Uniaxial strain tuning of polar lattice vibrations in KTaO$_3$ and SrTiO$_3$

TL;DR

This study investigates how uniaxial strain influences polar lattice vibrations in SrTiO$_3$ and KTaO$_3$, revealing a first-order quantum phase transition and violations of classical phonon-dielectric relations, with implications for understanding ferroelectric fluctuations.

Contribution

It combines experimental and theoretical approaches to elucidate strain-induced ferroelectric transitions and mesoscale fluctuations in STO and KTO, challenging existing models and relations.

Findings

Modes remain underdamped at nonzero energy across transition

Strong violation of Lyddane-Sachs-Teller relation observed

First-order transition occurs at low critical stress in metallic STO

Abstract

The interplay of electronic and structural degrees of freedom is a prominent feature of many quantum materials and of particular interest in systems with strong ferroelectric fluctuations, such as SrTiO$_3$ (STO) and KTaO$_3$ (KTO). Both materials are close to a ferroelectric transition, but despite six decades of extensive research, pivotal questions regarding the nature of this transition and of the associated fluctuations remain debated. Here we combine inelastic neutron scattering, Raman spectroscopy, and ab initio calculations to study the evolution of soft polar phonons across the strain-induced ferroelectric transition in STO and KTO. We find that the modes remain underdamped and at nonzero energy, consistent with a first-order quantum phase transition. We also reveal a strong violation of the well-known Lyddane-Sachs-Teller relation between the phonon energies and static dielectric permittivities in insulating KTO and STO, which is not captured by ab initio calculations and points to the presence of slow mesoscale fluctuations induced by long-range interactions. In metallic STO, we uncover a first-order transition at a remarkably low critical stress, in qualitative agreement with recent theoretical predictions. The present work resolves several long-standing questions pertaining to the model systems STO and KTO and is relevant to numerous other materials with soft polar phonons.