Orbital-selective Mott transition in Sr₂Mn₃As₂O₂

Sr₂Mn₃As₂O₂ is a layered material composed of alternating cuprate-like MnO₂ layers and MnAs layers similar to iron pnictides [1]. A quasi-2D Neel-AFM ordering in the MnO₂ layer accompanies the material's low-temperature insulating behavior, whereas the MnAs layer orders above room temperature and has a negligible effect on the transport properties. To better understand the experimental findings, we have performed first-principles DFT+U calculations to explore the electronic structure. We find both layers to be Mott insulating for sufficiently high Hubbard interaction U, with the MnO₂ layer requiring a larger value of U to become insulating. We observe the Mott transition in the MnO₂ layer to arise from the e_g (d_x²_−y² and d_z²) orbitals. We also notice orbital selectivity, namely the d_z² orbital forming a Mott gap at a lower Hubbard U than the d_x²_−y² orbital. We construct an effective multi-orbital Hubbard model using Wannier functions to parametrize the DFT bands, and then study this model using Variational Cluster Approximation (VCA) and Cellular Dynamic Mean Field Theory (CDMFT). Our results show the metal-insulator phase transition is indeed orbital selective.
[1] C.-W. Chen et al., "Orbital selective Mott transition in layered Sr₂Mn₃As₂O₂ single crystals" (under review)