3,578 research outputs found
Commensuration and Interlayer Coherence in Twisted Bilayer Graphene
The low energy electronic spectra of rotationally faulted graphene bilayers
are studied using a long wavelength theory applicable to general commensurate
fault angles. Lattice commensuration requires low energy electronic coherence
across a fault and preempts massless Dirac behavior near the neutrality point.
Sublattice exchange symmetry distinguishes two families of commensurate faults
that have distinct low energy spectra which can be interpreted as
energy-renormalized forms of the spectra for the limiting Bernal and AA stacked
structures. Sublattice-symmetric faults are generically fully gapped systems
due to a pseudospin-orbit coupling appearing in their effective low energy
Hamiltonians.Comment: 4 pages RevTeX, 3 jpg figure
Continuum theory for the piezoelectric response of chiral nanotubes under uniaxial and torsional stresses
We develop and solve a continuum theory for the piezoelectric response of
nanotubes under applied uniaxial and torsional stresses. We find that the
piezoelectric response is controlled by the chiral angle, the aspect ratio, and
two dimensionless parameters specifying the ratio of the strengths of the
electrostatic and elastic energies. The model is solved in two limiting cases
and the solutions are discussed. These systems are found to have several
unexpected physical effects not seen in conventional bulk systems, including a
strong stretch-twist coupling and the development of a significant bound charge
density in addition to a surface charge density. The model is applied to
estimate the piezoelectric response of a boron-nitride nanotube under uniform
tensile stress.Comment: 8 pages, 4 figures. Submitted to Physical Review
Nonradiative Recombination of Excitons in Carbon Nanotubes Mediated by Free Charge Carriers
Free electrons or holes can mediate the nonradiative recombination of
excitons in carbon nanotubes. Kinematic constraints arising from the quasi
one-dimensional nature of excitons and charge carriers lead to a thermal
activation barrier for the process. However, a model calculation suggests that
the rate for recombination mediated by a free electron is the same order of
magnitude as that of two-exciton recombination. Small amounts of doping may
contribute to the short exciton lifetimes and low quantum yields observed in
carbon nanotubes.Comment: 18 pages, 4 figures. Submitted to Physical Review
Band Symmetries and Singularities in Twisted Multilayer Graphene
The electronic spectra of rotationally faulted graphene bilayers are
calculated using a continuum formulation for small fault angles that identifies
two distinct electronic states of the coupled system. The low energy spectra of
one state features a Fermi velocity reduction which ultimately leads to
pairwise annihilation and regeneration of its low energy Dirac nodes. The
physics in the complementary state is controlled by pseudospin selection rules
that prevent a Fermi velocity renormalization and produce second generation
symmetry-protected Dirac singularities in the spectrum. These results are
compared with previous theoretical analyses and with experimental data.Comment: 5 pages, 3 figure
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