Transport measurements of the two-dimensional electron gas at the LaAlO3-SrTiO3 interface have found a density of carriers much lower than expected from the "polar catastrophe" arguments. From a detail density-functional study, we suggest how this discrepancy may be reconciled. We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties. Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport.

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Abstract. The electronic structures of the two thermoelectric materials Bi 2 Te 3 and Bi 2 Se 3 are studied using density-functional theory with the spin-orbit interaction included. The electron states in the gap region and the chemical bonding can be described in terms of ppσ interaction between the atomic p orbitals within the 'quintuple' layer. For Bi 2 Se 3 , we find both the valenceband maximum as well as the conduction-band minimum, each with a nearly isotropic effective mass, to occur at the zone centre in agreement with experimental results. For Bi 2 Te 3 , we find that the six valleys for the valence-band maximum are located in the mirror planes of the Brillouin zone and they have a highly anisotropic effective mass, leading to an agreement between the de Haas-van Alphen data for the p-doped samples and the calculated Fermi surface. The calculated conduction band, however, has only two minima, instead of the six minima indicated from earlier experiments. The calculated Seebeck coefficients for both p-type and n-type materials are in agreement with the experiments.

We study the exchange interaction J between two magnetic impurities in graphene (the RKKY interaction) by directly computing the lattice Green's function for the tight-binding band structure for the honeycomb lattice. The method allows us to compute J numerically for much larger distances than can be handled by finite-lattice calculations as well as for small distances. In addition, we rederive the analytical long-distance behavior of J for linearly dispersive bands and find corrections to the oscillatory factor that were previously missed in the literature. The main features of the RKKY interaction in graphene are that unlike the J ∝ (2kF R) −2 sin(2kF R) behavior of an ordinary 2D metal in the long-distance limit, J in graphene falls off as 1/R 3 , shows the 1 + cos((K − K ′ ).R)-type oscillations with additional phase factors depending on the direction, and exhibits a ferromagnetic interaction for moments on the same sublattice and an antiferromagnetic interaction for moments on the opposite sublattices as required by particle-hole symmetry. The computed J with the full band structure agrees with our analytical results in the long-distance limit including the oscillatory factors with the additional phases.