Structural dynamics of LaVO3 on the nanosecond time scale.

Matthew Brahlek,Jason Lapano,John W Freeland,Venkatraman Gopalan,Lei Zhang,Donald A Walko,I-Cheng Tung,Vladimir A Stoica,Roman Engel-Herbert,Hirofumi Akamatsu,Haidan Wen

doi:10.1063/1.5045704

Abstract

Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO3 in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1% out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by ∼0.3° and La displacements decrease by ∼0.2 pm, which directly corresponds to an ∼0.8° increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of ∼0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments.

Highlights

Deformed to accommodate many combinations of cations with different ionic radii without losing the corner connectivity to adjacent octahedra; this is accomplished through specific rotations of the oxygen octahedron by angles a, b, or c about the pseudocubic x, y, or z axes, respectively.12–16 For coherently strained LaVO3 grown epitaxially on SrTiO3 (001), the compressive strain favors the orthorhombic c-axis to be in the film plane,17 which can be described using Glazer notation13,18 as aÀaþc
This indicates that the lattice parameters and octahedral rotation angles along the pseudocubic x- and y-axes are the same, with an out-of-phase rotation along the x-axis and an in-phase rotation along the y-axis; while along the pseudocubic z-axis, the lattice parameter is different with an out-of-phase rotation angle c, shown in Figs. 1(c) and 1(d)
Since a large out-of-plane expansion can be induced in epitaxial thin films, while not affecting the in-plane lattice parameters, as dictated by the substrate which is typically chosen to not directly adsorb at the pump wavelength, ultrafast excitation can be used to uniquely modify rotations of the oxygen octahedra in a way that does not follow the static thermal trajectory that is accessible by changing the temperature of the film and the substrate; this can modulate electronic properties and may be used to search for entirely new metastable phases

Summary

Introduction

Deformed to accommodate many combinations of cations with different ionic radii without losing the corner connectivity to adjacent octahedra; this is accomplished through specific rotations of the oxygen octahedron by angles a, b, or c about the pseudocubic x, y, or z axes, respectively.12–16 For coherently strained LaVO3 (space group Pbnm) grown epitaxially on SrTiO3 (001), the compressive strain favors the orthorhombic c-axis to be in the film plane,17 which can be described using Glazer notation13,18 as aÀaþc.

Results

Conclusion