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The nonideal effects in a quantum field-effect directional coupler where two quantum wires are coupled through a finite potential barrier are studied by adopting the lattice Green function method. The results show that the electron energy distribution, asymmetric geometry and finite temperature all have obvious influence on the electron transfer of the coupler. Only for the electrons with energies in a certain region, can the complete periodic transfer between two quantum wires take place. The conductance of these electrons as a function of the barrier length and potential height exhibits a fine periodic or quasi-periodic pattern. For the electrons with energies beyond the region, however, the complete periodic transfer does not hold any more since many irregular oscillations are superimposed on the conductance profile. In addition, the finite temperature and asymmetric geometry both can reduce the electron transfer efficiency.

By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant peak splitting of this kind of structure is different from that of `magnetic' and `electric' superlattices in two-dimensional electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.

Using Green's function method, we investigate the spin transport properties of armchair graphene nanoribbons (AGNRs) under magnetic field and uniaxial strain. Our results show that it is very difficult to transform narrow AGNRs directly from semiconductor to spin gapless semiconductors (SGS) by applying magnetic fields. However, as a uniaxial strain is exerted on the nanoribbons, the AGNRs can transform to SGS by a small magnetic field. The combination mode between magnetic field and uniaxial strain displays a nonmonotonic arch-pattern relationship. In addition, we find that the combination mode is associated with the widths of nanoribbons, which exhibits group behaviors.

Using nonequilibrium Green's function method, the thermal transport properties of zigzag graphene nanoribbons (ZGNR) embedding a finite (semi-infinite or infinite) long linear defect are investigated in this paper. The results show that defect type and defect length have significant influence on the thermal conductance of ZGNR. When the embedded linear defects have the same lengths, thermal conductance of ZGNR embedding t5t7 defect is lower than that of ZGNR embedding Stone-Wales defect. As for the ZGNR embedding finite and the same type defects, their thermal conductance reduce with the increase of the defect length. However, as the linear defect is long enough, the thermal conductance is insensitive to the change of length. By comparing the ZGNRs embedding finite, semi-infinite and infinite long defects, we find that the thermal conductance of ZGNR embedding an infinite long defect is higher than that of ZGNR embedding a semi-infinite defect, while the thermal conductance of the latter is higher than that of ZGNR embedding a finite long defect. This is due to the fact that different structures possess different numbers of scattering interfaces in the phonon transmission direction. The more the scattering interfaces, the lower the thermal conductance is. These thermal transport phenomena are explained by analyzing transmission coefficient and local density of states. These results indicate that linear defects can tune thermal transport property of ZGNR efficiently.

In practical applications of graphene-based electronic devices, they may have some pore defects under energetic particle bombardment, or chemical corrosion, which will inevitably affect their electrical properties. These problems have recently aroused great concern and interest. In this paper, we systematically study the influence of shape (tripartite, tetragonal and hexagonal) of hole defect on the electrical property of zigzag graphene nanoribbon (ZGNR). The results show that the influence of the shape of the pore defects on the conductance and current characteristics of ZGNRs is significant, whicl may result from electron scattering for the different shapes of the poredefect boundary. In addition, due to defects in suspension adsorbed hydrogen or nitrogen atoms, caused by defects of the pore shape changes, it also affects the electrical properties of ZGNRs. This study will supply valuable theoretical guidances for graphene-based electronic device failure analysis and the design of the graphene pore structure.

Based on the Anderson tight-binding model, the electronic properties of disordered bilayer hexagonal boron nitride quantum films are investigated. Our numerical results show that the electrons in a disordered bilayer hexagonal boron nitride quantum film are localized, presenting an insulating property. However, for the monolayer disordered bilayer hexagonal boron nitride quantum film, the energy spectrum has persistent mobility edges which are independent of the disorder strength. This indicates that a metal-insulator transition occurs in the monolayer disorder structure. This is similar to the case in an order-disorder separated quantum film. The results could offer useful information for understanding and manipulating the electronic properties of bilayer hexagonal boron nitride quantum films.

By using nonequilibrium Green’s function method, the thermal transport properties of L-shaped graphene nano-junctions consisting of a semi-infinite armchair-edged nanoribbon and a semi-infinite zigzag-edged nanoribbon were studied. It is shown that the thermal conductance of the L-shaped graphene nano-junctions depends on the included angles and the widths of the graphene nanoribbons. As the angle of L-shaped graphene nano-junctions increases from 30° to 90° and further to 150°, the thermal conductance obviously increases. For the right-angle L-shape graphene nano-junction, the thermal conductance undergoes a transition with the increasing of the widths of the armchair nanoribbons. The thermal conductance decreases at low temperature region and increases at high temperature region. Meanwhile the thermal conductance of L-shape graphene nano-junction with included angle 150° decreases by increasing the widths of zigzag-edged nanoribbons in both low and high temperature regions. These thermal transport phenomena can be reasonably explained by analyzing the phonon transmission coefficient. We illustrate the mechanisms of thermal transport for different L-shaped graphene nano-junctions. The results provide significant physical models and theoretical basis for designing the thermal devices based on the graphene nano-junctions.

The effect of stacked graphene flakes (GFs) on the electronic transport property of zigzag-edged graphene nanoribbon (ZGNR) is investigated. By using the Greens function method, we calculate the conductances of ZGNRs with two different stacked-type GFs. It is found that the coupling effect between ZGNRs and GFs can induce dips at the conductance profiles in two different stacked-types. For both stacked-types, the dips far away from the Fermi level are nearly overlapped. However, the position of conductance dip near the Fermi level depends on the stacked-type. In addition, we discuss the effect of geometric size of GF on the electronic transport property. The results show that with the increase of the size of GF, the dips far away the Fermi level in two stacked-types gradually move toward the Fermi level, while the discrepancy of the dips near the Fermi level is much evident. Our results indicate that the stacked GFs can effectively tune the electronic transport of ZGNR.

Using first-principles method, we studied the electronic structure and the bonding mechanism of La-Ir-Si materials. The results of the band structure and the density of the states indicated that the superconducting property of the La-Ir-Si system is determined by the p-d coupling strength between the transition element Ir-d and Si-p states of the material. In order to quantitatively describe the p-d coupling strength, we calculated the charge transfer during the bond process of the materials using the atom-in-molecule method. The results revealed that the superconducting transition temperature TC is linerly proportional to the atomic basin charges of Ir.