Ultrafast strain dynamics in SrCoO2.5 thin films

Abstract

<sec>In order to understand the relationship between the structure of materials and its function, it is necessary to investigate the changes of the transient structure of materials over time. Laser-based plasma X-ray sources are currently widely used in the study of ultrafast structure dynamics in condensed matter due to their miniaturization and ultrahigh spatial-temporal resolution. Strongly correlated transition-metal oxides have attracted enormous attention due to their peculiar properties, among them Co-based oxides has now become one of the most promising candidates for renewable energy applications. With the variation of the oxygen stoichiometry, the physical properties of SrCoO_3–<i>x</i>, ferromagnetic metal perovskite SrCoO₃ and antiferromagnetic insulator brownmillerite SrCoO_2.5 can be reversibly transferred. Besides, the various complex physical properties make SrCoO_2.5 quite popular for fundamental research, the development of solid oxide fuel cells, etc. However, the research of its dynamic behavior under transient photo-excitation is still limited. Therefore, it is necessary to study the strain fields of SrCoO_2.5 films with different thickness. </sec><sec>This report focuses on the structural dynamics of SrCoO_2.5 films induced by ultrashort laser pulses. The ultrafast X-ray diffraction simulations exhibit transient changes of Bragg peak positions of the SrCoO_2.5 excited by laser. By studying the 40 nm- and 60 nm-thick samples, we observe a continuous shift of the Bragg peak towards lower angels at first and then a backshift until it reaches a new equilibrium. In contrast, the 100 nm-thick SrCoO_2.5 film exhibits a transient splitting of Bragg peak into two distinct parts until the initial peak disappears. For further research, we use Thomsen model to simulate the generation and evolution of acoustic deformation of SCO_2.5 thin film on a substrate supporting the LaAlO₃ film. In the case of the thicker film, we find that an inhomogeneity of temperature distribution will lead its thermal stress characteristics to change, and result in the transient splitting of Bragg peak. We believe that this work is important for analyzing the laser excited ultrafast dynamics of cobalt-based perovskite materials.</sec>