Observation of polarization density waves in SrTiO3

TL;DR

This study uses advanced femtosecond x-ray and THz techniques to visualize polarization density waves in SrTiO3, revealing mesoscopic modulated phases and collective modes near the quantum critical point.

Contribution

It introduces a novel experimental approach combining XFEL and THz control to directly observe polarization density waves in SrTiO3.

Findings

Detection of soft polar-acoustic collective modes at nanometer scales

Visualization of mesoscopic polarization modulations

Evidence supporting the existence of a modulated phase in SrTiO3

Abstract

The nature of the "failed" ferroelectric transition in SrTiO3 has been a long-standing puzzle in condensed matter physics. A compelling explanation is the competition between ferroelectricity and an instability with a mesoscopic modulation of the polarization. These polarization density waves, which should become especially strong near the quantum critical point, break local inversion symmetry and are difficult to probe with conventional x-ray scattering methods. Here we combine a femtosecond x-ray free electron laser (XFEL) with THz coherent control methods to probe inversion symmetry breaking at finite momenta and visualize the instability of the polarization on nanometer lengthscales in SrTiO3. We find polar-acoustic collective modes that are soft particularly at the tens of nanometer lengthscale. These precursor collective excitations provide evidence for the conjectured mesoscopic modulated phase in SrTiO3.