Two Dimensional Spin-Polarized Electron Gas at the Oxide Interfaces

The formation of a novel spin-polarized 2D electron gas at the LaMnO$_3$ monolayer embedded in SrMnO$_3$ is predicted from the first-principles density-functional calculations. The La (d) electrons become confined in the direction normal to the interface in the potential well of the La layer, serving as a positively-charged layer of electron donors. These electrons mediate a ferromagnetic alignment of the Mn t$_{2g}$ spins near the interface via the Anderson-Hasegawa double exchange and become, in turn, spin-polarized due to the internal magnetic fields of the Mn moments.Comment: 5 pages, 6 figure

Strain and Electric Field Modulation of the Electronic Structure of Bilayer Graphene

We study how the electronic structure of the bilayer graphene (BLG) is changed by electric field and strain from {\it ab initio} density-functional calculations using the LMTO and the LAPW methods. Both hexagonal and Bernal stacked structures are considered. The BLG is a zero-gap semiconductor like the isolated layer of graphene. We find that while strain alone does not produce a gap in the BLG, an electric field does so in the Bernal structure but not in the hexagonal structure. The topology of the bands leads to Dirac circles with linear dispersion in the case of the hexagonally stacked BLG due to the interpenetration of the Dirac cones, while for the Bernal stacking, the dispersion is quadratic. The size of the Dirac circle increases with the applied electric field, leading to an interesting way of controlling the Fermi surface. The external electric field is screened due to polarization charges between the layers, leading to a reduced size of the band gap and the Dirac circle. The screening is substantial in both cases and diverges for the Bernal structure for small fields as has been noted by earlier authors. As a biproduct of this work, we present the tight-binding parameters for the free-standing single layer graphene as obtained by fitting to the density-functional bands, both with and without the slope constraint for the Dirac cone.Comment: 7 pages, 7 figure

Intertwined Lattice Deformation and Magnetism in Monovacancy Graphene

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground state planar configuration with a saturated local moment of 1.5 $\mu_B$, and a metastable non-planar configuration with a vanishing magnetic moment, at a modest energy expense of ~50 meV. Though non-planarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (v$\sigma$, v$\pi$). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of v$\pi$ yielding a fractional moment, which is aligned parallel to the unpaired v$\sigma$ electron through Hund's coupling. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between v$\sigma$ and local d$\pi$ states, to form a hybridized v$\sigma^\prime$ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v$\sigma$ and v$\pi$ lowers the energy. The gradual spin reversal of v$\pi$ with increasing non-planarity opens up the possibility of an intermediate structure with balanced v$\pi$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v$\sigma$ based spin-1/2 paramagnetism.Comment: Published version - URL http://link.aps.org/doi/10.1103/PhysRevB.93.16540