Nonlinear dc transport in graphene

Plasmon Damping Rates in Coulomb-Coupled 2D Layers in a Heterostructure

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

Unsaturated Drift Velocity of Monolayer Graphene

We observe that carriers in graphene can be accelerated to the Fermi velocity without heating the lattice. At large Fermi energy | E_F| > 110 meV, electrons excited by a high-power terahertz pulse E_THz relax by emitting optical phonons, resulting in heating of the graphene lattice and optical-phonon generation. This is owing to enhanced electron-phonon scattering at large Fermi energy, at which the large phase space is available for hot electrons. The emitted optical phonons cause carrier scattering, reducing the drift velocity or carrier mobility. However, for | E_F| ≤ 110 meV, electron-phonon scattering rate is suppressed owing to the diminishing density of states near the Dirac point. Therefore, E_THz continues to accelerate carriers without them losing energy to optical phonons, allowing the carriers to travel at the Fermi velocity. The exotic carrier dynamics does not result from the massless nature, but the electron-optical-phonon scattering rate depends on Fermi level in the graphene. Our observations provide insight into the application of graphene for high-speed electronics without degrading carrier mobility.

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

THz saturable absorption in turbostratic multilayer graphene on silicon carbide

We investigated the room-temperature Terahertz (THz) response as saturable absorber of turbostratic multilayer graphene grown on the carbon-face of silicon carbide. By employing an open-aperture z-scan method and a 2.9 THz quantum cascade laser as source, a 10% enhancement of transparency is observed. The saturation intensity is several W/cm2, mostly attributed to the Pauli blocking effect in the intrinsic graphene layers. A visible increase of the modulation depth as a function of the number of graphene sheets was recorded as consequence of the low nonsaturable losses. The latter in turn revealed that crystalline disorder is the main limitation to larger modulations, demonstrating that the THz nonlinear absorption properties of turbostratic graphene can be engineered via a proper control of the crystalline disorder and the layers number.

Tuning the electronic and optical properties of graphene and boron-nitride quantum dots by molecular charge-transfer interactions: a theoretical study

Spin-polarized first-principles calculations have been performed to tune the electronic and optical properties of graphene (G) and boron-nitride (BN) quantum dots (QDs) through molecular charge-transfer using tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) as dopants. From our results, based on the formation energy and the distance between QDs and dopants, we infer that both the dopants are physisorbed on the QDs. Also, we find that GQDs interact strongly with the dopants compared to the BNQDs. Interestingly, although the dopants are physisorbed on QDs, their interactions lead to a decrement in the HOMO-LUMO gap of QDs by more than half of their original value. We have found a spin-polarized HOMO-LUMO gap in certain QD-dopant complexes. Mülliken population analysis, generation of density of states (DOS) and projected DOS (pDOS) plots, and optical conductivity calculations have been performed to support and understand the reasons behind our findings.

Hydrodynamic model for conductivity in graphene

Based on the recently developed picture of an electronic ideal relativistic fluid at the Dirac point, we present an analytical model for the conductivity in graphene that is able to describe the linear dependence on the carrier density and the existence of a minimum conductivity. The model treats impurities as submerged rigid obstacles, forming a disordered medium through which graphene electrons flow, in close analogy with classical fluid dynamics. To describe the minimum conductivity, we take into account the additional carrier density induced by the impurities in the sample. The model, which predicts the conductivity as a function of the impurity fraction of the sample, is supported by extensive simulations for different values of ε, the dimensionless strength of the electric field, and provides excellent agreement with experimental data.

Size effects in Aharonov-Bohm graphene rings

Aharonov-Bohm (AB) effects in mesoscopic metal rings have been extensively studied. In this paper, we investigate these effects on the persistent currents (PCs) in a closed graphene ring with broken time-reversal symmetry. A hard boundary condition is introduced to describe the Dirac electrons moving along such a ring-shaped configuration, and then the induced persistent currents are numerically calculated. Differing from the properties of PCs revealed in the metal AB rings, we show that the present PCs neither show the regular saw-tooth-like features nor present the odd-even symmetry of the electron number. More interestingly, we show that the energy difference between the two valleys and the amplitude of the oscillating PCs increase with the decrease (increase) of the radius (width) of the graphene ring. Our results imply that the AB effect and size-dependent PCs in ring-shaped microstructures could be tested at room temperature.