Interplay between lattice, strain, spin and electronic degrees of freedom in eg1 perovskites

ABO3 perovskites, with a B cation in a eg1 electronic configuration, have long been the focus of extensive research due to their promising metal-insulator transition (MIT) and related technological applications. However, because of the complex interplay between charge, orbital, spin and lattice degrees of freedom, distinct families of compounds like RMnO3, RNiO3, and AFeO3 perovskites, with R = rare-earth and A = alkaline-earth element, can exhibit quite different behaviors showing either orbital ordering and Jahn-Teller distortions or charge ordering and breathing distortions. What makes eg1 perovskites even more interesting is that the MIT property is extremely sensitive to external stimuli (e.g., strain, pressure, dimensionality, and stoichiometry). This provides an ideal platform to purposely tune related properties for versatile applications and has recently fueled significant research interest. In particular, epitaxial thin films and superlattices are two effective and promising approaches to tune MIT temperature (TMI ) in device applications. The great and recent advances in the synthesis of high-quality epitaxial thin films and superlattices spark rapidly increasing interest in the control of TMI. However, despite the effectiveness of tailoring TMI, their
actual influence is quite complex, and different studies may give inconsistent conclusions. Over the past years, continuous efforts have been devoted to understanding the microscopic mechanism behind the evolution of TMI in eg1 perovskites thin films and superlattices. Nevertheless, for the thin films, there is still a lack of a reliable and general explanation to simultaneously describe the
evolution of TMI under different strain conditions. In terms of superlattices, the roles of interface effect including the structural and electronic coupling, dimensionality, and confinement effect remain to be understood. In this thesis, combining first-principles calculations and Landau theory analysis, we explore comprehensively the interplay between lattice, strain, spin, and charge degrees of freedom in eg1 perovskites thin films and superlattices. We firstly investigate the origin of breathing distortions and charge ordering induced MIT in CaFeO3 from lattice coupling and electron-phonon coupling. Another aim is to rationalize the origin of a huge increase of TMI recently observed in CaFeO3 thin film under large tensile strain. We reveal that the breathing distortions, which are the origin of MIT, are triggered by the mode coupling with oxygen octahedral rotations. We further highlight that
epitaxial strain can tune the balance between charge ordering and orbital ordering phases. More especially, epitaxial tensile strain favors the Jahn-Teller distortions and drives the phase transition from charge ordering to orbital ordering. Since the orbital ordering system like LaMnO3 tends to have a high value of TMI, we infer that the new orbital ordering phase results in the unusual
increase of TMI observed in thin film under tensile strain. Then, we revisit the fundamental role of strain and pressure in several charge ordering perovskites. We propose that the effect of strain and pressure can be unified by the activation or tuning of specific strain modes, which in turn affect material properties through the direct and indirect strain-phonon-phonon coupling. Taking
widely investigated NdNiO3 thin film as an example, we develop a new perspective on the mechanical control of TMI and formulate a reliable and general theoretical guidance for the rational control of TMI in experiments.
Finally, we explore the superlattices composed of eg1 perovskites. The first purpose is to induce MIT in metallic eg1 perovskites like SrFeO3 through the superlattices strategy. The roles of cation ordering induced confinement effect, lattice distortions and strain have been carefully investigated, allowing us to reveal that both charge ordering and orbital ordering phases are unstable and capable to induce MIT by collaborating with cation ordering. Furthermore, we point out that the stability of the orbital ordering phase with Jahn-Teller distortions is very sensitive to epitaxial strain, while the charge ordering phase is insensitive to strain. Basing on this knowledge, we highlight that strain engineering can be utilized to switch between orbital ordering and charge ordering phases. More interestingly, we can purposely design charge ordering and orbital ordering ferromagnetic and ferroelectric multiferroics by selecting specific superlattices and strain conditions. Another aim is to rationalize the experimental findings of a collaborated work on (NdNiO3)m/(SmNiO3)m superlattices, which show layer thickness dependent MIT behaviors. From the view of first-principles calculations, we firstly helped to rule out the structural coupling at the interface as the key factor that controls the TMI evolution in the superlattices. Then we helped to verify the phase boundary energy between metallic and insulating phases as the dominant factor affecting the TMI proposed by our collaborators. Combining first-principles calculations, Landau model, and the experimental results, we convincingly propose a new paradigm to tune the TMI through controlling the layer thickness in the superlattices. Our findings not only extend the
knowledge of tuning MIT in eg1 perovskites but also are important for practical applications.