V. Lien Nguyen

Controllable spin-dependent transport in armchair graphene nanoribbon structures

Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.

Resonant tunneling and negative transconductance in single barrier bilayer graphene structure

Using the nonequilibrium Green’s function method, the electronic transport in a gate-induced barrier bilayer graphene structure is investigated. Strong resonant effects are shown to result in high amplitude oscillation of conductance as a function of Fermi energy and barrier height. Beyond a small effect of negative differential conductance (with peak to valley ratio less than 2), strong oscillations of transconductance are achieved. The amplitude of such oscillations between positive and negative values may exceed 5u2002mS/μm. This effect might be helpful for further development of graphene-based nanoelectronics.

Shot noise in metallic double dot structures with a negative differential conductance

The shot noise of current through a metallic double quantum dot structure exhibiting negative differential conductance is studied. We can exactly solve the master equation and derive an analytical expression of the spectral density of current fluctuations as a function of frequency in the first Coulomb staircase region. For a large range of bias voltage the noise is calculated by Monte Carlo simulation. We show that the noise is always sub-Poissonian though it is considerably enhanced in the negative differential conductance regime.

Coulomb blockade and negative differential conductance in metallic double-dot devices

We systematically analyze the stability diagrams and simulate the finite temperature current-voltage characteristics for metallic double-dot devices with cross couplings between dots and gates. The Coulomb blockade is described with respect to each device parameter. The negative differential conductance observed is essentially suppressed by increasing the temperature and/or introducing the offset charge and is very sensitive to the device parameters.

Negative differential conductance in metallic double quantum dot structures

We show that negative differential conductance (NDC) can be observed in metallic quantum dot structures. For a simple model at zero temperature we have derived an analytical expression for the current–voltage characteristics and a condition for observing NDC. For devices with gates at finite temperatures, using the Monte Carlo method we have suggested diagrams describing a correlation between the gate capacitance or temperature and the inter-dot coupling in producing (or removing) NDC.

Super-Poissonian noise in a Coulomb-blockade metallic quantum dot structure

The shot noise of the current through a single electron transistor (SET), coupled capacitively with an electronic box, is calculated, using the master equation approach. We show that the noise may be sub-Poissonian or strongly super-Poissonian, depending mainly on the box parameters and the gate. The study also supports the idea that not negative differential conductance, but charge accumulation in the quantum dot, responds for the super-Poissonian noise observed.

Spin-dependent transport in armchair graphene nanoribbon structures with edge roughness effects

We analyze the spin-dependent transport in single ferromagnetic gate structures based on armchair graphene nanoribbon (GNR) using the non-equilibrium Greens function method in a tight binding model. It is shown that the spin polarized current oscillates as a function of the gate-induced barrier height. For perfect GNRs, the larger the energy band gap, the stronger the oscillation of the spin polarization. However, though the edge roughness of the ribbons tends to enlarge the band gap, it also strongly reduces the conductance which finally degrades the spin polarized current.

Coulomb blockade, current and shot noise in parallel double metallic quantum dot structures

We systematically study sequential electron tunnelling through parallel double metallic quantum dot structures, focusing on the role of inter-dot coupling and parameter asymmetry. It is shown that the evolution of Coulomb blockade charging diagrams, induced by only the inter-dot capacitance, describes the existing experimental data quite well. Both the inter-dot capacitance and the resistance strongly affect the shot noise, making it super-Poissonian even in fully symmetric structures. An asymmetry of parameters, enhancing the role of inter-dot coupling, may produce a variety of current–voltage characteristics behaviours, including negative differential conductance regions, and a very large noise. For all structures studied the noise is shown to be more sensitive than the conductance to the inter-dot coupling as well as the parameter asymmetry.

Cotunnelling versus sequential tunnelling in Coulomb blockade metallic double quantum dot structures

The cotunnelling is systematically studied in comparison to sequential tunnelling in Coulomb blockade metallic double quantum dot structures, using the standard master equation approach. In the case of zero gate voltage, we are able to derive analytical expressions of threshold voltages for both tunnelling processes: Vth(s) for the sequential tunnelling and Vth(2) for the lowest-order inelastic macroscopic quantum tunnelling (cotunnelling). Taking into account the gate and temperature effects, numerical solutions of the master equation show that an increase of the inter-dot capacitance leads to a decrease of the ratio Vth(2)/Vth(s) and at the same time to a decrease of cotunnelling conductance compared to the sequential one. An oscillation of cotunnelling conductance is observed in classical Coulomb blocked regions. In comparison with the sequential tunnelling conductance spectroscopy, the peak height of cotunnelling conductance is about three orders of magnitude smaller and the peak spacing distribution is far from regular. Increase of temperature raises the current and destroys the Coulomb gap. Its relative influence is more important at lower bias.