Unconventional co-existence of insulating nano-regions and conducting filaments in reduced SrTiO3: mode softening, local piezoelectricity and metallicity

TL;DR

This paper investigates how reduced SrTiO3 exhibits coexisting insulating nano-regions and conducting filaments, revealing mode softening, local piezoelectricity, and metallicity due to inhomogeneous oxygen vacancy distribution.

Contribution

It introduces a polarizability model explaining the coexistence of polar nano-domains and metallic filaments in doped SrTiO3 at low carrier concentrations.

Findings

Mode softening occurs with polar nano-domain formation.

Metallic filaments coexist with polar regions below a critical doping.

Inhomogeneous oxygen vacancy distribution leads to filamentary metallicity.

Abstract

Doped SrTiO3 becomes a metal at extremely low doping concentrations n and is even superconducting with the superconducting transition temperature adopting a dome-like shape with the carrier concentration. It is shown here within the polarizability model that up to a well-defined carrier concentration transverse optic mode softening takes place together with polar nano-domain formation which evidences inhomogeneity and a two-component type behavior with metallicity coexisting with polarity. Beyond this region a conventional metal is formed where superconductivity as well as mode softening is absent. For this regime the effective electron-phonon coupling follows the superconducting transition temperature. Effusion measurements as well as macroscopic and nanoscopic conductivity measurements indicate that the distribution of oxygen vacancies is local and inhomogeneous from which it is concluded that metallicity stems from filaments which are embedded in a polar matrix as long as the carrier concentration is less than the critical one.