5,839 research outputs found
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Electron spin relaxation in graphene with random Rashba field: Comparison of D'yakonov-Perel' and Elliott-Yafet--like mechanisms
Aiming to understand the main spin relaxation mechanism in graphene, we
investigate the spin relaxation with random Rashba field induced by both
adatoms and substrate, by means of the kinetic spin Bloch equation approach.
The charged adatoms on one hand enhance the Rashba spin-orbit coupling locally
and on the other hand serve as Coulomb potential scatterers. Both effects
contribute to spin relaxation limited by the D'yakonov-Perel' mechanism. In
addition, the random Rashba field also causes spin relaxation by spin-flip
scattering, manifesting itself as an Elliott-Yafet--like mechanism. Both
mechanisms are sensitive to the correlation length of the random Rashba field,
which may be affected by the environmental parameters such as electron density
and temperature. By fitting and comparing the experiments from the Groningen
group [J\'ozsa {\it et al.}, Phys. Rev. B {\bf 80}, 241403(R) (2009)] and
Riverside group [Pi {\it et al.}, Phys. Rev. Lett. {\bf 104}, 187201 (2010);
Han and Kawakami, {\it ibid.} {\bf 107}, 047207 (2011)] which show either
D'yakonov-Perel'-- (with the spin relaxation rate being inversely proportional
to the momentum scattering rate) or Elliott-Yafet--like (with the spin
relaxation rate being proportional to the momentum scattering rate) properties,
we suggest that the D'yakonov-Perel' mechanism dominates the spin relaxation in
graphene. The latest experimental finding of a nonmonotonic dependence of spin
relaxation time on diffusion coefficient by Jo {\it et al.} [Phys. Rev. B {\bf
84}, 075453 (2011)] is also well reproduced by our model.Comment: 13 pages, 9 figures, to be published in New J. Phy
Disorder and Electronic Transport in Graphene
In this review, we provide an account of the recent progress in understanding
electronic transport in disordered graphene systems. Starting from a
theoretical description that emphasizes the role played by band structure
properties and lattice symmetries, we describe the nature of disorder in these
systems and its relation to transport properties. While the focus is primarily
on theoretical and conceptual aspects, connections to experiments are also
included. Issues such as short versus long-range disorder, localization (strong
and weak), the carrier density dependence of the conductivity, and conductance
fluctuations are considered and some open problems are pointed out.Comment: 18 pages, 5 figures, Topical Revie
Orbital symmetry fingerprints for magnetic adatoms in graphene
In this paper, we describe the formation of local resonances in graphene in
the presence of magnetic adatoms containing localized orbitals of arbitrary
symmetry, corresponding to any given angular momentum state. We show that
quantum interference effects which are naturally inbuilt in the honeycomb
lattice in combination with the specific orbital symmetry of the localized
state lead to the formation of fingerprints in differential conductance curves.
In the presence of Jahn-Teller distortion effects, which lift the orbital
degeneracy of the adatoms, the orbital symmetries can lead to distinctive
signatures in the local density of states. We show that those effects allow
scanning tunneling probes to characterize adatoms and defects in graphene.Comment: 15 pages, 11 figures. Added discussion about the multi-orbital case
and the validity of the single orbital picture. Published versio
Elliot-Yafet mechanism in graphene
The differences between spin relaxation in graphene and in other materials
are discussed. For relaxation by scattering processes, the Elliot-Yafet
mechanism, the relation between the spin and the momentum scattering times
acquires a dependence on the carrier density, which is independent of the
scattering mechanism and the relation between mobility and carrier
concentration. This dependence puts severe restrictions on the origin of the
spin relaxation in graphene. The density dependence of the spin relaxation
allows us to distinguish between ordinary impurities and defects which modify
locally the spin-orbit interaction.Comment: 4 pages + \epsilon + S
Magnetic and Kohn-Luttinger instabilities near a Van Hove singularity: monolayer versus twisted bilayer graphene
We investigate the many-body instabilities of electrons interacting near Van
Hove singularities arising in monolayer and twisted bilayer graphene. We show
that a pairing instability must be dominant over the tendency to magnetic order
as the Fermi level is tuned to the Van Hove singularity in the conduction band
of graphene. As a result of the extended character of the saddle points in the
dispersion, we find that the pairing of the electrons takes place
preferentially in a channel of f-wave symmetry, with an order parameter
vanishing at the position of the saddle points along the Fermi line. In the
case of the twisted bilayers, the dispersion has instead its symmetry reduced
down to the C_{3v} group and, most importantly, it leads to susceptibilities
that diverge at the saddle points but are integrable along the Fermi line. This
implies that a ferromagnetic instability becomes dominant in the twisted
graphene bilayers near the Van Hove singularity, with a strength which is
amplified as the lowest subband of the electron system becomes flatter for
decreasing twist angle.Comment: 16 pages, 10 figure
Graphene via large N I: Renormalization
We analyze the competing effects of moderate to strong Coulomb
electron-electron interactions and weak quenched disorder in graphene. Using a
one-loop renormalization group calculation controlled within the large-N
approximation, we demonstrate that, at successively lower energy (temperature
or chemical potential) scales, a type of non-Abelian vector potential disorder
always asserts itself as the dominant elastic scattering mechanism for generic
short-ranged microscopic defect distributions. Vector potential disorder is
tied to both elastic lattice deformations ("ripples") and topological lattice
defects. We identify several well-defined scaling regimes, for which we provide
scaling predictions for the electrical conductivity and thermopower, valid when
the inelastic lifetime due to interactions exceeds the elastic lifetime due to
disorder. Coulomb interaction effects should figure strongly into the physics
of suspended graphene films, where rs > 1; we expect vector potential disorder
to play an important role in the description of transport in such films.Comment: 25 pages, 21 figure
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