379 research outputs found
Topological aspects of quantum spin Hall effect in graphene: Z topological order and spin Chern number
For generic time-reversal invariant systems with spin-orbit couplings, we
clarify a close relationship between the Z topological order and the spin
Chern number proposed by Kane and Mele and by Sheng {\it et al.}, respectively,
in the quantum spin Hall effect. It turns out that a global gauge
transformation connects different spin Chern numbers (even integers) modulo 4,
which implies that the spin Chern number and the Z topological order yield
the same classification. We present a method of computing spin Chern numbers
and demonstrate it in single and double plane of graphene.Comment: 5 pages, 3 figure
Band engineering in graphene with superlattices of substitutional defects
We investigate graphene superlattices of nitrogen and boron substitutional
defects and by using symmetry arguments and electronic structure calculations
we show how such superlattices can be used to modify graphene band structure.
Specifically, depending on the superlattice symmetry, the structures considered
here can either preserve the Dirac cones (D_{6h} superlattices) or open a band
gap (D_{3h}). Relevant band parameters (carriers effective masses, group
velocities and gaps, when present) are found to depend on the superlattice
constant n as 1/n^{p} where p is in the range 1-2, depending on the case
considered. Overall, the results presented here show how one can tune the
graphene band structure to a great extent by modifying few superlattice
parameters.Comment: accepted, J. Phys. Chem.
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