Electron confinement, orbital ordering, and orbital moments in d⁰-d¹ oxide heterostructures

The (SrTiO₃)_m/(SrVO₃)_n d⁰-d¹ multilayer system is studied with first-principles methods through the observed insulator-to-metal transition with increasing thickness of the SrVO₃ layer. When correlation effects with reasonable magnitude are included, crystal-field splittings from the structural relaxations together with spin-orbit coupling (SOC) determines the behavior of the electronic and magnetic structures. These confined slabs of SrVO₃ prefer Q_orb=(π,π) orbital ordering of ℓ_z=0 and ℓ_z=−1 (j_z=−1/2) orbitals within the plane, accompanied by Q_spin=(0,0) spin order (ferromagnetic alignment). The result is a SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75 μ_B strongly compensates the spin moment on the ℓ_z=−1 sublattice. The insulator-metal transition for n=1→5 (occurring between n=4 and n=5) is reproduced. Unlike in the isoelectronic d⁰-d¹ TiO₂/VO₂ (rutile structure) system and in spite of some similarities in orbital ordering, no semi-Dirac point [ V. Pardo and W. E. Pickett Phys. Rev. Lett. 102 166803 (2009)] is encountered but the insulator-to-metal transition occurs through a different type of unusual phase. For n=5 this system is very near (or at) a unique semimetallic state in which the Fermi energy is topologically determined and the Fermi surface consists of identical electron and hole Fermi circles centered at k=0. The dispersion consists of what can be regarded as a continuum of radially directed Dirac points, forming a “Dirac circle.”