203 research outputs found
Low field phase diagram of spin-Hall effect in the mesoscopic regime
When a mesoscopic two dimensional four-terminal Hall cross-bar with Rashba
and/or Dresselhaus spin-orbit interaction (SOI) is subjected to a perpendicular
uniform magnetic field , both integer quantum Hall effect (IQHE) and
mesoscopic spin-Hall effect (MSHE) may exist when disorder strength in the
sample is weak. We have calculated the low field "phase diagram" of MSHE in the
plane for disordered samples in the IQHE regime. For weak disorder,
MSHE conductance and its fluctuations vanish identically
on even numbered IQHE plateaus, they have finite values on those odd numbered
plateaus induced by SOI, and they have values and
on those odd numbered plateaus induced by Zeeman energy. For moderate disorder,
the system crosses over into a regime where both and are
finite. A larger disorder drives the system into a chaotic regime where
while is finite. Finally at large disorder both
and vanish. We present the physics behind this ``phase
diagram".Comment: 4 page, 3 figure
Electronic Highways in Bilayer Graphene
Bilayer graphene with an interlayer potential difference has an energy gap
and, when the potential difference varies spatially, topologically protected
one-dimensional states localized along the difference's zero-lines. When
disorder is absent, electronic travel directions along zero-line trajectories
are fixed by valley Hall properties. Using the Landauer-B\"uttiker formula and
the non-equilibrium Green's function technique we demonstrate numerically that
collisions between electrons traveling in opposite directions, due to either
disorder or changes in path direction, are strongly suppressed. We find that
extremely long mean free paths of the order of hundreds of microns can be
expected in relatively clean samples. This finding suggests the possibility of
designing low power nanoscale electronic devices in which transport paths are
controlled by gates which alter the inter-layer potential landscape.Comment: 8 pages, 5 figure
Microscopic theory of quantum anomalous Hall effect in graphene
We present a microscopic theory to give a physical picture of the formation
of quantum anomalous Hall (QAH) effect in graphene due to a joint effect of
Rashba spin-orbit coupling and exchange field . Based on a
continuum model at valley or , we show that there exist two distinct
physical origins of QAH effect at two different limits. For ,
the quantization of Hall conductance in the absence of Landau-level
quantization can be regarded as a summation of the topological charges carried
by Skyrmions from real spin textures and Merons from \emph{AB} sublattice
pseudo-spin textures; while for , the four-band low-energy
model Hamiltonian is reduced to a two-band extended Haldane's model, giving
rise to a nonzero Chern number at either or . In the
presence of staggered \emph{AB} sublattice potential , a topological phase
transition occurs at from a QAH phase to a quantum valley-Hall phase. We
further find that the band gap responses at and are different when
, , and are simultaneously considered. We also show that the
QAH phase is robust against weak intrinsic spin-orbit coupling ,
and it transitions a trivial phase when
. Moreover, we use a tight-binding
model to reproduce the ab-initio method obtained band structures through doping
magnetic atoms on and supercells of graphene, and explain
the physical mechanisms of opening a nontrivial bulk gap to realize the QAH
effect in different supercells of graphene.Comment: 10pages, ten figure
Universal spin-Hall conductance fluctuations in two dimensions
We report a theoretical investigation on spin-Hall conductance fluctuation of
disordered four terminal devices in the presence of Rashba or/and Dresselhaus
spin-orbital interactions in two dimensions. As a function of disorder, the
spin-Hall conductance shows ballistic, diffusive and insulating
transport regimes. For given spin-orbit interactions, a universal spin-Hall
conductance fluctuation (USCF) is found in the diffusive regime. The value of
the USCF depends on the spin-orbit coupling , but is independent of
other system parameters. It is also independent of whether Rashba or
Dresselhaus or both spin-orbital interactions are present. When is
comparable to the hopping energy , the USCF is a universal number . The distribution of crosses over from a Gaussian distribution
in the metallic regime to a non-Gaussian distribution in the insulating regime
as the disorder strength is increased.Comment: to be published in Phys. Rev. Lett., 4 figure
Stabilizing topological phases in graphene via random adsorption
We study the possibility of realizing topological phases in graphene with
randomly distributed adsorbates. When graphene is subjected to periodically
distributed adatoms, the enhanced spin-orbit couplings can result in various
topological phases. However, at certain adatom coverages, the intervalley
scattering renders the system a trivial insulator. By employing a finite-size
scaling approach and Landauer-B\"{u}ttiker formula, we show that the
randomization of adatom distribution greatly weakens the intervalley
scattering, but plays a negligible role in spin-orbit couplings. Consequently,
such a randomization turns graphene from a trivial insulator into a topological
state.Comment: 5 pages and 3 figure
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