Hengyi Xu

Edge disorder induced Anderson localization and conduction gap in graphene nanoribbons

We study the effect of the edge disorder on the conductance of the graphene nanoribbons (GNRs).We find that only very modest edge disorder is sufficient to induce the conduction energy gap inthe ot ...

Quantized Magnetic Confinement in Quantum Wires

Ballistic quantum wires are exposed to longitudinal profiles of perpendicular magnetic fields composed of a spike and a homogeneous part. An asymmetric magnetoconductance peak as a function of the homogeneous magnetic field is found, comprising quantized conductance steps in the interval where the homogeneous magnetic field and the magnetic barrier have identical polarities, and a characteristic shoulder with several resonances in the interval of opposite polarities. The observations are interpreted in terms of inhomogeneous diamagnetic shifts of the quantum wire modes leading to magnetic confinement.

Transition from ballistic to diffusive behavior of graphene ribbons in the presence of warping and charged impurities

We study the effects of the long-range disorder potential and warping on the conductivity and mobility of graphene ribbons using the Landauer formalism and the tight-binding p-orbital Hamiltonian. We demonstrate that as the length of the structure increases the system undergoes a transition from the ballistic to the diffusive regime. This is reflected in the calculated electron-density dependencies of the conductivity and the mobility. In particular, we show that the mobility of graphene ribbons varies as mu(n)similar to n(-lambda), with 0 andlt;lambda less than or similar to 0.5. The exponent lambda depends on the length of the system with lambda=0.5 corresponding to short structures in the ballistic regime, whereas the diffusive regime lambda=0 (when the mobility is independent on the electron density) is reached for sufficiently long structures. Our results can be used for the interpretation of experimental data when the value of lambda can be used to distinguish the transport regime of the system (i.e., ballistic, quasiballistic, or diffusive). Based on our findings we discuss available experimental results.

Drag-induced particle segregation with vibrating boundaries

We consider a system composed of two different types of particles that have different radii, but equal density. Both particles experience gravity and a linear drag force from the interstitial fluid. They are excited by a boundary that vibrates with high frequency and adds sufficient energy that the particles near the boundary become highly dilated. For moderate energy input rates we show that a single large particle introduced into a large number of small particles will rapidly move to a fixed height and remain approximately stationary. In particular, the large particle will never come into contact with the vibrating base. If there are a large number of large particles, then this behavior leads to a very distinct segregation in which the large particles are sandwiched between two layers of small particles. We show that this behavior occurs as a direct result of non-equilibrium effects and develop a simple phenomenological model that gives good predictions of the height at which the sandwiched layer occurs.

Impurity and edge roughness scattering in graphene nanoribbons: the Boltzmann approach

The conductivity of graphene nanoribbons in the presence of bulk impurities and edge roughness is studied theoretically using the Boltzmann transport equation for quasi-one-dimensional systems. As the number of occupied subbands increases, the conductivity due to bulk impurities converges towards the two-dimensional case. It is shown that the dependence of the conductivity generated by edge roughness scattering depends in a distinctly different way on the sample parameters than the conductivity due to bulk scattering. The Boltzmann model furthermore predicts the amplitude of the edge-roughness-induced magnetoconductance dip as a function of the amplitude and the correlation length of the edge roughness.

Conductivity and scattering in graphene bilayers: Numerically exact results versus Boltzmann approach

We derive analytical expressions for the conductivity of bilayer graphene (BLG) using the Boltzmann approach within the the Born approximation for a model of Gaussian disorders describing both short- and long-range impurity scattering. The range of validity of the Born approximation is established by comparing the analytical results to exact tight-binding numerical calculations. A comparison of the obtained density dependencies of the conductivity with experimental data shows that the BLG samples investigated experimentally so far are in the quantum scattering regime where the Fermi wavelength exceeds the effective impurity range. In this regime both short-and long-range scattering lead to the same linear density dependence of the conductivity. Our calculations imply that bilayer and single-layer graphene have the same scattering mechanisms. We also provide an upper limit for the effective, density-dependent spatial extension of the scatterers present in the experiments.

Resonant reflection at magnetic barriers in quantum wires

The conductance of a quantum wire containing a single magnetic barrier is studied numerically by means of the recursive Greens function technique. For sufficiently strong and localized barriers, F ...

Electronic properties of quantum dots formed by magnetic double barriers in quantum wires

The transport through a quantum wire exposed to two magnetic spikes in series is modeled. We demonstrate that quantum dots can be formed this way which couple to the leads via magnetic barriers. Conceptually, all quantum dot states are accessible by transport experiments. The simulations show Breit-Wigner resonances in the closed regime, while Fano resonances appear as soon as one open transmission channel is present. The system allows one to tune the dot’s confinement potential from subparabolic to superparabolic by experimentally accessible parameters.

Interactions and screening in gated bilayer graphene nanoribbons

The effects of Coulomb interactions on the electronic properties of bilayer graphene nanoribbons (BGNs) covered by a gate electrode are studied theoretically. The electron-density distribution and ...

Magnetic-barrier-induced conductance fluctuations in quantum wires

Quasiballistic semiconductor quantum wires are exposed to localized perpendicular magnetic fields, also known as magnetic barriers. Pronounced, reproducible conductance fluctuations as a function of the magnetic barrier amplitude are observed. The fluctuations are strongly temperature dependent and remain visible up to temperatures of approximate to 10 K. Simulations based on recursive Greens functions suggest that the conductance fluctuations originate from parametric interferences of the electronic wave functions, which experience scattering between the magnetic barrier and the electrostatic potential landscape.