Yisong Zheng

Tunable pure spin currents in a triple-quantum-dot ring

Electronic transport through a triple-quantum-dot ring with three terminals is theoretically studied. By introducing local Rashba spin-orbit interaction on an individual quantum dot, we calculate the charge and spin currents in one lead. We find that a pure spin current without an accompanying charge current appears even at zero magnetic field case. The polarization direction of the spin current can be inverted by altering the bias voltage. In addition, by tuning the magnetic field strength, the charge and spin currents reach their respective peaks alternately.

Detection of a Majorana fermion zero mode by a T-shaped quantum-dot structure

We investigate the electron transport through the T-shaped quantum-dot (QD) structure theoretically, by coupling a Majorana zero mode to the terminal QD. It is found that in the double-QD configuration, the presence of the Majorana zero mode can efficiently dissolve the antiresonance point in the conductance spectrum while inducing a conductance peak to appear at the same energy position. In the case of asymmetric QD-lead coupling, such a valley-to-peak transition induced by the Majorana zero mode still exists. Next, we observe in the multi-QD case that at the zero-bias limit, the conductance values are always the same as the double-QD result, independent of the parity of the QD number. We believe that all these results can be helpful for understanding the properties of Majorana bound states.

A simple tight-binding model for typical graphyne structures

A π-electronic tight-binding (TB) model with, at most, three independent parameters is found to well fit the density functional theory results about the dispersions of the conduction and valence bands of α-, β-, γ- and (6,6,12)-graphyne. By means of such a toy model, the electron–hole symmetry in these graphynes is demonstrated. An explicit expression of the dispersion relation of α-graphyne is obtained. The position of the Dirac point on a particular Γ–M line in the Brillouin zone of β-graphyne is analytically determined. The absence of Dirac cones in γ-graphyne is intuitively explained. Based on these interesting results, it is believed that this TB model provides a simple but effective theoretical approach for further study of the electronic and transport properties of these typical graphynes.

Generalized transfer matrix theory of electronic transport through a graphene waveguide

In the effective-mass approximation, electronic property in graphene can be characterized by the relativistic Dirac equation. Within such a continuum model we investigate the electronic transport through graphene waveguides formed by connecting multiple segments of armchair-edged graphene nanoribbons of different widths. By using appropriate wave function connection conditions at the junction interfaces, we generalize the conventional transfer matrix approach to formulate the linear conductance of the graphene waveguide in terms of the structure parameters and the incident electron energy. In comparison with the tight-binding calculation, we find that the generalized transfer matrix method works well in calculating the conductance spectrum of a graphene waveguide even with a complicated structure and relatively large size. The calculated conductance spectrum indicates that the graphene waveguide exhibits a well-defined insulating band around the Dirac point, even though all the constituent ribbon segments are gapless. We attribute the occurrence of the insulating band to the antiresonance effect which is intimately associated with the edge states localized at the shoulder regions of the junctions. Furthermore, such an insulating band can be sensitively shifted by a gate voltage, which suggests a device application of the graphene waveguide as an electric nanoswitch.

Spin polarized electron transport through a graphene nanojunction

The electronic transmission spectrum of a graphene nanojunction formed by interconnecting two armchair-edged graphene nanoribbons is obtained by the first principle calculation. We find that in such a simple structure the electronic transmission is remarkably spin-polarized when its size is not very small. By calculating the local density of states and electron occupation numbers on some typical atoms at the scattering region, we conclude that the origin of the spin-polarization is the antiresonance effect, generated by the edge state localized at the zigzag-edged shoulder of the nanojunction.

Suppressed conductance in a metallic graphene nanojunction

The linear conductance spectrum of a metallic graphene junction formed by interconnecting two gapless graphene nanoribbons is calculated. A strong conductance suppression appears in the vicinity of the Dirac point. We found that such a conductance suppression arises from the antiresonance effect associated with an edge state localized at the zigzag-edged shoulder of the junction. The conductance valley due to the antiresonance is rather robust in the presence of the impurity and vacancy scattering. Also the center of the conductance valley can be readily tuned by an electric field exerted on the wider nanoribbon.

