Transport Properties Of Bilayer Graphene Research Articles

Controllable pure spin currents in bilayer graphene grown on monolayer Cr2X2Te6 hybrid structures: Layer-dependent magnetism

We investigate the spin transport properties of bilayer graphene grown on a monolayer Cr2X2Te6 hybrid structure. Here, X can be Ge, Si, or Sn, all of which are van der Waals layered materials with magnetic properties. Our results show that the magnetism in bilayer graphene is layer-dependent, affecting the spin-dependent energy gap and Fermi level. Focusing specifically on the X = Ge case, we reveal that pure spin currents can be precisely controlled by applying gate voltage and a perpendicular electric field across the barrier. By varying these parameters, we achieve perfect switching of pure spin current from spin-down to spin-up due to the system’s layer-dependent magnetism. Moreover, due to differing gaps between spin-up and spin-down electrons, spin polarization can sharply jump from 0 % to 100 % with changes in gate voltage. The splitting of spin conductance curves, creating a fully functional spin-filtering junction, is also predicted under a varying perpendicular electric field. This model can be extended to cases where X is Si or Sn due to their similar electronic band structures. Our research demonstrates the potential of bilayer graphene on Cr2X2Te6 hybrid structures, which exhibit distinctive layer-dependent magnetism ideal for spintronic applications.

Transport properties of vertical heterostructures under light irradiation

Electronic and transport properties of bilayer heterostructure under light irradiation are of fundamental interest to improve functionality of optoelectronic devices. We theoretically study the modification of transport properties of bilayer graphene and bilayer heterostructures under a time-periodic external light field. The bulk electronic and transport properties are studied in a Landauer-type configuration by using the nonequilibrium Green's function formalism. To illustrate the behavior of the differential conductance of a bilayer contact under light illumination, we consider tight-binding models of bilayer graphene and graphene/hexagonal boron-nitride heterostructures. The non-adiabatic driving induces sidebands of the original band structure and opening of gaps in the quasienergy spectrum. In transport properties, the gap openings are manifested in a suppression of the differential conductance. In addition to suppression, an external light field induces an enhancement of the differential conductance if photoexcited electrons tunnel into or out of a Van~Hove singularity.

Transport in Bilayer Graphene near Charge Neutrality: Which Scattering Mechanisms Are Important?

Using the semiclassical quantum Boltzmann equation (QBE), we numerically calculate the dc transport properties of bilayer graphene near charge neutrality. We find, in contrast to prior discussions, that phonon scattering is crucial even at temperatures below 40K. Nonetheless, electron-electron scattering still dominates over phonon collisions allowing a hydrodynamic approach. We introduce a simple two-fluid hydrodynamic model of electrons and holes interacting via Coulomb drag and compare our results to the full QBE calculation. We show that the two-fluid model produces quantitatively accurate results for conductivity, thermopower, and thermal conductivity.

Magnetotransport properties of graphene layers decorated with colloid quantum dots**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0300601 and 2017YFA0303304) and the National Natural Science Foundation of China (Grant Nos. 11774005, 11874071, 91221202, and 91421303).

The hybrid graphene-quantum dot devices can potentially be used to tailor the electronic, optical, and chemical properties of graphene. Here, the low temperature electronic transport properties of bilayer graphene decorated with PbS colloid quantum dots (CQDs) have been investigated in the weak or strong magnetic fields. The presence of the CQDs introduces additional scattering potentials that alter the magnetotransport properties of the graphene layers, leading to the observation of a new set of magnetoconductance oscillations near zero magnetic field as well as the high-field quantum Hall regime. The results bring about a new strategy for exploring the quantum interference effects in two-dimensional materials which are sensitive to the surrounding electrostatic environment, and open up a new gateway for exploring the graphene sensing with quantum interference effects.

Electronic structure and transport properties of bilayer graphene adsorbed by LiF2 super-halogen clusters and Li3O super-alkali clusters

The lack of a band gap limits the practical applications of graphene in electronic devices. Using first-principles calculations with a van der Waals correction, we investigated the electronic structure and transport properties of bilayer graphene (BLG) adsorbed by super-halogen clusters and super-alkali clusters. BLG sandwiched between a pair of LiF2 super-halogen and Li3O super-alkali molecules enables the energy band to open a gap of about 0.36 eV and 0.26 eV near the Fermi energy, making the system exhibit semiconducting properties, which are attributable to the strong dipole electric field between the LiF2 super-halogen and Li3O super-alkali molecules. We also found that Li3O and the adjacent layer of graphene provide an energy band at the bottom of the conduction band and that LiF2 and the adjacent layer of graphene provide an energy band at the top of the valence band. The spatial separation of the electrons and holes is highly suitable for obtaining highly efficient photoelectric conversion in photovoltaic cells. We used the density functional theory and the non-equilibrium Green’s function to determine the transport properties of the pristine BLG and the molecule-adsorbing BLG. A negative differential resistance effect occurred and an asymmetrical IV curve was observed under positive and negative bias, which is due to the inherent electric field effect between the graphene layers in the molecule-adsorbed BLG.

