Liu Nian-hua

First-principles study on transport properties of zigzag graphene nanoribbon with different spin-configurations

The current-voltage (I -V ) characteristics and the transmission spectra of zigzag graphene nanoribbon with different spin-configurations are investigated by using first-principles calculations. It is shown that the I -V curves and transmission spectra strongly depend on the spin-configurations of the two sides of the ribbon. For the spin-parallel configuration structure, the curve is linear under lower bias voltage; for the spin-antiparallel config- uration structure, there is a strong spin-polarization-dependent transmission which implies that the ribbon can be used as a spin filter; while for other spin-configuration structures, the curve has the characteristics of a semicon- ductor. It is found that there is a large magneto-resistance (MR) when the bias voltage is small. The impurity in the central scattering region significantly influences the spin-dependent current and the spin filter efficiency, which may lead the large MR to disappear.

Large-Area Self-Assembly of Rubrene on Au(111) Surface

Large-area self-assembly of rubrene has been fabricated on Au(111) surface and studied by scanning tunnelling microscopy. The rubrene monolayer on Au(111) surface is characterized by well-ordered row-like structures similar to the a – b plane of rubrene single crystal. However, the directions of the neighboured molecular rows are opposite to each other. In a two-layer film of rubrene on Au(111) surface, the arrangements of the top molecular rows are determined by the underneath rows, suggesting an orthorhombic crystal structure with a = 1.26 nm, b = 4.36 nm, c = 0.17nm for multilayer of rubrene on Au(111).

Negative Differential Resistance in Atomic Carbon Chain-Graphene Junctions

We investigate the electronic transport properties of atomic carbon chain-graphene junctions by using the density-functional theory combining with the non-equilibrium Greens functions. The results show that the transport properties are sensitively dependent on the contact geometry of carbon chain. From the calculated I-V curve we find negative differential resistance (NDR) in the two types of junctions. The NDR can be considered as a result of molecular orbitals moving related to the bias window.

Inter-Well Coupling and Resonant Tunneling Modes of Multiple Graphene Quantum Wells

We investigate the inter-well coupling of multiple graphene quantum well structures consisting of graphene superlattices with different periodic potentials. The general form of the eigenlevel equation for the bound states of the quantum well is expressed in terms of the transfer matrix elements. It is found that the electronic transmission exhibits resonant tunneling peaks at the eigenlevels of the bound states and shifts to the higher energy with increasing the incident angle. If there are N coupled quantum wells, the resonant modes have N-fold splitting. The peaks of resonant tunneling can be controlled by modulating the graphene barriers.

Electromagnetic Bloch oscillation in one-dimensional multiple microcavities composed of metamaterials

We propose a photonic structure stacked sequentially by one-dimensional photonic crystals and cavities. The whole structure is composed of single-negative and double-negative materials. The optical Wannier?Stark ladder (WSL) can be obtained in a low frequency region by modulating the widths of the cavities in order. We simulate the dynamical behavior of the electromagnetic wave passing through the proposed photonic structure. Due to the dispersive characteristics of the metamaterials, a very narrow WSL can be obtained. The long-period electromagnetic Bloch oscillation is demonstrated theoretically to have a period on a microsecond time scale.

Electronic Energy Band and Transport Properties in Monolayer Graphene with Periodically Modulated Magnetic Vector Potential and Electrostatic Potential

We investigated the electronic energy band and transport features of graphene superlattice with periodically modulated magnetic vector potential and electrostatic potential. It is found that both parallel magnetic vector potential and electrostatic potential can decisively shift Dirac point in a different way, which may be an efficient way to achieve electron or hole filter. We also find that applying modulated parallel and anti-parallel magnetic vector potential to the electrons can efficiently change electronic states between pass and stop states, which can be useful in designing electron or hole switches and lead to large magneto-resistance.

Electronic structures and transport properties of BN nanodot superlattices of armchair graphene nanoribbons

The electronic and transport properties of embedded boron nitride (BN) nanodot superlattices of armchair graphene nanoribbons are studied by first-principles calculations. The band structure of the graphene superlattice strongly depends on the geometric shape and size of the BN nanodot, as well as the concentration of nanodots. The conduction bands and valence bands near the Fermi level are nearly symmetric, which is induced by electron-hole symmetry. When B and N atoms in the graphene superlattices with a triangular BN nanodot are exchanged, the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry. In addition, the hybridization of π orbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level. Our results also show a series of resonant peaks appearing in the conductance. This strongly depends on the distance of the two BN nanodots and on the shape of the BN nanodot. Controlling these parameters might allow the modulation of the electronic response of the systems.

Proposal of an Ultracompact Triplexer Using Photonic Crystal Waveguide with an Air Holes Array

We propose an ultracompact triplexer based on a shift of the cutoff frequency of the fundamental mode in a planar photonic crystal waveguide (PCW) with a triangular lattice of air holes. The shift is realized by modifying the radii of the border holes adjacent to the PCW core. Some defect holes are introduced to control the beam propagation. The numerical results obtained by the finite-difference time-domain method show that the presented triplexer can separate three specific wavelengths, i.e. 1310, 1490 and 1550 nm with the extinction ratios higher than −18 dB. The designed device with a size as compact as 12 μm × 6.5 μm is feasible for the practical application, and can be utilized in the system of fiber to the home.

Tunable Pair Transmission in One-Dimensional Photonic Crystals Consisting of Single-Negative Materials

We study the transmission of one-dimensional photonic crystals consisting of single-negative permittivity and single-negative permeability media by using transfer matrix method. A pair of transmission modes is found in the gap. The transmission modes are dependent only on the ratio of the thicknesses of the two alternating layers. The separation of a pair of transmission modes can be tuned by varying the thickness of the defect layer or the ratio of thicknesses of the two alternating layers.

Engineering Photonic Crystal Impurity Bands for Multiple Channelled Optical Switches

We report theoretically on the engineering of photonic crystal impurity bands to realize multiple channelled optical switches. The mechanism is based on the confinement of the impurity band in a photonic quantum-well structure, leading to N-quantized confined states coming from N-coupled defects. Due to the strong localization of electromagnetic wave at defect regions, the transmission of confined states are greatly dependent on the defects and then multiple channelled optical switches can be realized by slightly changing the defects by a control light. The dependence of the threshold of such a switch on the layer number of photonic barriers is also given.