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Register a new userBallistic transport in graphene antidot lattices
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Graphene samples can have a very high carrier mobility if influences from the substrate and the environment are minimized. Embedding a graphene sheet into a heterostructure with hexagonal boron nitride (hBN) on both sides was shown to be a particularly efficient way of achieving a high bulk mobility. Nanopatterning graphene can add extra damage and drastically reduce sample mobility by edge disorder. Preparing etched graphene nanostructures on top of an hBN substrate instead of SiO2 is no remedy, as transport characteristics are still dominated by edge roughness. Here we show that etching fully encapsulated graphene on the nanoscale is more gentle and the high mobility can be preserved. To this end, we prepared graphene antidot lattices where we observe magnetotransport features stemming from ballistic transport. Due to the short lattice period in our samples we can also explore the boundary between the classical and the quantum transport regime.
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The excitation spectrum and the collective modes of graphene antidot lattices (GALs) are studied in the context of a $pi$-band tight-binding model. The dynamical polarizability and dielectric function are calculated within the random phase approximation. The effect of different kinds of disorder, such as geometric and chemical disorder, are included in our calculations. We highlight the main differences of GALs with respect to single-layer graphene (SLG). Our results show that, in addition to the well-understood bulk plasmon in doped samples, inter-band plasmons appear in GALs. We further show that the static screening properties of undoped and doped GALs are quantitatively different from SLG.
We theoretically study the inelastic scattering rate and the carrier mean free path for energetic hot electrons in graphene, including both electron-electron and electron-phonon interactions. Taking account of optical phonon emission and electron-electron scattering, we find that the inelastic scattering time $tau sim 10^{-2}-10^{-1} mathrm{ps}$ and the mean free path $l sim 10-10^2 mathrm{nm}$ for electron densities $n = 10^{12}-10^{13} mathrm{cm}^{-2}$. In particular, we find that the mean free path exhibits a finite jump at the phonon energy $200 mathrm{meV}$ due to electron-phonon interaction. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.
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.
We have investigated antidot lattices, which were prepared on exfoliated graphene single layers via electron-beam lithography and ion etching, by means of scanning Raman spectroscopy. The peak positions, peak widths and intensities of the characteristic phonon modes of the carbon lattice have been studied systematically in a series of samples. In the patterned samples, we found a systematic stiffening of the G band mode, accompanied by a line narrowing, while the 2D mode energies are found to be linearly correlated with the G mode energies. We interpret this as evidence for p-type doping of the nanostructured graphene.
We report on the fabrication and characterization of a device which allows the formation of an antidot lattice (ADL) using only electrostatic gating. The antidot potential and Fermi energy of the system can be tuned independently. Well defined commensurability features in magnetoresistance as well as magnetothermopower are obsereved. We show that the thermopower can be used to efficiently map out the potential landscape of the ADL.
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