polarized at $\alpha \approx 13^\circ$ relative to the crystal axes, consistent with the angular dependence of the anharmonic coefficients derived from DFT.
Coherent Control: Using the double-pulse protocol, the authors demonstrated deterministic control over the induced polarization:
- Suppression: When the second pulse arrives out-of-phase ( $|\tau| = (n + 1/2)T_{TO}$ ), the FE steady state is switched off.
- Amplification: When the second pulse arrives in-phase ( $|\tau| = nT_{TO}$ ), the polarization is coherently amplified.
- The oscillation of the signal amplitude with delay $\tau$ confirms the coherent nature of the manipulation.

Significance and Claims
The paper claims to establish a general strategy for controlling mode-mediated functionalities in quantum materials by unifying the preparation of a symmetry-broken state with its coherent steering.

Beyond Quantum Paraelectrics: The work extends light-induced states beyond the restrictive regime of quantum paraelectrics, demonstrating that a centrosymmetric material with no intrinsic ferroelectric instability can be driven into a robust FE state solely through phonon anharmonicity.
Mechanism Clarity: Unlike previous systems where the origin was debated, the mechanism here is clearly identified as the nonlinear coupling of degenerate modes, independent of pre-existing local dipoles.
Ultrafast Functional Control: The ability to coherently amplify or suppress the induced polarization on ultrafast timescales (picoseconds) via double-pulse protocols suggests a pathway for ultrafast information processing and logic operations.
Probing Hidden Order: The strong THz field excitation serves as a sensitive probe of equilibrium local hidden order in PbTe, revealing enhanced phonon damping near 100 K that was previously accessible only via neutron measurements.
Future Directions: The authors suggest that the doubly degenerate TO mode's ability to support circular ionic motion (axial phonons) could be exploited with circularly polarized THz fields to generate effective magnetic fields, potentially enabling the dynamic induction of multiple ferroic orders and advancing THz phonon-polaritonics.