Polar Catastrophe, Orbital Reconstruction, and Emergent Ferromagnetic Exchange Coupling at the SrFeO₂(001) Surface

The FeO₂-terminated (001) surface of the stable antiferromagnetic insulating phase of the infinite-layer oxide SrFeO₂ is found to undergo a magnetic reconstruction consisting of a spin-flip process at the surface: Each Fe spin at the surface flips to pair with one in the subsurface layer. In spite of the polar catastrophe inherent at the polar surface, the behavior is driven by a reduced intra-atomic 3d_<i>z</i>²–4s hybridization and the enhanced Hund’s coupling due to surface symmetry lowering. These results, based on density-functional theory, show a surface-driven ferromagnetic exchange coupling in transition-metal oxides and provide an effective avenue to realize unusual electronic and magnetic properties

Single-Spin Dirac Fermion and Chern Insulator Based on Simple Oxides

It is highly desirable to combine recent advances in the topological quantum phases with technologically relevant materials. Chromium dioxide (CrO₂) is a half-metallic material, widely used in high-end data storage applications. Using first-principles calculations, we show that a novel class of half semimetallic Dirac electronic phase emerges at the interface CrO₂ with TiO₂ in both thin film and superlattice configurations, with four spin-polarized Dirac points in momentum-space (<b><i>k</i></b>-space) band structure. When the spin and orbital degrees of freedom are allowed to couple, the CrO₂/TiO₂ superlattice becomes a Chern insulator without external fields or additional doping. With topological gaps equivalent to 43 K and a Chern number ±2, the ensuing quantization of Hall conductance to ±2<i>e</i>²/<i>h</i> will enable potential development of these highly industrialized oxides for applications in topologically high fidelity data storage and energy-efficient electronic and spintronic devices