Si(001)-(2×1) surface is one of the many two-dimensional systems of scientific and applied interest. It has two surface state bands (1) anti-bonding π * band, which has acceptor states and (2) bonding π band, which has donor states. Due to its asymmetric dimer reconstruction, transport through this surface can be considered in two distinct directions, i.e. along and perpendicular to the paired dimer rows. We calculate the zero bias conductance of these surface states under flat-band condition and find that conduction along the dimer row direction is significant due to strong orbital hybridization. We also find that the surface conductance is orders of magnitude higher than the bulk conductance close to the band edges for the unpassivated surface at room temperature. Therefore, we propose that the transport through these surface states may be the dominant conduction mechanisms in the recently reported scanning tunneling microscopy of silicon nanomembranes. We also calculate the zero bias conductance under flat-band condition for the weakly interacting dangling bond wires along and perpendicular to the dimer row direction and find similar trends. Extended Hückel theory is used for the electronic structure calculations, which is benchmarked with the GW approximation for Si and has been successfully applied to Si systems in past.
Although the theory of tunnel magnetoresistance (TMR) in Fe/MgO/Fe heterostructures is well known, there is a discrepancy between the values predicted by ab initio calculations with a band gap of 5.2 eV and the ones predicted by other methods, e.g., empirical tight-binding with a band gap of 7.6 eV. To our knowledge, no one has yet used the same theory to explore the reasons behind this discrepancy. In this work, we report a three-dimensional atomistic nonequilibrium Green’s function transport model with two set of transferable extended Hückel theory parameters for MgO; one with the experimental band gap of 7.8 eV and the other with the local density approximation of the density functional theory band gap of 5.2 eV. To capture the symmetry filtering property of MgO, we parameterize using the k-resolved orbital projected density of states as the benchmark. We show that the band gap has a significant effect on the barrier width dependence and the bias dependence of the transport quantities. By using the experimental band gap, the TMR is much smaller than the one observed with a band gap of 5.2 eV.
We present a computationally efficient transferable single-band tight-binding model (SBTB) for spin polarized transport in heterostructures with an effort to capture the band structure effects. As an example, we apply it to study transport through Fe-MgO-Fe(100) magnetic tunnel junction devices. We propose a novel approach to extract suitable tight-binding parameters for a material by using the energy resolved transmission as the benchmark, which inherently has the bandstructure effects over the two dimensional transverse Brillouin zone. The SBTB parameters for each of the four symmetry bands for bcc Fe(100) are first proposed which are complemented with the transferable tight-binding parameters for the MgO tunnel barrier for the ∆1 and ∆5 bands. The non-equilibrium Green's function formalism is then used to calculate the transport. Features like I-V characteristics, voltage dependence and the barrier width dependence of the tunnel magnetoresistance ratio are captured quantitatively and the trends match well with the ones observed by ab initio methods. 75.47.Jn,
We report the use of bilayer graphene as an atomically smooth contact for nanoscale devices. A two-terminal bucky-ball (C60) based molecular memory is fabricated with bilayer graphene as a contact on the polycrystalline nickel electrode. Graphene provides an atomically smooth covering over an otherwise rough metal surface. The use of graphene additionally prohibits the electromigration of nickel into the C60 layer. The devices exhibit a low-resistance state in the first sweep cycle and irreversibly switch to a high-resistance state at 0.8 to 1.2 V bias. In the subsequent cycles, the devices retain the high-resistance state, thus making it write-once read-many memory.
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