Transport In Bilayer Graphene Research Articles

Conductance and shot noise in strained bilayer graphene

We explore the effect of trigonal warping and of elastic deformations on the electronic spectrum of bilayer graphene devices, on their ballistic conductance as well as on the shot noise. Uniaxial strain distorts the lattice creating a uniform fictitious gauge field in the electronic Dirac Hamiltonian which ultimately causes a dramatic reconstruction in the trigonally warped electronic spectrum, inducing topological transitions in the Fermi surface. In this paper we present results of ballistic transport in bilayer graphene in the absence and presence of strain, with particular focus on noise and the Fano factor F. The inclusion of trigonal warping preserves the pseudo-diffusive value of F = 1/3 at the Dirac point, as calculated in the absence of trigonal warping terms. However, the range of energies which show pseudo-diffusive transport increases by orders of magnitude compared to the results stemming out of a parabolic spectrum and the applied strain acts to increase this energy range further.

Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene

Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport.

Asymmetric Effect of Oxygen Adsorption on Electron and Hole Mobilities in Bilayer Graphene: Long- and Short-Range Scattering Mechanisms

We probe electron and hole mobilities in bilayer graphene under exposure to molecular oxygen. We find that the adsorbed oxygen reduces electron mobilities and increases hole mobilities in a reversible and activated process. Our experimental results indicate that hole mobilities increase due to the screening of long-range scatterers by oxygen molecules trapped between the graphene and the substrate. First principle calculations show that oxygen molecules induce resonant states close to the charge neutrality point. Electron coupling with such resonant states reduces the electron mobilities, causing a strong asymmetry between electron and hole transport. Our work demonstrates the importance of short-range scattering due to adsorbed species in the electronic transport in bilayer graphene on SiO2 substrates.

Effect of temperature, electric and magnetic field on spin relaxation in bilayer graphene

In spintronics, spin degree of freedom of an electron is used to store and process information and thus can provide numerous advantages over conventional electronics by providing new functionalities. In this paper, we employ the semiclassical Monte Carlo approach to study the spin polarized transport in bilayer graphene. Due to lower spin orbit interaction (SOI) and higher spin relaxation lengths, graphene is considered as suitable material for spintronics application. Spin relaxation in bilayer graphene is caused by D'yakonov---Perel (DP) relaxation and Elliott---Yafet (EY) relaxation. The effect of temperature, magnetic field and driving electric field on spin relaxation length is studied. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction.

Effect of Zeeman splitting and interlayer bias potential on electron transport in bilayer graphene

Motivated by the recent experiments [Weitz et al., Science 330, 812 (2010) and Kim et al., Phys. Rev. Lett. 107, 016803 (2011)], we present here a theoretical analysis for the Hall resistance, the longitudinal resistance, and the Hall conductance of a six-terminal bilayer graphene Hall bar under a perpendicular magnetic field. The Landauer-B\uttiker formalism combined with the nonequilibrium Green function method is applied. In the presence of both the Zeeman splitting and interlayer bias potential $U$, the spin and valley degeneracies of Landau levels are lifted, which leads to the result that the filling factor $\ensuremath{\nu}$ can be an arbitrary integer number and the Hall resistance exhibits a quantum plateau at $1/\ensuremath{\nu}(h/{e}^{2})$. While $\ensuremath{\nu}=0$, the system can be in both the spin-polarized and valley-polarized regions. For the valley-polarized $\ensuremath{\nu}=0$ region, the longitudinal resistance is predicted to have a very large value which means that the system is a quantum Hall insulator even though there are four states in the Fermi surface. However, in the spin-polarized $\ensuremath{\nu}=0$ region, the system is predicted to display a quantum spin Hall effect, in which the spin-up and spin-down edge states are counterpropagating. In both the spin-polarized and valley-polarized $\ensuremath{\nu}=0$ regions, the Hall conductances show zero quantum plateaus, although the Hall resistances are very different. In addition, due to the counterpropagating edge states, the longitudinal resistance exhibits some fractional quantum plateaus with values $2/9(h/{e}^{2})$, $1/4(h/{e}^{2})$, $1/2(h/{e}^{2})$, etc.

