Session B26: Graphene and Graphene/hexagonal Bonron Nitride Superlattices

High-frequency current oscillations in graphene-boron nitride resonant tunnel diodes

The successful realisation of multilayer graphene-hBN-graphene resonant tunnelling diodes (graphene- RTDs) with negative differential conductance (NDC) and MHz current oscillations offers the exciting possibility of exploiting them as high-frequency oscillators and mixers [1, 2]. In this paper, we examine their potential for generating higher frequencies by simulating the oscillations in the tunnel current and charge that arise when the device is biased in the NDC region and placed in a resonant circuit. Using the Bardeen transfer Hamiltonian method, we examine the effect on the device characteristics of the twist angle, $\theta$, between the two graphene electrodes, the hBN barrier thickness and of the carrier density in the graphene electrodes, which can be adjusted by chemical doping or by an applied bias voltage. The simulations accurately reproduce our recently-reported measurements on these RTDs (Fig. 4, [2]). The results of simulations show that frequencies of tens of GHz are achievable by optimising the device parameters [3]. References: [1] L. Britnell et al., Nature Communications {\bf 4}, 1794 (2013). [2] A. Mishchenko et al., Nature Nanotechnology {\bf 9}, 808 (2014). [3] J. Gaskell et al., Applied Physics Letters {\bf 107}, 103105 (2015)   

Band structure engineering of graphene by a local gate defined periodic potential

Recent improvements in 2-dimensional (2D) material layering have resulted in enhanced device quality and created pathways for new device architectures. We fabricate periodic arrays from a patterned local back gate and a uniform top gate on hBN encapsulated graphene channels. The symmetry and lattice size of the periodic potential can be determined by state-of-art electron beam lithography and etching, achieving a lattice constant of 35 nm. The strength of the electric potential modulation can be controlled through applied voltage on the patterned gate. We observe signatures of superlattice modulation near the main Dirac peak in the density dependent resistance measurement at zero magnetic field. Current studies focus on the exploration of Hofstadter fractal band structures under magnetic fields. Our nano-patterned engineered superlattices on graphene hold great promise for wider applications.   

Layer resolved capacitive probing of graphene bilayers

Compared to single layer graphene, graphene bilayers have an additional “which-layer” degree of freedom that can be controlled by an external electric field in a dual-gated device geometry. We describe capacitance measurements capable of directly probing this degree of freedom. By performing top gate, bottom gate, and penetration field capacitance measurements, we directly extract layer polarization of both Bernal and twisted bilayers. We will present measurements of hBN encapsulated bilayers at both zero and high magnetic field, focusing on the physics of the highly degenerate zero-energy Landau level in the high magnetic field limit where spin, valley, and layer degeneracy are all lifted by electronic interactions.   

Topologically-driven valley polarization in twisted graphene/hexagonal boron nitride heterostructures

Valley polarization, that is, selective electronic localization in a momentum valley, has been proposed on materials presenting either a strong spin-orbit coupling (SOC) or with a weak SOC but in the presence of electric and magnetic fields. In this talk, we identify a non-centro symmetric system which can also present valley polarization purely by topological means without the necessity of SOC. We find that twisted bilayers of graphene/hexagonal boron nitride heterostructures have different absorption for right- and left- circular polarized light, indicating valley polarization. This induced polarization occurs due to band folding of the electronic bands, i.e., it has a topological origin.   

Bandgap opening in bilayer graphene at metal contacts

A bandgap is opened in bilayer graphene (BLG) by introducing a potential difference between the two graphene layers, raising expectations for its application to a transistor channel. The potential difference can be introduced, for example by charge transfer from surface adsorbates. Thus, a finite bandgap is expected to be opened also at a metal contact, an inevitable component of transistors, where interfacial charge transfer occurs to align the Fermi levels of the metallic electrode and the underlying BLG. The bandgap at the metal-BLG interface can be detected by the superlinear current-voltage characteristics in back-gate field-effect transistors, caused by carriers propagating through the bandgap, i.e., by the band-to-band transport [1]. The superlinearity was higher in the positively-gated region, attributed to hole doping from the Cr/Au electrodes. The control experiments using single-layer graphene (SLG) did not have a superlinearity, which is consistent with the fact that a sizeable bandgap is not expected at the metal-SLG interface. The current transport through the bandgap should be an additional source of electrode-contact resistance. [1] R. Nouchi, Appl. Phys. Lett. 105, 223106 (2014).   

