Transport In Bilayer Graphene Research Articles

Controllable ion transport in bilayer graphene with charged nanopores

Heterogeneous membrane structures exhibiting ion diode effects have attracted significant interest in energy conversion and storage. Herein, asymmetric double-layer graphene stacks with charged nanopores are constructed and placed in a KCl solution to study the controllable ion transport properties. Ion current rectification (ICR) is observed and the switching ratio is up to 102 by leveraging the asymmetric structure, charge, and interlayer distance. The trapping behavior of cations (K+) and anions (Cl−) in the nanochannels is investigated by applying bias voltages, and peak ion capture occurs at an electric field of E = 0.02 V/Å. The underlying mechanism of ICR is elucidated according to the dependence between the ion current and carrier concentration in the nanopores. The results provide valuable insights into the ICR mechanism and reveal high potential in the energy field.

Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor.

Electronic transport in the regime where carrier-carrier collisions are the dominant scattering mechanism has taken on new relevance with the advent of ultraclean two-dimensional materials. Here, we present a combined theoretical and experimental study of ambipolar hydrodynamic transport in bilayer graphene demonstrating that the conductivity is given by the sum of two Drude-like terms that describe relative motion between electrons and holes, and the collective motion of the electron-hole plasma. As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics.

Ratchet effect in spatially modulated bilayer graphene: Signature of hydrodynamic transport

The authors study here the transition between drift-diffusion and hydrodynamic regimes of ac electron transport in bilayer graphene, focusing on one of the most general photovoltaic phenomenon: the ratchet effect. Compared to conventional dc transport measurements, this effect offers a number of additional possibilities to probe hydrodynamic flow of the electron fluid. A signature of the hydrodynamic regime --- a very sharp frequency dependence of the generated ratchet current --- is clearly demonstrated experimentally, in excellent agreement with the developed theory.

Tuning the phonon transport in bilayer graphene to an anomalous regime dominated by electron-phonon scattering

Recent studies have revealed the significance of electron-phonon interaction (EPI) in phonon transport at intermediate temperatures. In some metals, the EPI can even dominate over the anharmonic phonon-phonon (ph-ph) scattering, leading to an anomalous phonon transport regime in which the lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ becomes nearly temperature ($T$) independent in contrast to the usual $1/T$ dependence. However, the experimental verification of this anomalous transport regime is very challenging due to the difficulty in separating the phonon contributions from the dominating electron ones to the measured total thermal conductivity in metals. In this work, using first-principles calculations, we predict that in bilayer graphene, the phonon transport can be driven to the anomalous regime by tuning the doping level. At high doping levels close to the Van Hove singularity, the EPI can result in a fivefold reduction of ${\ensuremath{\kappa}}_{L}$ at room temperature, and ${\ensuremath{\kappa}}_{L}$ becomes $T$ independent. This anomalous behavior is found to have its origin in three aspects: (i) mirror symmetry breaking enables direct coupling between flexural phonons, the dominant carriers of ${\ensuremath{\kappa}}_{L}$, and electrons; (ii) dominance of normal processes in the anharmonic ph-ph scattering facilitates the EPI to be more prominent; (iii) dominance of these normal ph-ph processes induces the indirect effect of EPI on ${\ensuremath{\kappa}}_{L}$. This is distinct from monolayer graphene, where the mirror symmetry prohibits the direct scattering of the flexural phonons by electrons and only the indirect EPI affects ${\ensuremath{\kappa}}_{L}$. This work gives insight into the manipulation of heat conduction via externally induced EPI in two-dimensional materials in which mirror symmetry breaks and normal processes dominate the ph-ph scattering.

Tunable phononic thermal transport in two-dimensional C6CaC6 via guest atom intercalation

The graphite intercalation compounds have attracted wide interest due to the superconductivity. In this work, the thermal transport in bilayer graphene intercalated with Ca atoms (C6CaC6) at room temperature is studied by using non-equilibrium molecular dynamics simulations. Our simulation results show that the in-plane lattice thermal conductivity (κL) of C6CaC6 is significantly lower than that of the bilayer graphene. The detailed phonon mode analysis reveals that the reduction of κL is because of the mode hybridization and flatbands induced by the intercalated Ca atoms, leading to the decrease in phonon group velocity and the enhancement of phonon scattering. Unlike the role of van der Waals interactions in multilayer graphene and supported graphene, increasing coupling strength between intercalated Ca atoms and graphene brings an enhanced κL in C6CaC6. The spectral phonon analysis uncovers that such anomalous phenomenon is caused by the redistribution of phonon scattering phase space originated from the shift of the flatbands. This study indicates that atom intercalation is an effective way to regulate the heat transport in two-dimensional materials.