Unipolar transport in bilayer graphene controlled by multiple p-n interfaces

Unipolar transport is demonstrated in a bilayer graphene with a series of p-n junctions and is controlled by electrostatic biasing by a comb-shaped top gate. The OFF state is induced by multiple barriers in the p-n junctions, where the band gap is generated by applying a perpendicular electric field to the bilayer graphene, and the ON state is induced by the p-p or n-n configurations of the junctions. As the number of the junction increases, current suppression in the OFF state is pronounced. The multiple p-n junctions also realize the saturation of the drain current under relatively high source-drain voltages.

Electron transport properties of bilayer graphene

Electron transport in bilayer graphene is studied by using a first-principles analysis and the Monte Carlo simulation under conditions relevant to potential applications. While the intrinsic properties are found to be much less desirable in bilayer than in monolayer graphene, with significantly reduced mobilities and saturation velocities, the calculation also reveals a dominant influence of extrinsic factors such as the substrate and impurities. Accordingly, the difference between two graphene forms is more muted in realistic settings, although the velocity-field characteristics remain substantially lower in the bilayer. When bilayer graphene is subject to an interlayer bias, the resulting changes in the energy dispersion lead to stronger electron scattering at the bottom of the conduction band. The mobility decreases significantly with the size of the generated band gap, whereas the saturation velocity remains largely unaffected.

Multiband transport in bilayer graphene at high carrier densities

We report a multiband transport study of bilayer graphene at high carrier densities. Employing a poly(ethylene)oxide-CsClO$_4$ solid polymer electrolyte gate we demonstrate the filling of the high energy subbands in bilayer graphene samples at carrier densities $|n|\geq2.4\times 10^{13}$ cm$^{-2}$. We observe a sudden increase of resistance and the onset of a second family of Shubnikov de Haas (SdH) oscillations as these high energy subbands are populated. From simultaneous Hall and magnetoresistance measurements together with SdH oscillations in the multiband conduction regime, we deduce the carrier densities and mobilities for the higher energy bands separately and find the mobilities to be at least a factor of two higher than those in the low energy bands.

Influence of Band-Gap Opening on Ballistic Electron Transport in Bilayer Graphene and Graphene Nanoribbon FETs

Although a graphene is a zero-gap semiconductor, band-gap energy values up to several hundred millielectronvolts have been introduced by utilizing quantum-mechanical confinement in nanoribbon structures or symmetry breaking between two carbon layers in bilayer graphenes (BLGs). However, the opening of a band gap causes a significant reduction in carrier velocity due to the modulation of band structures in their low-energy spectra. In this paper, we study intrinsic effects of the band-gap opening on ballistic electron transport in graphene nanoribbons (GNRs) and BLGs based on a computational approach, and discuss the ultimate device performances of FETs with those semiconducting graphene channels. We have shown that an increase in the external electric field in BLG-FETs to obtain a larger band-gap energy degrades substantially its electrical characteristics because of deacceleration of electrons due to a Mexican hat structure; therefore, GNR-FETs outperform in principle BLG-FETs.

Observation of Long Spin-Relaxation Times in Bilayer Graphene at Room Temperature

We report on the first systematic study of spin transport in bilayer graphene (BLG) as a function of mobility, minimum conductivity, charge density, and temperature. The spin-relaxation time τ(s) scales inversely with the mobility μ of BLG samples both at room temperature (RT) and at low temperature (LT). This indicates the importance of D'yakonov-Perel' spin scattering in BLG. Spin-relaxation times of up to 2 ns at RT are observed in samples with the lowest mobility. These times are an order of magnitude longer than any values previously reported for single-layer graphene (SLG). We discuss the role of intrinsic and extrinsic factors that could lead to the dominance of D'yakonov-Perel' spin scattering in BLG. In comparison to SLG, significant changes in the carrier density dependence of τ(s) are observed as a function of temperature.