RKKY interaction in AB-stacked multilayer graphene

The RKKY interaction between two magnetic impurities absorbed on the surface layer of half-filled AB-stacked multilayer graphene (ABSMLG) is theoretically studied based on the lattice Greens function technique. In comparison with the case of monolayer graphene, the RKKY interaction in such multilayer graphene presents distinct properties in some aspects. Firstly, from the numerical results, we find that the thickness of the ABSMLG influences the RKKY interaction in a complicated manner, depending on the odd/even parity of the number of layers and the sublattice attribution of the positions of the two magnetic impurities. Then, we derive the asymptotic expressions of the RKKY interactions in ABSMLG in the long-distance limit. For even-layered ABSMLG, we find that the RKKY interactions of the 1A-1A, 1B-1A and 1B-1B couplings fall off as 1/R(2), 1/R(4) and 1/R(6) (1A and 1B stand for, respectively, the sublattice points in the surface layer, which are positioned directly on the plaquette and on a lattice point of the layer underneath). On the other hand, in odd-layered ABSMLG, the decays of these interactions follow the 1/R(2), 1/R(3) and 1/R(3) power laws respectively. In addition, we also find that these analytical expressions are quantitatively valid to describe the RKKY interaction in ABSMLG when the distance between the two magnetic impurities is larger than the lattice constant of graphene by one order of magnitude.

Calculation of the effective dielectric function of composites with periodic geometry

A method for calculating the effective dielectric function of a two-component periodic composite is described. Using a simple Fourier expansion technique, we obtain an explicit power series expression of H(t), which is one of the characteristic geometric functions of the two-component composite proposed by Bergman. The relation between the series of H(t) and that of another characteristic geometric function of composite F(s) is studied. The dielectric function of composite F, Of two kinds of model systems is calculated by using both H(t) and F(s) for finite-size reciprocal lattice. The deviations of the numerical results of epsilon (e) from the exact ones, which are caused by the limited size of the reciprocal lattice used, are investigated. It is found that the: accuracies of the numerical results of F(s) differ from those of H(t). For simple cubic arrays of nonoverlapping spheres, the results of epsilon (e), obtained from H(t) are closer to the exact ones, especially when the volume fraction of the inclusions is larger and the dielectric contrast of the composite is higher. For 2-D prisms, the averages of the results of epsilon (e) obtained from using F(s) and those from H(t) are closer to the exact ones

Odd-even effect of transport through a chain of Majorana bound states in a T-shaped junction

We investigate the transport characteristics of a chain of Majorana bound states (MBSs) in a T-shaped junction, where two normal leads are coupled to the same terminate MBS. We find the apparent odd-even effect in the transport process. Namely, when the MBS number is odd, the conductance spectrum exhibits a peak in the zero-bias limit. Besides, the shot noise Fano factor in the zero-bias limit and the conductance maximum are related by equation with . Otherwise, in the case of even-numbered MBSs, at the zero-bias limit, the conductance encounters its zero value, and the relation between F0 and changes as . Further investigation shows that these two kinds of relations are caused by the different interplay mechanisms between the crossed Andreev reflection and the local Andreev reflection. In addition, it is observed that the fluctuation of the inter-MBS couplings contributes little to the transport results. We ascertain that these results are helpful for understanding the MBS signature in transport spectra.

Andreev Reflection in a T-Shaped Double-Quantum-Dot Structure Induced by Majorana Bound States

The Andreev reflection in a T-shaped double-quantum-dot structure induced by Majorana bound states is theoretically investigated. It is found that due to the Fano effect, the Andreev conductance spectrum presents a well-defined insulating band in the low-bias region. By discussing the influences of quantum dot levels, interdot coupling, and the coupling between the quantum dot and the Majorana bound state, the properties of the insulating band are analyzed in detail. We believe that all the results can help the researchers to understand the nontrivial role of the Majorana bound states in manipulating the Andreev reflection.