Transport properties of bilayer graphene decorated by K adatoms in the framework of Thomas-Fermi screening

We report the transport properties of bilayer graphene (BLG) due to the increase of Coulomb potential fluctuations and the decrease of K adatom concentration $({\mathrm{n}}_{\mathrm{K}})$ induced by diffusion and cluster formation of the K adatom as a function of temperature (20--300 K) and time at room temperature (RT). Upon K adatom deposition on BLG at $T=20$ K, conductivity decreased due to the emergence of Coulomb scatterers, and the dependence of conductivity transformed from linear $(\ensuremath{\sigma}\ensuremath{\sim}{n}^{\ensuremath{\alpha}},\phantom{\rule{0.16em}{0ex}}\ensuremath{\alpha}\ensuremath{\sim}1)$ to superlinear $(\ensuremath{\alpha}\ensuremath{\sim}1.5)$. As the inhomogeneity of the Coulomb potential on BLG increased by cluster formation of K adatoms, the magnitude of the conductivity increased, and the dependence of conductivity on charge carrier density (curvature) shifted towards linear behavior. We fit the experimental data with Boltzmann transport theory to show that the changes in the transport properties of BLG in the presence of charged impurities originate from two factors, ${\mathrm{n}}_{\mathrm{K}}$ and the Thomas-Fermi screening vector $({\mathrm{q}}_{\mathrm{TF}})$, which are both affected by cluster formation of K adatoms on BLG and the subsequent increase of Coulomb potential inhomogeneity.

Transport properties of bilayer graphene due to charged impurity scattering: Temperature-dependent screening and substrate effects

We calculate the zero-temperature conductivity of bilayer graphene (BLG) impacted by Coulomb impurity scattering using four different screening models: unscreened, Thomas–Fermi (TF), overscreened and random phase approximation (RPA). We also calculate the conductivity and thermal conductance of BLG using TF, zero- and finite-temperature RPA screening functions. We find large differences between the results of the models and show that TF and finite-temperature RPA give similar results for diffusion thermopower S[Formula: see text]. Using the finite-temperature RPA, we calculate temperature and density dependence of S[Formula: see text] in BLG on SiO2, HfO2 substrates and suspended BLG for different values of interlayer distance c and distance between the first layer and the substrate d.

Quantum transport across van der Waals domain walls in bilayer graphene

Bilayer graphene can exhibit deformations such that the two graphene sheets are locally detached from each other resulting in a structure consisting of domains with different van der Waals inter-layer coupling. Here we investigate how the presence of these domains affects the transport properties of bilayer graphene. We derive analytical expressions for the transmission probability, and the corresponding conductance, across walls separating different inter-layer coupling domains. We find that the transmission can exhibit a valley-dependent layer asymmetry and that the domain walls have a considerable effect on the chiral tunnelling properties of the charge carriers. We show that transport measurements allow one to obtain the strength with which the two layers are coupled. We perform numerical calculations for systems with two domain walls and find that the availability of multiple transport channels in bilayer graphene significantly modifies the conductance dependence on inter-layer potential asymmetry.

Stacking defect‐induced electronic cloaking of confined states and Fano resonance in zero‐energy shifted bilayer graphene

We show that a stacking defect or a shift has a striking effect on transport properties of bilayer graphene. The tunneling through a ballistic n–neutral–n junction of shifted bilayer graphene may result at normal incidence in a zero‐energy cloaking effect. States normally incident on the barrier are confined under it, depending on their pseudospin orientation. These confined states are perfectly decoupled from continuum states having the opposite pseudospin orientation. The barrier at normal incidence acts as a cloak for confined states. We also show a shift‐dependent asymmetric zero‐energy Fano resonance at near normal incidence arising from the interference between continuum and confined states. These zero‐energy cloaking effect and Fano resonance are unique since they are achieved due to the stacking defect. They do not exist in the same setup of perfectly Bernal‐stacked bilayer graphene.

Transport properties of bilayer graphene in a strong in-plane magnetic field

A strong in-plane magnetic field drastically alters the low-energy spectrum of bilayer graphene by separating the parabolic energy dispersion into two linear Dirac cones. The effect of this dramatic change on the transport properties strongly depends on the orientation of the in-plane magnetic field with respect to the propagation direction of the charge carriers and the angle at which they impinge on the electrostatic potentials. For magnetic fields oriented parallel to the potential boundaries an additional propagating mode that results from the splitting into Dirac cones enhances the transmission probability for charge carriers tunneling through the potentials and increases the corresponding conductance. Our results show that the chiral suppression of transmission at normal incidence is turned into a chiral enhancement when the magnetic field increases, thus indicating a transition from a bilayer to a monolayer-like system at normal incidence. Further, we find that the typical transmission resonances stemming from confinement in a potential barrier are shifted to higher energy and are eventually transformed into anti-resonances with increasing magnetic field. For magnetic fields oriented perpendicular to the potential boundaries we find a very pronounced transition from a bilayer system to two separated monolayer-like systems with Klein tunneling emerging at certain incident angles symmetric around 0, which also leaves a signature in the conductance. For both orientations of the magnetic field, the transmission probability is still correctly described by pseudospin conservation. Finally, to motivate the large in-plane magnetic field, we show that its energy spectrum can be mimicked by specific lattice deformations such as a relative shift of one of the layers. With this equivalence we introduce the notion of an in-plane pseudo-magnetic field.

