Transport in multiterminal graphene nanodevices

We study the transport properties of multiterminal graphene nanodevices using the Landauer-Buttiker approach and the tight binding model. We consider a four-terminal device made at the crossing of a zigzag and armchair nanoribbons and two types of T-junction devices. The transport properties of graphene multiterminal devices are highly sensitive to the details of the junction region. Thus the properties are drastically different from those on the armchair and zigzag counterparts. In the cross-junction device, we see a conductance dip in the armchair lead associated with a conductance peak in the zigzag lead. We find that this effect is enhanced in a T-junction device with one armchair sidearm.     

Synthesis and electrical characterization of magnetic bilayer graphene intercalate

We report synthesis and transport properties of the minimal graphite intercalation compound, a ferric chloride (FeCl(3))(n) island monolayer inside bilayer graphene. Chemical doping by the intercalant is simultaneously probed by micro-Raman spectroscopy and Hall measurements. Quantum oscillations of conductivity originate from microscopic domains of intercalated and unintercalated regions. A slight upturn in resistance related to magnetic transition is observed. Two-dimensional intercalation in bilayer graphene opens new possibilities to engineer two-dimensional properties of intercalates.     

Irradiated bilayer graphene

We describe the gated bilayer graphene system when it is subjected to intense terahertz frequency electromagnetic radiation. We examine the electron band structure and density of states via exact diagonalization methods within Floquet theory. We find that dynamical states are induced which lead to modification of the band structure. We first examine the situation where there is no external magnetic field. In the unbiased case, dynamical gaps appear in the spectrum which manifest as dips in the density of states. For finite inter-layer bias (where a static gap is present in the band structure of unirradiated bilayer graphene), dynamical states may be induced in the static gap. These states can show a high degree of valley polarization. When the system is placed in a strong magnetic field, the radiation induces coupling between the Landau levels which allows dynamical levels to exist. For strong fields, this means the Landau levels are smeared to form a near-continuum of states.     

Giant two-photon absorption in bilayer graphene

We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 μm) with a resonant 2PA coefficient of up to ～0.2 cm/W located at half of the bandgap energy, γ(1) = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of ω(-3) in bilayer graphene, while it is proportional to ω(-4) for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.     

Tunable quantum dots in bilayer graphene

We demonstrate theoretically that quantum dots in bilayers of graphene can be realized. A position-dependent doping breaks the equivalence between the upper and lower layer and lifts the degeneracy of the positive and negative momentum states of the dot. Numerical results show the simultaneous presence of electron and hole confined states for certain doping profiles and a remarkable angular momentum dependence of the quantum dot spectrum, which is in sharp contrast with that for conventional semiconductor quantum dots. We predict that the optical spectrum will consist of a series of nonequidistant peaks.     

Spin-polarized quantum pumping in bilayer graphene

We study adiabatic quantum pumping in bilayer graphene where two-barrier potentials are weakly modulated as pumping parameters. Comparing the results with those for a normal quantum pump of non-chiral quasiparticles, we find that the chirality of quasiparticles in bilayer graphene heavily affects the pumped current through chiral tunnelling. When an exchange splitting induced by the proximity of a ferromagnetic insulator is introduced, the pumped current becomes spin-polarized. It is interesting that an almost 100% polarized charge current and a pure spin current with vanishing charge current can all be achieved under suitable conditions. The experimental feasibility and the interlayer asymmetric effect in bilayer graphene caused by the gate and the ferromagnet structures are also discussed. The results are useful for spintronics applications based on graphene.     

Dual-gated bilayer graphene hot-electron bolometer

Graphene is an attractive material for use in optical detectors because it absorbs light from mid-infrared to ultraviolet wavelengths with nearly equal strength. Graphene is particularly well suited for bolometers-devices that detect temperature-induced changes in electrical conductivity caused by the absorption of light-because its small electron heat capacity and weak electron-phonon coupling lead to large light-induced changes in electron temperature. Here, we demonstrate a hot-electron bolometer made of bilayer graphene that is dual-gated to create a tunable bandgap and electron-temperature-dependent conductivity. The bolometer exhibits a noise-equivalent power (33?fW?Hz(-1/2) at 5?K) that is several times lower, and intrinsic speed (>1?GHz at 10?K) three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.     

Vibrational characteristics of bilayer graphene sheets

Bilayer graphene sheets (BLGSs) are currently receiving increasing attention. In this paper, the vibration characteristics of BLGSs are investigated using analytical and atomistic finite element approaches. Various possible scenarios, namely different geometrical configuration (armchair and zigzag), boundary conditions, and aspect ratio are considered in the present study. The dynamic characteristics of BLGS studied have shown dependence on aspect ratio and the boundary conditions. The unique vibrational properties and large stiffness of BLGS identified in the present work make them suitable candidates for manufacturing nanosensors; electromechanical resonators also will aid the nanomaterials research community to design nanodevices.     

Weak localization in monolayer and bilayer graphene

We demonstrate quantitative experimental evidence for a weak localization correction to the conductivity in monolayer and bilayer graphene systems. We show how inter- and intra-valley elastic scattering control the correction in small magnetic fields in a way which is unique to graphene. A clear difference in the forms of the correction is observed in the two systems, which shows the importance of the interplay between the elastic scattering mechanisms and how they can be distinguished. Our observation of the correction at zero-net carrier concentration in both systems is clear evidence of the inhomogeneity engendered into the graphene layers by disorder.     

