Photoinduced suppression of the ferroelectric instability in PbTe

TL;DR

This study demonstrates that photoexcitation can stabilize the paraelectric state in PbTe by weakening long-range forces, highlighting the role of electronic states near the band edges in lattice stability.

Contribution

It provides experimental and theoretical evidence that photo-injection of carriers suppresses ferroelectric instability in PbTe, a material near phase transition.

Findings

Photoexcitation stabilizes the paraelectric state in PbTe.

Weakening of long-range forces is linked to electronic excitation.

Electronic states near band edges influence lattice stability.

Abstract

The interactions between electrons and phonons drive a large array of technologically relevant material properties including ferroelectricity, thermoelectricity, and phase-change behaviour. In the case of many group IV-VI, V, and related materials, these interactions are strong and the materials exist near electronic and structural phase transitions. Their close proximity to phase instability produces a fragile balance among the various properties. The prototypical example is PbTe whose incipient ferroelectric behaviour has been associated with large phonon anharmonicity and thermoelectricity. Experimental measurements on PbTe reveal anomalous lattice dynamics, especially in the soft transverse optical phonon branch. This has been interpreted in terms of both giant anharmonicity and local symmetry breaking due to off-centering of the Pb ions. The observed anomalies have prompted renewed theoretical and computational interest, which has in turn revived focus on the extent that electron-phonon interactions drive lattice instabilities in PbTe and related materials. Here, we use Fourier-transform inelastic x-ray scattering (FT-IXS) to show that photo-injection of free carriers stabilizes the paraelectric state. With support from constrained density functional theory (CDFT) calculations, we find that photoexcitation weakens the long-range forces along the cubic direction tied to resonant bonding and incipient ferroelectricity. This demonstrates the importance of electronic states near the band edges in determining the equilibrium structure.