Lifshitz transition and modulation of electronic and transport properties of bilayer graphene by sliding and applied normal compressive strain

Using density functional theory (DFT) we investigate the changes in electronic and transport properties of graphene bilayer caused by sliding one of the layers. Change in stacking pattern breaks the lattice symmetry, which results in Lifshitz transition together with the modulation of the electronic structure. Going from AA to AB stacking by sliding along armchair direction leads to a drastic transition in electronic structure from linear to parabolic dispersion. Our transport calculations show a significant change in the overall transmission value for large sliding distances along zigzag direction. The increase in interlayer coupling with normal compressive strain increases the overlapping of conduction and valence band, which leads to further shift in the Dirac points and an enhancement in the Lifshitz transition. The ability to tune the topology of band structure by sliding and/or applying normal compressive strain will open doors for controlled tuning of many physical phenomenon such as Landau levels and quantum Hall effect in graphene.

Carrier Transport Properties of Bilayer Graphene Obtained via Hall Measurements.

The electron transport characteristics of bilayer graphene were investigated in terms of changes in the temperature from 2 to 300 K. The purpose of this study was a confirming the reported values5 of the carrier density and the mobility for using a ballistic quantum transport experiment. The mechanical exfoliation method was adopted to get the best quality of bilayer graphene and the number of layers of graphene was identified by the Raman spectra. Our bilayer graphene was a slightly n-doped state with the neutrality point at Vg = -7.5 V. Hall measurements provided an accurate means to evaluate the carrier density and the mobility. In magnetic field of -9 to 9 T, clear Shubnikov-de Haas oscillations were measured in bilayer graphene, indicating the presence of a clear electron quantum transport mechanism. The mobility and the carrier density of the bilayer graphene at 2 K were measured to be 9,400 cm2/V·s and 3 x 10(11) cm(-2), respectively.

A comparison of the transport properties of bilayer graphene, monolayer graphene, and two-dimensional electron gas

We studied and compared the transport properties of charge carriers in bilayer graphene, monolayer graphene, and the conventional semiconductors (the two-dimensional electron gas (2DEG)). It is elucidated that the normal incidence transmission in the bilayer graphene is identical to that in the 2DEG but totally different from that in the monolayer graphene. However, resonant peaks appear in the non-normal incidence transmission profile for a high barrier in the bilayer graphene, which do not occur in the 2DEG. Furthermore, there are tunneling and forbidden regions in the transmission spectrum for each material, and the division of the two regions has been given in the work. The tunneling region covers a wide range of the incident energy for the two graphene systems, but only exists under specific conditions for the 2DEG. The counterparts of the transmission in the conductance profile are also given for the three materials, which may be used as high-performance devices based on the bilayer graphene.

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.

Finite-temperature pseudospin torque effect in graphene bilayers

We use self-consistent quantum transport theory to investigate the influence of layer Fermi energy and temperature on the interlayer transport properties of bilayer graphene in the regime of excitonic superfluidity. We conclude that at low temperature the critical tunneling currents and quasiparticle penetration depths are well explained by the existing zero-temperature pseudospin torque model. However, when the thermal broadening associated with finite-temperature transport is included, we find that modes injected above the gap lead to increased critical interlayer currents and quasiparticle penetration in to the gapped regime beyond what current analytical models predict. We find that the increases in the critical tunneling current are due to thermal smearing smoothing out the quantum interference effects present at low temperature leading to a slower pseudospin precession about the pseudospin Bloch sphere.

Boltzmann transport and residual conductivity in bilayer graphene

A Drude-Boltzmann theory is used to calculate the transport properties of bilayer graphene. We find that for typical carrier densities accessible in graphene experiments, the dominant scattering mechanism is overscreened Coulomb impurities that behave like short-range scatterers. We anticipate that the conductivity $\ensuremath{\sigma}(n)$ is linear in $n$ at high density and has a plateau at low density corresponding to a residual density of ${n}^{*}=\sqrt{{n}_{\mathrm{imp}}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}$, where $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}$ is a constant which we estimate, using a self-consistent Thomas-Fermi screening approximation, to be $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}\ensuremath{\approx}0.01$ ${q}_{\mathrm{TF}}^{2}\ensuremath{\approx}140\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Analytic results are derived for the conductivity as a function of the charged impurity density. We also comment on the temperature dependence of the bilayer conductivity.