Lamellar fluctuations melt ferroelectricity

TL;DR

This paper investigates how thermal and quantum fluctuations destabilize long-range modulated order in ferroelectrics with flexoelectric coupling, suggesting that materials like SrTiO3 and KTaO3 are melted remnants of an underlying modulated phase.

Contribution

It introduces a self-consistent phonon approach to demonstrate the instability of long-range modulated order due to fluctuations in flexoelectric ferroelectrics.

Findings

Long-range modulated order is unstable at all temperatures.

Thermal fluctuations dominate at finite temperatures, destabilizing the modulated phase.

Materials like SrTiO3 and KTaO3 are likely melted versions of an underlying modulated state.

Abstract

We consider a standard Ginzburg-Landau model of a ferroelectric whose electrical polarization is coupled to gradients of elastic strain. At the harmonic level, such flexoelectric interaction is known to hybridize acoustic and optic phonon modes and lead to phases with modulated lattice structures that precede the symmetry broken state for sufficiently large couplings. Here, we use the we use the self-consistent phonon approximation to calculate the effects of thermal and quantum polarization fluctuations on the bare modes to show that such long-range modulated order is unstable at all temperatures. We discuss the implications for the nearly ferroelectric SrTiO$_3$ and KTaO$_3$, and propose that these systems are melted versions of an underlying modulated state which is dominated by non-zero momentum thermal fluctuations except at the very lowest temperatures.