The recent proposal of cascading spin logic in low-dimensional carbon materials by Friedman et al.  has provided a new paradigm to perform efficient high-performance computing. In this scenario, graphene nanoribbon (GNR) transistors, in concert with metallic carbon nanotubes (CNT) interconnect, provide compact logic, low current requirements, and fast switching times. The idea is based on the recent discovery of CNT unzipping mechanisms and exceptional negative magnetoresistance in GNRs for which partially unzipped CNTs permit the development of an all-carbon spintronic logic family.
In this talk, we describe the physical model that predicts the onset of spin polarization on zig-zag GNR edges in the presence of magnetic field. The model is based on a tight-binding Hamiltonian in the presence of non-local magnetic field with site-to-site interaction up to the third nearest neighbor. Coupled to a non-equilibrium green function (NEGF) formalism, our model shows electrons of opposing spin result in two stable states: a global antiferromagnetic (AFM) ground state with zero net magnetization, and a ferromagnetic (FM) state at a slightly higher energy with a net magnetization. Both magnetic states have local AFM ordering and large magnetization at the edge sites. A magnetic field arising from a current passing through a CNT close to the edge alters the GNR's magnetic energy via the Zeeman interaction. When the field is strong enough, the change of the GNR ground state from AFM to FM switches its electrical behavior from insulating to conducting.
1. J.S. Friedman et al., Nature Communications 8, 15635 (2017).
The single-atom thickness of monolayer graphene makes it an ideal candidate for DNA sequencing as it can scan molecules passing through a nanopore at high resolution. Additionally, unlike most insulating membranes, graphene is electrically active, and this property can be exploited to control and electronically sense biomolecules. We show that the shape of the edge as well as the shape and position of the nanopore can strongly affect the electronic conductance through a lateral constriction in a graphene nanoribbon as well as its sensitivity to external charges. In this context the geometry of the graphene membrane can be tuned to detect the rotational and positional conformation of a charge distribution inside the nanopore. We show that a quantum point contact (QPC) geometry is suitable for the electrically-active graphene layer and propose a viable design for a graphene-based DNA sequencing device.
Properties of the stability diagram and exchange energy of a few-electron laterally coupled quantum dots in magnetic fields are investigated. The calculations are performed by numerically exact diagonalization of the many-body Schroedinger equation. We show variations of the energy separation between the single-particle ground and first excited states, and the exchange energy with biases on the two dots at different magnetic fields. Two-dimensional single-particle wavefunction and electron density profiles show electron localization with magnetic fields. From the extracted double-triple point separation on the stability diagram, we also show that the coupling strength decrease as the magnetic field increases.