A textured polar phase in strained SrTiO3

TL;DR

This study reveals a hidden textured polar phase in strained SrTiO3, characterized by a finite-wavevector vibrational mode, challenging the conventional view of SrTiO3 as a uniform quantum paraelectric.

Contribution

The paper uncovers a nanometre-scale polar texture in SrTiO3 under strain, demonstrating a new type of ordered state distinct from traditional ferroelectricity.

Findings

A new vibrational mode appears at finite wavevector under tensile strain.

Unstrained SrTiO3 is a disordered precursor to the textured phase.

Finite-momentum excitations reveal hidden phases in quantum materials.

Abstract

Quantum materials can harbour hidden phases whose microscopic structures differ from conventional ordered states while reproducing their macroscopic signatures, making them easy to miss. Strontium titanate is a longstanding puzzle of this kind: on cooling it shows every hallmark of an incipient ferroelectric, yet never orders, and is usually described as a quantum paraelectric in which fluctuations suppress ferroelectricity. Here we combine uniaxial strain, single-cycle terahertz excitation and femtosecond x-ray scattering to measure the polar collective modes of strontium titanate as a function of momentum and strain. Under modest tensile strain, we observe a new vibrational mode that emerges not at the Brillouin zone centre, as a ferroelectric transition would require, but at finite wavevector, identifying the ordered state as a polar texture on nanometre length scales rather than a uniform ferroelectric. Unstrained quantum paraelectric strontium titanate is then naturally understood as the disordered precursor of this textured phase, offering a resolution to a decades-old puzzle and illustrating how finite-momentum collective excitations can unmask hidden phases in quantum materials.