Time-reversal odd transport in bilayer graphene: Hall conductivity and Hall viscosity

We consider the time-reversal odd dynamics of the bilayer graphene at low energies in the quantum Hall regime. A generating functional for the effective action that captures the electromagnetic response to all orders in momentum and frequency is presented and evaluated to the third order in the space-time gradient $\mathcal O(\partial^3)$. In addition, we calculate the Hall viscosity and derive an explicit relationship with the $q^2$ coefficient of the Hall conductivity. It is reminiscent of the Hoyos--Son relation in the Galilean invariant systems, which can be recovered in the limit of large filling factor $N$.

Quantizing viscous transport in bilayer graphene

The momentum transport in ultraclean bilayer graphene is characterized by the viscous transport. In quantizing magnetic field the momentum current passes through the guiding center of the cyclotron orbit. In this study we derive the formula of the quantized Hall viscosity for bilayer graphene. This can be detected in the non-local magnetoresistivity measurements that varies with the quantized step. For weak magnetic field the Landau levels start overlapping and lead to the Shubnikov–de-Haas oscillations, superimposed on the classical formulae, reference Steinberg (1958 Phys. Rev. 109 1486). These oscillations are present in the longitudinal and Hall viscosities.

Valley pumping via edge states and the nonlocal valley Hall effect in two-dimensional semiconductors

Recent experiments have studied the temperature and gate voltage dependence of nonlocal transport in bilayer graphene, identifying features thought to be associated with the two-dimensional semiconductor's bulk intrinsic valley Hall effect. Here, we use both simple microscopic tight-binding ribbon models and phenomenological bulk transport equations to emphasize the impact of sample edges on the nonlocal voltage signals. We show that the nonlocal valley Hall response is sensitive to electronic structure details at the sample edges, and that it is enhanced when the local longitudinal conductivity is larger near the sample edges than in the bulk. We discuss recent experiments in light of these findings and also discuss the close analogy between electron pumping between valleys near two-dimensional sample edges in the valley Hall effect, and bulk pumping between valleys due to the chiral anomaly in three-dimensional topological semimetals.

Combined effect of 13C isotope and vacancies on the phonon properties in AB stacked bilayer graphene

The combined effects of 13C isotope and vacancies on the phonon properties in AB stacked bilayer graphene (BLG) are explored theoretically. We have calculated the phonon density of states (PDOS) by varying the isotope contents (0–100%) and vacancies (0–30%) in both layers and only in the upper layer of the BLG using forced vibrational method. We found that both isotope and vacancy or merging of these two defects significantly affect the PDOS, especially, E2g mode phonon, which is responsible for the Raman G band, shifted downward with the increase of defect concentrations. Moreover, when 13C isotopes are induced only in the upper layer, E2g peak splits into two peaks which corresponds well with the experimental results of 13C/12C dependence G peak splitting in the Raman spectra of BLG. We also explored the defect induced phonon localization in BLG. Our calculated typical mode patterns show that high frequency optical phonons are strongly localized in the vacancy as well as merging 13C isotope and vacancy defected BLG. The calculated average localization length noticed that strong phonon localization exists at 60% 13C isotope concentration. These findings are important for understanding the experimentally observed Raman spectra as well as thermal transport in BLG.

Study the metal-insulator transitions of bilayer graphene: Abelian group schemes approach

Abstract Bilayer graphene (BLG), as a two-dimensional crystalline form of carbon, with a controllable band gap, has been proposed as an alternative to graphene for nanoscale electronics. Through modeling single-electron transport in BLG, using Abelian group schemes Z m × Z m as a novel approach, the present paper has numerically studied metal-insulator transition using random matrix theory in doped AA-stacking BLG. In this mathematical technique, initially, Abelian group schemes and the underlying graph of honeycomb periodic lattice of the single-layer graphene were constructed. Then, the tight-binding Hamiltonian matrix of this layer was represented by the adjacency matrices of the graph. Through using these matrices, it is possible to generate m-dimensional Bose-Mesner algebra. Finally, by applying the wreath product of these matrices, the Hamiltonian was calculated. In fact, instead of the hopping integral between individual layers, the mathematical combination of their underlying graphs was considered. This would be generalized to more than two layers. Numerical results indicated that insulator to metal transition occurred at the threshold doping value C = 0.5 % , which has already been achieved. After this threshold value, by increasing doping, a fast quantum chaotic transition from Poisson to Wigner energy-level statistic distribution was observed. The paper has suggested that a sufficient difference in the concentration of dopant atoms at individual layers would be an efficient way of controlling metal-insulator transitions.

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.

Topological valley currents in bilayer graphene/hexagonal boron nitride superlattices

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a “hot spot” at the charge-neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 kΩ, which obeys a scaling relation. This nonlocal resistance evolves from an analog of the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way for developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking.

