31 research outputs found
Effect of contact induced states on minimum conductivity in graphene
The objective of this paper is to point out that contact induced states can
help explain the structure dependence of the minimum conductivity observed
experimentally even if the samples were purely ballistic. Contact induced
states are similar to the well-known metal induced gap states (MIGS) in
metal-semiconductor Schottky junctions, which typically penetrate only a few
atomic lengths into the semiconductor, while the depth of penetration decreases
with increasing band gap. However, in graphene we find that these states
penetrate a much longer distance of the order of the width of the contacts. As
a result, ballistic graphene samples with a length less than their width can
exhibit a resistance proportional to length that is not Ohmic in origin, but
arises from a reduced role of contact-induced states. While actual samples are
probably not ballistic and involve scattering processes, our results show that
these contact induced effects need to be taken into account in interpreting
experiments and minimum conductivity depends strongly on the structure and
configuration (two- vs. four-terminal) used.Comment: 4 pages, 4 figure
Tunable Band Structure Effects on Ballistic Transport in Graphene Nanoribbons
Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic
fields are shown to exhibit dramatic changes in their band structure and
electron transport properties. A strong electric field across the ribbon
induces multiple chiral Dirac points, closing the semiconducting gap in
armchair GNR's. A perpendicular magnetic field induces partially formed Landau
levels as well as dispersive surface-bound states. Each of the applied fields
on its own preserves the even symmetry of the subband
dispersion. When applied together, they reverse the dispersion parity to be odd
and gives and mix the electron and hole subbands within
the energy range corresponding to the change in potential across the ribbon.
This leads to oscillations of the ballistic conductance within this energy
range
Conductance of graphene nanoribbon junctions and the tight binding model
Planar carbon-based electronic devices, including metal/semiconductor junctions, transistors and interconnects, can now be formed from patterned sheets of graphene. Most simulations of charge transport within graphene-based electronic devices assume an energy band structure based on a nearest-neighbour tight binding analysis. In this paper, the energy band structure and conductance of graphene nanoribbons and metal/semiconductor junctions are obtained using a third nearest-neighbour tight binding analysis in conjunction with an efficient nonequilibrium Green’s function formalism. We find significant differences in both the energy band structure and conductance obtained with the two approximations
Kinetic investigation on extrinsic spin Hall effect induced by skew scattering
The kinetics of the extrinsic spin Hall conductivity induced by the skew
scattering is performed from the fully microscopic kinetic spin Bloch equation
approach in GaAs symmetric quantum well. In the steady state, the
extrinsic spin Hall current/conductivity vanishes for the linear-
dependent spin-orbit coupling and is very small for the cubic-
dependent spin-orbit coupling. The spin precession induced by the
Dresselhaus/Rashba spin-orbit coupling plays a very important role in the
vanishment of the extrinsic spin Hall conductivity in the steady state. An
in-plane spin polarization is induced by the skew scattering, with the help of
the spin-orbit coupling. This spin polarization is very different from the
current-induced spin polarization.Comment: 5 pages, 2 figures, to be published in JPC
How close can one approach the Dirac point in graphene experimentally?
The above question is frequently asked by theorists who are interested in
graphene as a model system, especially in context of relativistic quantum
physics. We offer an experimental answer by describing electron transport in
suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with
the onset of Landau quantization occurring in fields below 5 mT. The observed
charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state
with a few charge carriers per entire micron-scale device. Above liquid helium
temperatures, the electronic properties of such devices are intrinsic, being
governed by thermal excitations only. This yields that the Dirac point can be
approached within 1 meV, a limit currently set by the remaining charge
inhomogeneity. No sign of an insulating state is observed down to 1 K, which
establishes the upper limit on a possible bandgap
Photocurrent imaging and efficient photon detection in a graphene transistor
We measure the channel potential of a graphene transistor using a scanning
photocurrent imaging technique. We show that at a certain gate bias, the impact
of the metal on the channel potential profile extends into the channel for more
than 1/3 of the total channel length from both source and drain sides, hence
most of the channel is affected by the metal. The potential barrier between the
metal controlled graphene and bulk graphene channel is also measured at various
gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the
original p-type graphene channel turns into a p-n-p channel. When light is
focused on the p-n junctions, an impressive external responsivity of 0.001 A/W
is achieved, given that only a single layer of atoms are involved in photon
detection.Comment: 24 pages, 4 figure