Gate-induced insulating state in bilayer graphene devices

The potential of graphene-based materials consisting of one or a few layers of graphite for integrated electronics originates from the large room-temperature carrier mobility in these systems (approximately 10,000 cm2 V(-1) s(-1)). However, the realization of electronic devices such as field-effect transistors will require controlling and even switching off the electrical conductivity by means of gate electrodes, which is made difficult by the absence of a bandgap in the intrinsic material. Here, we demonstrate the controlled induction of an insulating state--with large suppression of the conductivity--in bilayer graphene, by using a double-gate device configuration that enables an electric field to be applied perpendicular to the plane. The dependence of the resistance on temperature and electric field, and the absence of any effect in a single-layer device, strongly suggest that the gate-induced insulating state originates from the recently predicted opening of a bandgap between valence and conduction bands.     

Achieving large transport bandgaps in bilayer graphene

Since opening sizable bandgaps in bilayer graphene (BLG) was proven possible, BLG has attracted considerable attention as a promising high-mobility candidate material for many electronic and optoelectronic applications. However, the bandgaps observed in the transport experiments reported in the literature are far smaller than both the theoretical predictions and the bandgaps extracted from optical measurements. In this study, we investigate the factors preventing the formation of large bandgaps and demonstrate that a ~200-meV transport bandgap can be opened in BLG by scaling the gate dielectric and employing a ribbon channel to suppress the percolative transport. This is the largest transport bandgap that has been achieved in BLG to date.

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New Dirac fermions in periodically modulated bilayer graphene

We investigate the effect of periodic potentials on the electronic structure of bilayer graphene and show that there is a critical value of the external potential below which new Dirac fermions are generated in the low-energy band structure, and above which a band gap is opened in the system. Our results, obtained from a self-consistent tight-binding calculation, can be simply explained by a two-band continuum model as a consequence of the pseudospin physics in graphene. The findings are robust against changes in the form of the potential, as well as bias voltages between the layers.     

Quantum-enhanced tunable second-order optical nonlinearity in bilayer graphene

Second order optical nonlinear processes involve the coherent mixing of two electromagnetic waves to generate a new optical frequency, which plays a central role in a variety of applications, such as ultrafast laser systems, rectifiers, modulators, and optical imaging. However, progress is limited in the mid-infrared (MIR) region due to the lack of suitable nonlinear materials. It is desirable to develop a robust system with a strong, electrically tunable second order optical nonlinearity. Here, we demonstrate theoretically that AB-stacked bilayer graphene (BLG) can exhibit a giant and tunable second order nonlinear susceptibility χ((2)) once an in-plane electric field is applied. χ((2)) can be electrically tuned from 0 to ~10(5) pm/V, 3 orders of magnitude larger than the widely used nonlinear crystal AgGaSe(2). We show that the unusually large χ((2)) arise from two different quantum enhanced two-photon processes thanks to the unique electronic spectrum of BLG. The tunable electronic bandgap of BLG adds additional tunability on the resonance of χ((2)), which corresponds to a tunable wavelength ranging from ~2.6 to ~3.1 μm for the up-converted photon. Combined with the high electron mobility and optical transparency of the atomically thin BLG, our scheme suggests a new regime of nonlinear photonics based on BLG.     

Electric field control of spin rotation in bilayer graphene

The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin field-effect transistors. An alternative solution can be found in magnetic doping of graphene or, as discussed in the present work, in exploiting the proximity effect between graphene and ferromagnetic oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction, that acts as an effective Zeeman field for electrons in graphene, inducing a spin precession around the magnetization axis of the FO. Here we show that in an appropriately designed double-gate field-effect transistor, with a bilayer graphene channel and FO used as a gate dielectric, spin-precession of carriers can be turned ON and OFF with the application of a differential voltage to the gates. This feature is directly probed in the spin-resolved conductance of the bilayer.     

Transport spectroscopy of symmetry-broken insulating states in bilayer graphene

Bilayer graphene is an attractive platform for studying new two-dimensional electron physics, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron-electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source-drain bias as a spectroscopic tool to resolve a gap of ～2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ～15 mV nm(-1), but it increases monotonically with magnetic field, with an apparent particle-hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.     

Electronic excited states in bilayer graphene double quantum dots

We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features additional energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.     

Twinning and twisting of tri- and bilayer graphene

The electronic, optical, and mechanical properties of bilayer and trilayer graphene vary with their structure, including the stacking order and relative twist, providing novel ways to realize useful characteristics not available to single layer graphene. However, developing controlled growth of bilayer and trilayer graphene requires efficient large-scale characterization of multilayer graphene structures. Here, we use dark-field transmission electron microscopy for rapid and accurate determination of key structural parameters (twist angle, stacking order, and interlayer spacing) of few-layer CVD graphene. We image the long-range atomic registry for oriented bilayer and trilayer graphene, find that it conforms exclusively to either Bernal or rhombohedral stacking, and determine their relative abundances. In contrast, our data on twisted multilayers suggest the absence of such long-range atomic registry. The atomic registry and its absence are consistent with the two different strain-induced deformations we observe; by tilting the samples to break mirror symmetry, we find a high density of twinned domains in oriented multilayer graphene, where multiple domains of two different stacking configurations coexist, connected by discrete twin boundaries. In contrast, individual layers in twisted regions continuously stretch and shear independently, forming elaborate Moiré patterns. These results, and the twist angle distribution in our CVD graphene, can be understood in terms of an angle-dependent interlayer potential model.     

The electronic properties of graphene and its bilayer

We present a discussion of some of the physical properties of graphene and its bilayer. In particular, we focus our attention on the calculation of the transparency of graphene and on the dependence of the energy gap of the biased graphene bilayer on the electronic density. We show that the transparency of graphene is controlled by the value of the fine structure constant over a frequency range from the infra-red to the ultra-violet. We derive the dependence of the energy gap of the graphene bilayer on the external applied electric field.