Quantum transport in double-gated graphene devices

Landau level (LL) gaps are important parameters for understanding electronic interactions and symmetry-broken processes in bilayer graphene (BLG). Here we present transport spectroscopy measurements of LL gaps in double-gated suspended BLG with high mobilities in the quantum Hall regime. By using bias as a spectroscopic tool, we measure the gap Δ for the quantum Hall (QH) state at filling factors ν = ±4 and −2. The single-particle Δ ν=4 scales linearly with magnetic field B and is independent of the out-of-plane electric field E ⊥ . For the symmetry-broken ν = −2 state, the measured values of Δ ν=−2 are ∼1.1 meV/T and 0.17 meV/T for singly gated geometry and dual-gated geometry at E ⊥ = 0, respectively. The difference between the two values arises from the E ⊥. dependence of Δ ν=−2 , suggesting that the ν = −2 state is layer polarized. Our studies provide the first measurements of the gaps of the broken symmetry QH states in BLG with well-controlled E ⊥ and establish a robust method that can be implemented for studying similar states in other layered materials.

Physics Reports, 2017

Two-dimensional (2D) materials for their versatile band structures and strictly 2D nature have attracted considerable attention over the past decade. Graphene is a robust material for spintronics owing to its weak spin–orbit and hyperfine interactions, while monolayer transition metal dichalcogenides (TMDs) possess a Zeeman effect-like band splitting in which the spin and valley degrees of freedom are nondegenerate. The surface states of topological insulators (TIs) exhibit a spin–momentum locking that opens up the possibility of controlling the spin degree of freedom in the absence of an external magnetic field. Nanostructures made of these materials are also viable for use in quantum computing applications involving the superposition and entanglement of individual charge and spin quanta. In this article, we review a selection of transport studies addressing the confinement and manipulation of charges in nanostructures fabricated from various 2D materials. We supply the entry-level knowledge for this field by first introducing the fundamental properties of 2D bulk materials followed by the theoretical background relevant to the physics of nanostructures. Subsequently, a historical review of experimental development in this field is presented, from the early demonstration of graphene nanodevices on SiO2 substrate to more recent progress in utilizing hexagonal boron nitride to reduce substrate disorder. In the second part of this article, we extend our discussion to TMDs and TI nanostructures. We aim to outline the current challenges and suggest how future work will be geared towards developing spin qubits in 2D materials.

Applied Physics Letters, 2012

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

Journal of Physics: Condensed Matter

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 inter-layer coupling. Here we investigate how the presence of these domains affect 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 domain. 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 performed numerical calculations for systems with two domain walls and find that the availability of multiple transport channels in bilayer graphene modifies significantly the conductance dependence on inter-layer potential asymmetry.

Nano Letters, 2011

The quantum transport properties of graphene nanoribbon networks are investigated using first-principles calculations based on density functional theory. Focusing on systems that can be experimentally realized with existing techniques, both in-plane conductance in interconnected graphene nanoribbons and tunneling conductance in out-of-plane nanoribbon intersections were studied. The characteristics of the ab initio electronic transport through in-plane nanoribbon cross-points is found to be in agreement with results obtained with semiempirical approaches. Both simulations confirm the possibility of designing graphene nanoribbon-based networks capable of guiding electrons along desired and predetermined paths. In addition, some of these intersections exhibit different transmission probability for spin up and spin down electrons, suggesting the possible applications of such networks as spin filters. Furthermore, the electron transport properties of out-of-plane nanoribbon cross-points of realistic sizes are described using a combination of firstprinciples and tight-binding approaches. The stacking angle between individual sheets is found to play a central role in dictating the electronic transmission probability within the networks.

Science, 2007

The unique band structure of graphene allows reconfigurable electric-field control of carrier type and density, making graphene an ideal candidate for bipolar nanoelectronics. We report the realization of a single-layer graphene p-n junction in which carrier type and density in two adjacent regions are locally controlled by electrostatic gating. Transport measurements in the quantum Hall regime reveal new plateaus of two-terminal conductance across the junction at 1 and 3 /2 times the quantum of conductance, e 2 /h, consistent with recent theory. Beyond enabling investigations in condensed-matter physics, the demonstrated local-gating technique sets the foundation for a future graphene-based bipolar technology.

Journal of Physics: Condensed Matter, 2010

Using the nonequilibrium Green function method, the electrical behavior of a double gate bilayer graphene structure is investigated. Due to energy bandgap opening when potential energies in the layers are different, a clear gap of electrical current is observed. The sensitivity of this phenomenon to device parameters (gate length, temperature) has been considered systematically. It appears that the threshold voltage can be controlled by tuning the gate voltages and/or the Fermi energy. Our obtained results may be useful and provide new suggestions for further experimental investigations.

Solid State Communications, 2012

We study the effect of local modulation of charge density and carrier type in graphene field effect transistors using a double top gate geometry. The two top gates lead to the formation of multiple p-n junctions. Electron transport measurements at low temperature and in the presence of magnetic field show various integer and fractionally quantized conductance plateaus. We explain these results based on the mixing of the edge channels and find that inhomogeneity plays an important role in defining the exact quantization of these plateaus, an issue critical for the metrology applications of p-n junctions.

Physical Review B

One of the key features of graphene is the chirality of the electrons originating from the lattice symmetry. Conservation of the chirality leads to Klein and anti-Klein tunneling in graphene and AB-stacked bilayer graphene, respectively. Introduction of an external perturbation (electric field, strain, twist, layer mismatch, etc.) can change the chirality and therefore significantly modify the quantum transport. We explore the tunneling in AB-stacked bilayer graphene in the presence of an in-plane pseudomagnetic field (IPMF) and an out-of-plane electric field (OEF). Employing a low-energy effective model, we study the energy spectrum, chirality, and wave functions to evaluate the transmission and conductance in an n-p-n junction. We demonstrate that the IPMF and OEF together induce an indirect band gap. The IMPF induces a layer polarization of propagating states (enhanced in the presence of the OEF) with the layer symmetry broken locally in a single valley but preserved globally. While the effect of the IPMF is negligible in the ballistic transport regime at the neutrality point, the OEF suppresses the ballistic transport (partially recovered in the presence of the IPMF). In the diffusive transport regime, the IPMF amplifies the Fabry-Pérot resonances, thus modifying the conductance, while the OEF induces an out-of-plane chirality component that enables Klein resonances.

Physical Review B, 2011

We report markedly different transport properties of ABA-and ABC-stacked trilayer graphenes. Our experiments in double-gated trilayer devices provide evidence that a perpendicular electric field opens an energy gap in the ABC trilayer, while it causes the increase of a band overlap in the ABA trilayer. In a perpendicular magnetic field, the ABA trilayer develops quantum Hall plateaus at filling factors of ν = 2, 4, 6... with a step of ∆ν = 2, whereas the inversion symmetric ABC trilayer exhibits plateaus at ν = 6 and 10 with 4-fold spin and valley degeneracy.