Electrically controllable spin transport in bilayer graphene with Rashba spin-orbit interaction

We study the spin transport in bilayer graphene nanoribbons (BGNs) in the presence of Rashba spin-orbit interaction (SOI) and external gate voltages. It is found that the spin polarization can be significantly enhanced by the interlayer asymmetry or longitudinal mirror asymmetry produced by external gate voltages. Rashba SOI alone in BGNs can only generate current with spin polarization along the in-plane y direction, but the polarization components can be found along the x, y and z directions when a gate voltage is applied. High spin polarization with flexible orientation is obtained in the proposed device. Our findings shed new light on the generation of highly spin-polarized current in BGNs without external magnetic fields, which could have useful applications in spintronics device design.

Breakdown of the Wiedemann-Franz law in AB -stacked bilayer graphene

We present a simple theory of thermoelectric transport in bilayer graphene and report our results for the electrical resistivity, the thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio as functions of doping density and temperature. In the absence of disorder, the thermal resistivity tends to zero as the charge neutrality point is approached; the electric resistivity jumps from zero to an intrinsic finite value, and the Seebeck coefficient diverges in the same limit. Even though these results are similar to those obtained for single-layer graphene, their derivation is considerably more delicate. The singularities are removed by the inclusion of a small amount of disorder, which leads to the appearance of a "window" of doping densities $0<n<n_c$ (with $n_c$ tending to zero in the zero-disorder limit) in which the Wiedemann-Franz law is severely violated.

Electron transport in the presence of valley-orbit interaction

New theoretical study investigates the valley aspect of lateral tunneling transport in bilayer graphene and calculates its valley dependence in electron transport in 2D material-based quantum structures.

Gate-tunable topological valley transport in bilayer graphene

Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands, is a distinct feature of two-dimensional Dirac materials. Similar to spin, the valley pseudospin is spanned by a time reversal pair of states, though the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interests. Bilayer graphene (BLG) is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable valley-topological transport. Here, we used a perpendicular gate electric field to break the inversion symmetry in BLG, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge contemporary understanding of topological transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.

Influence of interlayer coupling and intra-layer Coulomb interaction on electronic transport in bilayer graphene

Calculations of renormalized perpendicular conductivity within Kubo formula employing single particle temperature dependent Green's function formalism for bilayer graphene has been attempted. On the basis of numerical analysis, perpendicular conductivity as a function of temperature, interlayer coupling, onsite Coulomb interaction and carrier concentration per site has been analyzed for both AA- and AB-stacked bilayer graphene. It is found that perpendicular conductivity increases with interlayer coupling and also with temperature at low temperatures while at higher temperatures, there is saturation in perpendicular conductivity. Influences of onsite Coulomb interaction and carrier concentration per site on perpendicular conductivity is just opposite to each other while onsite Coulomb energy suppresses the rate of increase of σ⊥/σ⊥0 with temperature, on the other hand increase in carrier density per site enhance this rate significantly. Finally, theoretically obtained results on temperature dependent perpendicular conductivity are viewed in terms of electronic transport data as well as recent theoretical works available in bilayer graphene.

Limitations to carrier mobility and phase-coherent transport in bilayer graphene.

We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase-coherence times τϕ as well as the inter- and intravalley scattering times τi and τ*, respectively. While τϕ is in qualitative agreement with an electron-electron interaction-mediated dephasing mechanism, electron spin-flip scattering processes are limiting τϕ at low temperatures. The analysis of τi and τ* points to local strain fluctuation as the most probable mechanism for limiting the mobility in high-quality bilayer graphene.

Stacking Boundaries and Transport in Bilayer Graphene

Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few millielectronvolts. This behavior has been interpreted as the result of an intrinsic electronic instability induced by many-body correlations. Intriguingly, however, some samples of similar mobility exhibit good metallic properties with a minimal conductivity of the order of 2e(2)/h. Here, we propose an explanation for this dichotomy, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains ("solitons"). We argue, using a numerical analysis, that the hallmark features of the previously inferred many-body insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing.

Investigation of two-dimensional lattice thermal transport in bilayer graphene using phonon scattering mechanism

Two-dimensional lattice thermal transport in bilayer graphene is investigated using phonon scattering mechanism. In the plane layer of carbon atoms the thermal conductivity (κ) is demonstrated by incorporating the phonon–defect, phonon–electron, phonon–grain boundaries, phonon–phonon umklapp scatterings and out-of-plane phonon scattering process in the model Hamiltonian. A typical T1.5 dependence of thermal conductivity observed at low temperatures (lower than 150K) is the resultant of various operating phonon scattering mechanisms. Above room temperatures, the thermal conductivity decreases and follows almost T−2 dependence which is an artifact of the dominant Umklapp phonon scattering at higher temperatures. The phonon peak appear at around 225K is due to the competition between the increase in the phonon population and decrease in phonon mean free path due to umklapp phonon scattering with increasing temperature. The results obtained from present model are in good agreement with the available experimental data and reflect the two-dimensional nature of phonon transport in graphene which is dominated by phonon scatterings.