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Spin-orbit coupling in graphene structures D. Kochan, M. Gmitra, J. Fabian Stará Lesná, 25.8.2012

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Outline Предварительные сведения Bloch vs. Wannier Tight-binding approximation = LCAO Graphene Spin-orbit-interaction in Graphene What we are doing ….

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Bloch vs. Wannier Periodic structure Bloch Theorem Brillouin zone k set of good quantum numbers Direct lattice Dual lattice

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Bloch vs. Wannier Bloch states: – delocalized & orthogonal – labeled by the momentum k Wannier states: – localized & orthogonal – labeled by the lattice vector R

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Tight-binding approximation 1) Wannier states basis = local atomic orbitals 2) Bloch states basis = Bloch sum of local atomic orbitals

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Tight-binding approximation 3) General solution: 4) Matrix(-secular) equation: How to compute ??-matrix elements?

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Tight-binding approximation 5) The heart of TB approx: -nearest & next-nearest neighbors only few terms that are lowest in |R|

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Tight-binding approximation only few terms that are lowest in |R| 5) The heart of TB approx: -nearest & next-nearest neighbors

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Tight-binding approximation 6) Further simplification – point (local) group symmetries - elements – square lattice non-zero elementszero elements

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Tight-binding approximation

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7) Secular equation + fitting of TB parameters model parameters:

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Direct lattice Dual lattice Graphene

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Graphene – basic (orbital) energetics Konschuh, Gmitra, Fabian, PRB 82 245412 (2010) Gmitra, Konschuh, Ertler, Ambrosch-Draxl, Fabian, PRB 80 235431 (2009)

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Graphene – basic (orbital) model Basic TB-model with p z - orbitals Direct lattice Dual lattice structural function of the hexagonal lattice: low energy Hamiltonian: expansion at

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Graphene – basic (orbital) model “relativistic” Hamiltonian Direct lattice Dual lattice - acts in pseudospin degrees of freedom – what is that? - seemingly 2D massless fermions - linear dispersion relation - BUT no-spin degrees of freedom, (when spin ) pseudospin up/down – amplitude to find e - on sublattice A/B

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Spin-orbit coupling

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Spintronics - tunable & strong/week SOC spin relaxation (quantum) spin Hall effect - TI magneto-anisotropy weak (anti-)localization SOC - quintessence of Spin-orbit coupling

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Intra-atomic spin-orbit coupling Questions: How does SOC modify in periodically arrayed structures? Is (and by how much) SOC enhanced in carbon allotropes? How to further stimulate and control SOC?

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Graphene - Intrinsic SOC Gmitra et al., PRB 80 235431 (2009) symmetry arguments: Kane, Mele, PRL 95 226801 (2005) McClure, Yafet, Proc. of 5 th Conf. on Carbon, Pergamon, Vol.1, pp 22-28, 1962 physics behind d-orbitals Ab-initioTheory next-nearest neighbor interaction

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How to derive effective SOC? Direct lattice Dual lattice Group theory – invariance: - translations (obvious) - point group D 6h – symmetry group of hexagon - time-reversal: k -k, , - Graphene - Intrinsic SOC

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How to compute matrix elements? - go to atomic (Wannier) orbitals Direct lattice Dual lattice Graphene - Intrinsic SOC - employing all D 6h elements + TR one non-zero matr. elem.

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Full spin-orbit coupling Hamiltonian Direct lattice Dual lattice Graphene - Intrinsic SOC linearized SOC Hamiltonian at Gmitra et al., PRB 80 235431 (2009)

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Intrinsic SOC – atomism: - multi-TB perturbation theory Direct lattice Dual lattice Graphene - Intrinsic SOC Konschuh, Gmitra, Fabian, PRB 82 245412 (2010)

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What will happen if ….??? Direct lattice Dual lattice Graphene – as Topological Insulator Kane, Mele, PRL 95 226801 (2005)

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Graphene - Extrinsic SOC Graphene – always grown on substrate – background el. field 0 1.0 2.444.0 E [V/nm]

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How to derive effective SOC? Direct lattice Dual lattice Group theory – invariance: - translations (obvious) - point group C 6v – symmetry group of hexagon without the space inversion - time-reversal Graphene - Extrinsic SOC

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Full spin-orbit coupling Hamiltonian Graphene - Extrinsic SOC linearized SOC Hamiltonian at

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Extrinsic SOC – atomism: - multi-TB perturbation theory Direct lattice Dual lattice Graphene - Extrinsic SOC Konschuh, Gmitra, Fabian, PRB 82 245412 (2010)

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C O N C L U S I O N Graphene: - intrinsic SOC dominated by d-orbitals - detailed ab-initio and multi-TB-studies Bilayer graphene: - symmetry derived SO Hamiltonian - detailed ab-initio and model studies - band structure & SO-splittings - SOC comparable with single-layered graphene Hydrogenized graphene structures: SH & SI - detailed ab-initio, symmetry and TB-model studies - substantial SO-splittings compared to single-layered graphene Gmitra et al., PRB 80 235431 (2009) Konschuh et al., PRB 82 245412 (2010) Konschuh et al., PRB 85 1145423 (2012) Gmitra, Kochan, Fabian – work in progress

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