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Register a new userElectron-Hole Symmetry Breaking in Charge Transport in Nitrogen-Doped Graphene
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Hongqi Xu Professor
Publication date
2017
fields
Physics
and research's language is
English
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Graphitic nitrogen-doped graphene is an excellent platform to study scattering processes of massless Dirac fermions by charged impurities, in which high mobility can be preserved due to the absence of lattice defects through direct substitution of carbon atoms in the graphene lattice by nitrogen atoms. In this work, we report on electrical and magnetotransport measurements of high-quality graphitic nitrogen-doped graphene. We show that the substitutional nitrogen dopants in graphene introduce atomically sharp scatters for electrons but long-range Coulomb scatters for holes and, thus, graphitic nitrogen-doped graphene exhibits clear electron-hole asymmetry in transport properties. Dominant scattering processes of charge carriers in graphitic nitrogen-doped graphene are analyzed. It is shown that the electron-hole asymmetry originates from a distinct difference in intervalley scattering of electrons and holes. We have also carried out the magnetotransport measurements of graphitic nitrogen-doped graphene at different temperatures and the temperature dependences of intervalley scattering, intravalley scattering and phase coherent scattering rates are extracted and discussed. Our results provide an evidence for the electron-hole asymmetry in the intervalley scattering induced by substitutional nitrogen dopants in graphene and shine a light on versatile and potential applications of graphitic nitrogen-doped graphene in electronic and valleytronic devices.
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The electron-hole symmetry in the structure graphene - insulating substrate -semiconductor gate is violated due to an asymmetrical drop of potential in the semiconductor gate under positive or negative biases. The gate voltage dependencies of concentration and conductivity are calculated for the case of SiO_2 substrate placed over low- (moderate-) doped p-Si. Similar dependencies of the optical conductivity are analyzed for the case of high-kappa substrates (AlN, Al_2O_3, HfO_2, and ZrO_2). The comparison of our results with experimental data shows a good agreement for both cases.
A new type of self-similar potential is used to study a multibarrier system made of graphene. Such potential is based on the traditional middle third Cantor set rule combined with a scaling of the barriers height. The resulting transmission coefficient for charge carriers, obtained using the quantum relativistic Dirac equation, shows a surprising self-similar structure. The same potential does not lead to a self-similar transmission when applied to the typical semiconductors described by the non-relativistic Schrodinger equation. The proposed system is one of the few examples in which a self-similar structure produces the same pattern in a physical property. The resulting scaling properties are investigated as a function of three parameters: the height of the main barrier, the total length of the system and the generation number of the potential. These scaling properties are first identified individually and then combined to find general analytic scaling expressions.
We study conductance across a twisted bilayer graphene coupled to single-layer graphene leads in two setups: a flake of graphene on top of an infinite graphene ribbon and two overlapping semi-infinite graphene ribbons. We find conductance strongly depends on the angle between the two graphene layers and identify three qualitatively different regimes. For large angles ($theta gtrsim 10^{circ}$) there are strong commensurability effects for incommensurate angles the low energy conductance approaches that of two disconnected layers, while sharp conductance features correlate with commensurate angles with small unit cells. For intermediate angles ($3^{circ}lesssim theta lesssim 10^{circ}$), we find a one-to-one correspondence between certain conductance features and the twist-dependent Van Hove singularities arising at low energies, suggesting conductance measurements can be used to determine the twist angle. For small twist angles ($1^{circ}lesssimthetalesssim 3^{circ}$), commensurate effects seem to be washed out and the conductance becomes a smooth function of the angle. In this regime, conductance can be used to probe the narrow bands, with vanishing conductance regions corresponding to spectral gaps in the density of states, in agreement with recent experimental findings.
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