562 research outputs found
Splitting of the Zero-Energy Landau Level and Universal Dissipative Conductivity at Critical Points in Disordered Graphene
We report on robust features of the longitudinal conductivity ()
of the graphene zero-energy Landau level in presence of disorder and varying
magnetic fields. By mixing an Anderson disorder potential with a low density of
sublattice impurities, the transition from metallic to insulating states is
theoretically explored as a function of Landau-level splitting, using highly
efficient real-space methods to compute the Kubo conductivities (both
and Hall ). As long as valley-degeneracy is
maintained, the obtained critical conductivity
is robust upon disorder increase (by almost one order of magnitude) and
magnetic fields ranging from about 2 to 200 Tesla. When the sublattice symmetry
is broken, eventually vanishes at the Dirac point owing to
localization effects, whereas the critical conductivities of pseudospin-split
states (dictating the width of a plateau) change to
, regardless of the splitting strength, superimposed
disorder, or magnetic strength. These findings point towards the non
dissipative nature of the quantum Hall effect in disordered graphene in
presence of Landau level splitting
Efficient Linear Scaling Approach for Computing the Kubo Hall Conductivity
We report an order-N approach to compute the Kubo Hall conductivity for
disorderd two-dimensional systems reaching tens of millions of orbitals, and
realistic values of the applied external magnetic fields (as low as a few
Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity
using a wavepacket propagation method and a continued fraction
expansion for the computation of diagonal and off-diagonal matrix elements of
the Green functions. The validity of the method is demonstrated by comparison
of results with brute-force diagonalization of the Kubo formula, using
(disordered) graphene as system of study. This approach to mesoscopic system
sizes is opening an unprecedented perspective for so-called reverse engineering
in which the available experimental transport data are used to get a deeper
understanding of the microscopic structure of the samples. Besides, this will
not only allow addressing subtle issues in terms of resistance standardization
of large scale materials (such as wafer scale polycrystalline graphene), but
will also enable the discovery of new quantum transport phenomena in complex
two-dimensional materials, out of reach with classical methods.Comment: submitted PRB pape
Spin Valve Effect in ZigZag Graphene Nanoribbons by Defect Engineering
We report on the possibility for a spin valve effect driven by edge defect
engineering of zigzag graphene nanoribbons. Based on a mean-field spin
unrestricted Hubbard model, electronic band structures and conductance profiles
are derived, using a self-consistent scheme to include gate-induced charge
density. The use of an external gate is found to trigger a semiconductor-metal
transition in clean zigzag graphene nanoribbons, whereas it yields a closure of
the spin-split bandgap in the presence of Klein edge defects. These features
could be exploited to make novel charge and spin based switches and field
effect devices.Comment: 4 pages, 4 figure
Backscattering in carbon nanotubes : role of quantum interference effects
For similar disorder, the backscattering contribution to the conductivity,
irrelevant for metallic single-walled carbon nanotubes, is proved to become
more significant for doped semiconducting systems, as found in experiments. In
the case of multi-walled nanotubes, the intershell coupling is further shown to
enhance the contribution of backscattering for "metallic" double-walled,
whereas it remains insignificant for "metallic/semiconducting" double-walled
systems. This supports that MWNTs are long ballistic conductors close to the
charge neutrality point.Comment: 8 pages, 3 figure
Valley-Polarized Quantum Transport Generated by Gauge Fields in Graphene
We report on the possibility to simultaneously generate in graphene a {\it
bulk valley-polarized dissipative transport} and a {\it quantum valley Hall
effect} by combining strain-induced gauge fields and real magnetic fields. Such
unique phenomenon results from a resonance/anti-resonance effect driven by the
superposition/cancellation of superimposed gauge fields which differently
affect time reversal symmetry. The onset of a valley-polarized Hall current
concomitant to a dissipative valley-polarized current flow in the opposite
valley is revealed by a Hall conductivity plateau. We employ efficient
linear scaling Kubo transport methods combined with a valley projection scheme
to access valley-dependent conductivities and show that the results are robust
against disorder.Comment: 2D Materials (accepted for publication
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