25 research outputs found
Unimpeded tunneling in graphene nanoribbons
We studied the Klein paradox in zigzag (ZNR) and anti-zigzag (AZNR) graphene
nanoribbons. Due to the fact that ZNR (the number of lattice sites across the
nanoribbon (N is even) and AZNR (N is odd) configurations are indistinguishable
when treated by the Dirac equation, we supplemented the model with a
pseudo-parity operator whose eigenvalues correctly depend on the sublattice
wavefunctions for the number of carbon atoms across the ribbon, in agreement
with the tight-binding model. We have shown that the Klein tunneling in zigzag
nanoribbons is related to conservation of the pseudo-parity rather than
pseudo-spin in infinite graphene. The perfect transmission in the case of
head-on incidence is replaced by perfect transmission at the center of the
ribbon and the chirality is interpreted as the projection of the pseudo-parity
on momentum at different corners of the Brillouin zone
Spectroscopic Characterization of Gapped Graphene in the Presence of Circularly Polarized Light
We present a description of the energy loss of a charged particle moving
parallel to a graphene layer and graphene double layers. Specifically, we
compare the stopping power of the plasma oscillations for these two
configurations in the absence as well as the presence of circularly polarized
light whose frequency and intensity can be varied to yield an energy gap of
several hundred between the valence and conduction bands. The
dressed states of the Dirac electrons by the photons yield collective plasma
excitations whose characteristics are qualitatively and quantitatively
different from those produced by Dirac fermions in gapless graphene, due in
part to the finite effective mass of the dressed electrons. For example, the
range of wave numbers for undamped self-sustaining plasmons is increased as the
gap is increased, thereby increasing the stopping power of graphene for some
range of charged particle velocity when graphene is radiated by circularly
polarized light
Anomalous Photon-Assisted Tunneling in Graphene
We investigated the Dirac electrons transmission through a potential barrier
in the presence of circularly polarized light. An anomalous photon-assisted
enhanced transmission is predicted and explained in a comparison with the
well-known Klein paradox. It is demonstrated that the perfect transmission for
nearly-head-on collision in an infinite graphene is suppressed in gapped
dressed states of electrons, which is further accompanied by shift of peaks as
a function of the incident angle away from the head-on collision. In addition,
the perfect transmission in the absence of potential barrier is partially
suppressed by a photon-induced gap in illuminated graphene. After the effect of
rough edges of the potential barrier or impurity scattering is included, the
perfect transmission with no potential barrier becomes completely suppressed
and the energy range for the photon-assisted perfect transmission is reduced at
the same time