Ultrafast relaxation dynamics of the antiferrodistortive phase in Ca doped SrTiO3

TL;DR

This study investigates the ultrafast relaxation of structural distortions in Ca-doped SrTiO3 using time-resolved x-ray diffraction, revealing a sub-picosecond response driven by photoinduced electronic changes.

Contribution

It demonstrates the ultrafast, temperature-independent relaxation dynamics of the antiferrodistortive phase in Ca:SrTiO3 and links it to electronic effects via density functional theory.

Findings

Ultrafast 0.2 ps relaxation of octahedral rotation observed.

Relaxation is independent of initial temperature.

Electronic excitation causes displacive soft-mode modification.

Abstract

The ultrafast dynamics of the octahedral rotation in Ca:SrTiO3 is studied by time resolved x-ray diffraction after photo excitation over the band gap. By monitoring the diffraction intensity of a superlattice reflection that is directly related to the structural order parameter of the soft-mode driven antiferrodistortive phase in Ca:SrTiO3, we observe a ultrafast relaxation on a 0.2 ps timescale of the rotation of the oxygen octahedron, which is found to be independent of the initial temperaure despite large changes in the corresponding soft-mode frequency. A further, much smaller reduction on a slower picosecond timescale is attributed to thermal effects. Time-dependent density-functional-theory calculations show that the fast response can be ascribed to an ultrafast displacive modification of the soft-mode potential towards the normal state, induced by holes created in the oxygen 2p states.