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Register a new userElectronegativity in quantum electronic transport
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Electronegativity is shown to control charge transfer, energy level alignments, and electron currents in single molecule tunnel junctions, all of which are governed by correlations contained within the density matrix. This is demonstrated by the fact that currents calculated from the one-electron reduced density matrix to second order in electron correlation are identical to the currents obtained from the Greens function corrected to second order in electron self-energy.
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For semimetal nanowires with diameters smaller than a few tens of nanometers, a semimetal-to-semiconductor transition is observed as the emergence of an energy band gap resulting from quantum confinement. Quantum confinement in a semimetal results in either lifting of the degeneracy of the conduction and valence bands in a zero gap semimetal, or shifting of bands with a negative energy overlap to form conduction and valence bands. For semimetal nanowires with diameters below 10 nanometer, the magnitude of the band gap can become significantly larger than the thermal energy at room temperature resulting in a new class of semiconductors relevant for nanoelectronics with critical dimensions on the order of a few atomic lengths. The smaller a nanowires diameter, the larger its surface-to-volume ratio thus leading to an increasing impact of surface chemistry on its electronic structure. Energy level shifts to states in the vicinity of the Fermi level due to the electronegativity of surface terminating species are shown to be comparable in magnitude to quantum confinement effects at nanowire diameters of a few nanometer; these two effects can be used to counteract one another leading to semimetallic behavior for nanowire cross sections at which the quantum confinement effect would otherwise dominate. Abruptly changing the surface terminating species along the length of a nanowire leads to an abrupt change in the surface electronegativity. This can result in the formation of a semimetal-semiconductor junction within a monomaterial nanowire, without the need for impurity doping nor requiring the formation of a heterojunction.
Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition, and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these collapsed wrinkle structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide folded wrinkles, we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.
We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berrys phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms.
The success of all-graphene electronics is severely hindered by the challenging realization and subsequent integration of semiconducting channels and metallic contacts. Here, we comprehensively investigate the electronic transport across width-modulated heterojunctions consisting of a graphene quantum dot of varying lengths and widths embedded in a pair of armchair-edged metallic nanoribbons, of the kind recently fabricated via on-surface synthesis. We show that the presence of the quantum dot enables the opening of a width-dependent transport gap, thereby yielding built-in one-dimensional metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity of the band edges, the conductance is subject to a smooth transition from an antiresonant to a resonant transport regime upon increasing the channel length. These results are rationalized in terms of a competition between quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work establishes graphene quantum dot nanoarchitectures as appealing platforms to seamlessly integrate gap-tunable semiconducting channels and metallic contacts into an individual nanoribbon, hence realizing self-contained carbon-based electronic devices.
In nearly compensated graphene, disorder-assisted electron-phonon scattering or supercollisions are responsible for both quasiparticle recombination and energy relaxation. Within the hydrodynamic approach, these processes contribute weak decay terms to the continuity equations at local equilibrium, i.e., at the level of ideal hydrodynamics. Here we report the derivation of the decay term due to weak violation of energy conservation. Such terms have to be considered on equal footing with the well-known recombination terms due to nonconservation of the number of particles in each band. At high enough temperatures in the hydrodynamic regime supercollisions dominate both types of the interaction). We also discuss the contribution of supercollisions to the heat transfer equation (generalizing the continuity equation for the energy density in viscous hydrodynamics).
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