Spin transport in bilayer graphene

In this work, we perform a study of spin transport in bilayer graphene using semiclassical Monte Carlo simulation. Both the D’yakonov–Perel’ (DP) and Elliot–Yafet (EY) mechanisms for spin relaxation are considered. A vertical field of varying magnitude is applied across the bilayer and the dependence of the spin relaxation length on the applied field is considered. It is found that the spin relaxation length is a function of the applied vertical field, due to the effects of the EY and DP mechanisms, and the relaxation length reaches a maximum for a particular value of the vertical field.

Influence of Disorder on Conductance in Bilayer Graphene under Perpendicular Electric Field

Electron transport in bilayer graphene placed under a perpendicular electric field is revealed experimentally. Steep increase of the resistance is observed under high electric field; however, the resistance does not diverge even at low temperatures. The observed temperature dependence of the conductance consists of two contributions: the thermally activated (TA) conduction and the variable range hopping (VRH) conduction. We find that for the measured electric field range (0-1.3 V/nm) the mobility gap extracted from the TA behavior agrees well with the theoretical prediction for the band gap opening in bilayer graphene, although the VRH conduction deteriorates the insulating state more seriously in bilayer graphene with smaller mobility. These results show that the improvement of the mobility is crucial for the successful operation of the bilayer graphene field effect transistor.

Charged impurity scattering in bilayer graphene

We have examined the impact of charged impurity scattering on charge carrier transport in bilayer graphene (BLG) by deposition of potassium in ultrahigh vacuum at low temperature. Charged impurity scattering gives a conductivity which is supralinear in carrier density with a magnitude similar to single-layer graphene for the measured range of carrier densities of $2--4\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$. Upon addition of charged impurities of concentration ${n}_{\text{imp}}$, the minimum conductivity ${\ensuremath{\sigma}}_{\text{min}}$ decreases proportional to ${n}_{\text{imp}}^{\ensuremath{-}1/2}$ while the electron and hole puddle carrier density increases proportional to ${n}_{\text{imp}}^{1/2}$. These results for the intentional deposition of potassium on BLG are consistent with theoretical predictions for charged impurity scattering assuming a gapless hyperbolic dispersion relation. However, our results also suggest that charged impurity scattering alone cannot explain the observed transport properties of pristine BLG on ${\text{SiO}}_{2}$ before potassium doping.

Theory of carrier transport in bilayer graphene

We develop a theory for density, disorder, and temperature dependent electrical conductivity of bilayer graphene in the presence of long-range charged impurity scattering as well as an additional short-range disorder of independent origin, establishing that both scattering mechanisms contribute significantly to determining bilayer transport properties. We find that although strong screening properties of bilayer graphene lead to qualitative differences with the corresponding single layer situation, both systems exhibit the linearly density dependent conductivity at high density and the minimum graphene conductivity behavior around the charge neutrality point due to the formation of inhomogeneous electron-hole puddles induced by the random charged impurity centers.

Electronic transport in bilayer graphene

We present theoretical studies on the transport properties and localization effects of bilayer graphene. We calculate the conductivity by using the effective mass model with the self-consistent Born approximation, in the presence and absence of an energy gap opened by the interlayer asymmetry. We find that, in the absence of the gap, the minimum conductivity approaches the universal value by increasing the disorder potential, and the value is robust in the strong disorder regime where mixing with high-energy states is considerable. The gap-opening suppresses the conductivity over a wide energy range, even in the region away from the gap.We also study the localization effects in the vicinity of zero energy in bilayer graphene. We find that the states are all localized in the absence of the gap, while the gap-opening causes a phase transition analogous to the quantum Hall transition, which is accompanied by electron delocalization.

Transport in bilayer graphene: Calculations within a self-consistent Born approximation

The transport properties of a bilayer graphene are studied theoretically within a self-consistent Born approximation. The electronic spectrum is composed of $k$-linear dispersion in the low-energy region and $k$-square dispersion as in an ordinary two-dimensional metal at high energy, leading to a crossover between different behaviors in the conductivity on changing the Fermi energy or disorder strengths. We find that the conductivity approaches $2{e}^{2}∕{\ensuremath{\pi}}^{2}\ensuremath{\hbar}$ per spin in the strong-disorder regime, independently of the short- or long-range disorder.