Anomalous conductivity noise in gapped bilayer graphene heterostructure

Bilayer graphene has unique electronic properties – it has a tunable band gap and also, valley symmetry and pseudospin degree of freedom like its single layer counterpart. In this work, we present a study of conductance fluctuations in dual gated bilayer graphene heterostructures by varying the Fermi energy and the band gap independently. At a fixed band gap, we find that the conductance fluctuations obtained by Fermi energy ensemble sampling increase rapidly as the Fermi energy is tuned to charge neutrality point (CNP) whereas the time-dependent conductance fluctuations diminish rapidly. This discrepancy is completely absent at higher number densities, where the transport is expected to be through the 2D bulk of the bilayer system. This observation indicates that near the CNP, electrical transport is highly sensitive to Fermi energy, but becomes progressively immune to time-varying disorder. A possible explanation may involve transport via edge states which becomes the dominant conduction mechanism when the bilayer graphene is gapped and Fermi energy is situated close to the CNP, thereby causing a dimensional crossover from 2D to 1D transport. Our experiment outlines a possible experimental protocol to probe intrinsic topological states in gapped bilayer graphene.   

Tuning the Band Gap of Bilayer Graphene by Sandwich-Like Stacking

As far as we know, graphene has been taken as a potential host material for next-generation electric devices. However, this attractive prospect has been blocked by the metallic character of graphene. Although many methods have been proposed to get a moderate energy gap, such as hydrogenated graphene (graphane), but all the intrinsic advantages (carrier's mobility, etc...) of graphene have been destroyed. Here, we report that a large energy gap of graphene bilayer can be opened without breaking its natural characters by sandwiching it between functionalized BN substrates. Also, we show that the band gap of graphene bilayer can be tuned from 0.35 eV to 0.50 eV, depending on the substrates. The gap value is much larger than any other methods, and the structure of graphene bilayer is perfectly kept. And the energy gap is robust, namely, once the sandwiched substrates are selected, the relative position of substrates and graphene bilayer hardly changes the energy gap. Since the proposed way is easy to be realized in experiments, our results will hopefully accelerate the application of graphene in semiconductor devices and promote the development of the graphene technology.   

Anomalous Coulomb drag in bilayer graphene double layers

Bilayer graphene double-layer structure consists of two layers of bilayer graphene separated by atomically thin hexagonal boron nitride (hBN). With a perfect Fermi surface nesting and strong electron-electron interaction (E$_{Coulomb}$ $>$ E$_{kinetic}$), such systems offer exciting platforms to study interaction driven phenomena, such as Coulomb drag and exciton condensation. We fabricate ultra-clean encapsulated bilayer graphene double layers with dry pick-up method. Room temperature drag measurement on our devices shows the sign of drag agree with the typical Fermi liquid behavior. However, at lower temperatures, the sign of drag reversed, indicating a new drag mechanism emerges and dominates. We measure this with different geometry, temperature, bias and gating to investigate the origin of such effect and discuss the implication of the drag sign changes.   

Coulomb Drag Measurements in Bilayer-Bilayer Graphene Device

We report Coulomb drag measurements on bilayer-bilayer graphene devices assembled using the Van De Waals transfer technique. The two bilayer graphene flakes are encapsulated and separated by hBN. High temperature measurements reveal positive drag response when the carrier types are different in two graphene layers, and negative when carrier types are the same, a result that is similar to previous measurements reported in monolayer graphene devices. However, upon cooling to low temperature, novel drag response is observed in the low density region. We also report a new device set-up which improves measurement quality for Coulomb drag measurements at low temperature.   

Planar Tunneling Spectroscopy of Graphene Nanodevices

2-D Van-der-Waals mesoscopic physics have seen a rapid development in the last 10 years, with new materials each year added to the toolbox. Stacking them like Lego enables the combination of their individual electronic properties. In particular, hexagonal boron nitride, which is an insulator, gives the possibility to perform planar (2-D to 2-D) tunneling spectroscopy within this type of heterostructures. Unlike standard transport measurements, tunneling spectroscopy enables to probe the electronic properties in the energy domain. Moreover, since planar tunneling probes a large area of the system, global quantum features such as quantum Hall effect, superconducting proximity effect or quantum confinement can be investigated. In this talk, we will present implementation of heterostructures consisting of graphene, hexagonal boron nitride, and graphite, fabricated for planar tunneling spectroscopy. In order to reveal the intrinsic properties of materials, the fabrication scheme aims at preserving the pristine nature of the 2-DEGS as well as minimizing the doping introduced by external probes. As a demonstration, measurements of these devices in normal states, high magnetic field environment, and induced superconducting state will be presented.   

