Electron Confinement, Orbital Ordering, and Orbital Moments in d-d Oxide Heterostructures

The (SrTiO3)m/(SrVO3)n d 0 − d 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 Qorb=(π, π) orbital ordering of lz = 0 and lz = −1 (jz = − 1 2 ) orbitals within the plane, accompanied by Qspin=(0,0) spin order (ferromagnetic alignment). The result is a SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75 μB strongly compensates the spin moment on the lz = −1 sublattice. The insulator-metal transition for n = 1 → 5 (occurring between n=4 and n=5) is reproduced. Unlike in the isoelectronic d − d TiO2/VO2 (rutile structure) system and in spite of some similarities in orbital ordering, no semi-Dirac point [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”.