Mesoscopic tunneling in strontium titanate

TL;DR

This paper reports the observation of large softening of transverse acoustic phonons in SrTiO3, linked to mesoscopic domains and quantum tunneling, revealing complex ground state phenomena in quantum paraelectrics.

Contribution

It uncovers the role of mesoscopic fluctuating domains in quantum tunneling and phonon behavior in SrTiO3, a novel insight into its quantum paraelectric ground state.

Findings

Large softening of transverse acoustic mode at tiny wave vector

Formation of mesoscopic fluctuating domains due to strain and ferroelectric fluctuations

Hybridized optical-acoustic phonons observed in SrTiO3

Abstract

Spatial correlation between atoms can generate a depletion in the energy dispersion of acoustic phonons. Two well known examples are rotons in superfluid helium and the Kohn anomaly in metals. Here we report on the observation of a large softening of the transverse acoustic mode in quantum paraelectric SrTiO$_3$ by means of inelastic neutron scattering. In contrast to other known cases, this softening occurs at a tiny wave vector implying spatial correlation extending over a distance as long as 40 lattice parameters. We attribute this to the formation of mesoscopic fluctuating domains due to the coupling between local strain and quantum ferroelectric fluctuations. Thus, a hallmark of the ground state of insulating SrTiO$_3$ is the emergence of hybridized optical-acoustic phonons. Mesoscopic fluctuating domains play a role in quantum tunneling, which impedes the emergence of a finite macroscopic polarisation.