Observation of Large Intrinsic Gap in Rhombohedral-Stacked Tetralayer Graphene

Few-layer graphene has attracted attention in the scientific community as a novel 2D material due to its observed quantum hall effect, high electronic mobility, high transparency and tensile strength, among other properties. In rhombohedral-stacked few-layer graphene, the very flat band near the charge neutrality point is unstable to electronic interactions, and gives rise to states with spontaneous broken symmetries. Intrinsic gaps of \textasciitilde 2 meV and 40 meV are observed in bilayer and trilayer graphene, respectively. Here, we report the observation of an even larger gap in suspended rhombohedral-stacked tetralayer graphene (r-4LG) samples. We will present the latest data of the evolution of the gapped state with temperature and external fields, and compare with theoretical models.   

Observation of Hysteresis in Rhombohedral-Stacked Trilayer Graphene

Few-layer graphene is an attractive platform for exploration of physical processes~confined to two dimensions. Diverging density of states in rhombohedral-stacked~trilayer graphene (r-TLG) leads to strong electronic correlation. Recently an intrinsic insulating phase with 40 meV gap has been observed at charge~neutrality point (CNP) in r-TLG, which is consistent with an layer antiferromagnetic state.~By using~dual-gated suspended r-TLG device, we observe hysteresis loops in conductance in the~vicinity of CNP, which suggests the possible evidence of spontaneous spin polarization or presence of domains with different anomalous Hall conductivities.   

Experimental realization of gate controlled topological conducting channels in bilayer graphene

Manipulating the valley degree of freedom in two-dimensional honeycomb lattices can potentially lead to a new type of electronics called valleytronics. In electrically gapped bilayer graphene, the broken inversion symmetry leads to non-zero and asymmetric Berry curvature $\Omega$ in the K and K$^\prime$ valleys of the Brillouin zone. Reversing the sign of $\Omega$ at the internal line junction of two oppositely gated bilayer graphene is predicted to yield counter-propagating edge modes, the so-called kink states, with quantized conductance of $4e^2/h$ in the absence of valley mixing. We have overcome fabrication challenges to implement high-quality hBN encapsulated, dual-split-gates structures necessary to observe the kink states. Here I present experimental evidences of the kink states. In the absence of a magnetic field, the kink states have a mean free path of a few hundred nm. Ballistic conductance of $4e^2/h$ is achieved in a perpendicular magnetic field. We discuss the potential valley-mixing mechanisms and the role of the magnetic field. Experimental results are supported by numerical studies. We will also discuss ongoing efforts in realizing valley-controlled transmission and guiding of the kink states, which is a significant step towards the development of valleytronics.   

Topological valley transport at bilayer graphene domain walls

Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. An external electric field can induce a tunable bandgap in bilayer graphene, and domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. In this talk, I will present our efforts on revealing the topologically protected edge states at AB-BA domain walls by combining near field infrared nanoscopy with electrical transport measurement. These one-dimensional valley-polarized conducting channels feature a ballistic length of about 400 nanometres at 4 kelvin.   

Electronic excitation spectrum of ABC-stacked multilayer graphene

The electronic properties of ABC graphene trilayers has attracted lot of attention recently due to their potential applications in engineering carbon-based devices with gate tunable electrical conductivity. Morever,ABC-stacked thin layers of graphite are predicted to host peculiar surface electronic states, with a flat dispersion over most of the Brillouin zone. The associated high density of states is likely to favour the emergence of exotic electronic phases, such as charge density waves or even superconductivity. We present a micro-magneto-Raman scattering study of a thin graphite flake produced by exfoliation of natural graphite, composed of $\sim$15graphene layers, and including a large ABC-stacked domain. Exploring the low temperature Raman scattering spectrum of this domain up to B=29T,we identify inter Landau level electronic excitations within the surface flat bands,together with electronic excitations involving the gapped states in the bulk. This interband electronic excitation at B=0T can be observed,up to room temperature, directly in the Raman scattering spectrum as a broad($\sim 180$cm$^{-1}$) feature. Because the energy gap strongly depends on the number of layers,this electronic excitation can be used to identify and characterize ABC-stacked graphite thin layers.