Dynamic Visualization and Control of Emergent Phases in Complex Oxide Heterostructures

Work at Northwestern University will use first-principles density functional theory (DFT) meth-ods, supplemented with phenomenological models, group theory (symmetry), materials informat-ics, and chemistry principles to: (1) Evaluate the phase stability of competing ground states owing to coupled order parame-ters in single phase materials; and (2) Explore routes to design perovskites oxide superlattices to be near functional electronic phase boundaries from control of lattice modes and interfacial structure. These efforts will allow for the selection of the optimal materials (chemistries and structures) to be synthesized by thin film team members and the corresponding experimental configurations for dynamical tuning of the electronic, spin, and orbital states with THz pulses. The planned investigations by Rondinelli form a key component of the proposed project. The technical proposal, therefore, is narrated in a manner to clearly delineate the specific objectives and computational methods that his group will use to achieve the target deliverables—a class of novel emergent (hidden) phases in oxides based on tuning electronic and steric interactions through real-space control of structure topology, chemical ordering, and polyhedral geometric connectivity. For that reason, planned tasks or only generally described and itemized below. Complete details are available in the Technical Narrative. The main tasks include to:  Identify suitable exchange-correlation potentials for density functional studies of correlated oxides  Compute phase stability of bulk and artificial oxides under various geometric constraints, including strain, superlattice periodicity, dimensionality, and cation ordering  Determine the equilibrium atomic structure, electronic band structure, magnetic order, and orbital polarizations for ground state, metastable, and trapped configurations through energy basin explorations  Formulate structure-based models of phase stability and changes in magnetic/orbital interactions  Perform first-principles calculations of interatomic force constants to obtain phonon dispersion curves  Examine changes in IR and Raman active modes with external perturbations  Create a predictive framework for the rational design of hidden states of matter in correlated oxides  Inform experimental design of experiment and cross-check experimental results Calculations will be performed by the budgeted personnel (1 graduate student) in concert with the PI, whom will be actively involved in supervising the entire project, analyzing results, mentoring research personnel, and exchanging information with team members. These computational simulations will inform experimental design and characterization by the team members while also contributing to a microscopic understanding of the non-equilibrium states in oxides under THz excitations. It is envisaged that this knowledge may ultimately pro-vide a platform for the design of dynamic materials with previously unknown transient or long-lived correlated states of matter. It is anticipated that a shared website and data server, bi-monthly virtual meetings, and visits to Penn State, Argonne National Laboratory, and other team institutions will facilitate the trans-mission of new data resulting directly from this project.