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Register a new userSymmetry of electron bands in graphene: (nearly) free electron vs. tight-binding
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We present the symmetry labelling of all electron bands in graphene obtained by combining numerical band calculations and analytical analysis based on group theory. The latter was performed both in the framework of the (nearly) free electron model, or in the framework of the tight-binding model. The predictions about relative positions of the bands which can be made on the basis of each of the models just using the group theory (and additional simple qualitative arguments, if necessary) are complimentary.
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Nearly free electron (NFE) state is an important kind of unoccupied state in low dimensional systems. Although it is intensively studied, a clear picture on its physical origin and its response behavior to external perturbations is still not available. Our systematic first-principles study based on graphene nanoribbon superlattices suggests that there are actually two kinds of NFE states, which can be understood by a simple Kronig-Penney potential model. An atom-scattering-free NFE transport channel can be obtained via electron doping, which may be used as a conceptually new field effect transistor.
Artificial graphene consisting of honeycomb lattices other than the atomic layer of carbon has been shown to exhibit electronic properties similar to real graphene. Here, we reverse the argument to show that transport properties of real graphene can be captured by simulations using theoretical artificial graphene. To prove this, we first derive a simple condition, along with its restrictions, to achieve band structure invariance for a scalable graphene lattice. We then present transport measurements for an ultraclean suspended single-layer graphene pn junction device, where ballistic transport features from complex Fabry-Perot interference (at zero magnetic field) to the quantum Hall effect (at unusually low field) are observed and are well reproduced by transport simulations based on properly scaled single-particle tight-binding models. Our findings indicate that transport simulations for graphene can be efficiently performed with a strongly reduced number of atomic sites, allowing for reliable predictions for electric properties of complex graphene devices. We demonstrate the capability of the model by applying it to predict so-far unexplored gate-defined conductance quantization in single-layer graphene.
Using a set of first-principles calculations, we studied the electronic structures of two-dimensional transition metal carbides and nitrides, so called MXenes, functionalized with F, O, and OH. Our projected band structures and electron localization function analyses reveal the existence of nearly free electron (NFE) states in variety of MXenes. The NFE states are spatially located just outside the atomic structure of MXenes and are extended parallel to the surfaces. Moreover, we found that the OH-terminated MXenes offer the NFE states energetically close to the Fermi level. In particular, the NFE states in some of the OH-terminated MXenes, such as Ti2C(OH)2, Zr2C(OH)2, Zr2N(OH)2, Hf2C(OH)2, Hf2N(OH)2, Nb2C(OH)2, and Ta2C(OH)2, are partially occupied. This is in remarkable contrast to graphene, graphane, and MoS2, in which their NFE states are located far above the Fermi level and thus they are unoccupied. As a prototype of such systems, we investigated the electron transport properties of Hf2C(OH)2 and found that the NFE states in Hf2C(OH)2 provide almost perfect transmission channels without nuclear scattering for electron transport. Our results indicate that these systems might find applications in nanoelectronic devices. Our findings provide new insights into the unique electronic band structures of MXenes.
Graphene has proven to host outstanding mesoscopic effects involving massless Dirac quasiparticles travelling ballistically resulting in the current flow exhibiting light-like behaviour. A new branch of 2D electronics inspired by the standard principles of optics is rapidly evolving, calling for a deeper understanding of transport in large-scale devices at a quantum level. Here we perform large-scale quantum transport calculations based on a tight-binding model of graphene and the non-equilibrium Greens function method and include the effects of $p-n$ junctions of different shape, magnetic field, and absorptive regions acting as drains for current. We stress the importance of choosing absorbing boundary conditions in the calculations to correctly capture how current flows in the limit of infinite devices. As a specific application we present a fully quantum-mechanical framework for the 2D Dirac fermion microscope recently proposed by B{o}ggild $et, al.$ [Nat. Comm. 8, 10.1038 (2017)], tackling several key electron-optical effects therein predicted via semiclassical trajectory simulations, such as electron beam collimation, deflection and scattering off Veselago dots. Our results confirm that a semiclassical approach to a large extend is sufficient to capture the main transport features in the mesoscopic limit and the optical regime, but also that a richer electron-optical landscape is to be expected when coherence or other purely quantum effects are accounted for in the simulations.
In this paper, a possible way to achieve lasing from THz to extreme UV domain due to stimulated scattering of graphene plasmons on the free electrons is considered. The analytical-quantitative description of the proposed FEL scheme is based on the self-consistent set of the Maxwell--Vlasov equations. We study the downconversion as well as the upconversion. It is shown that the coherent downconversion of infrared radiation to THz one can be achieved using a source of very non-relativistic electrons at the resonant coupling with the graphene plasmons. Due to the strongly confined graphene plasmons, the upconversion of mid-infrared to extreme UV radiation can be achieved with the mildly relativistic electron beams. The latter is a promising mechanism for the tabletop short-wavelength free electron nanolaser.
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