Anomalous Phase Transition in Strained SrTiO$_3$ Thin Films

TL;DR

This study reveals that biaxial strain in SrTiO$_3$ thin films significantly alters the cubic to tetragonal phase transition, decoupling internal octahedral rotations from lattice shape changes, unlike in bulk material.

Contribution

It demonstrates an anomalous phase transition behavior in strained SrTiO$_3$ films, highlighting the impact of epitaxial strain on transition nature and internal lattice dynamics.

Findings

Critical temperature varies strongly with strain

Lattice constants evolve smoothly through transition

Internal octahedral rotations uncoupled from lattice shape

Abstract

We have studied the cubic to tetragonal phase transition in epitaxial SrTiO$_3$ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO$_6$ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with T$_C$ as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO$_6$ rotations) have become uncoupled from the overall lattice shape.